i THE APPLICATION OF THE CRITICAL PATH METHOD TO

advertisement
i
THE APPLICATION OF THE CRITICAL PATH METHOD
TO AIRCRAFT MAINTENANCE
by
HAROLD ANGUS CHARLES SUMMERS
B.A., U n i v e r s i t y o f B r i t i s h Columbia, 1962
A THESIS SUBMITTED AS PARTIAL FULFILMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF BUSINESS ADMINISTRATION
i n the F a c u l t y
of
COMMERCE AND BUSINESS ADMINISTRATION
We accept t h i s
required
t h e s i s as conforming
t o the
standard
THE UNIVERSITY OF BRITISH COLUMBIA
March, 1965
In presenting
the
t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of
r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f
B r i t i s h Columbia, I agree that
the L i b r a r y
a v a i l a b l e f o r r e f e r e n c e and s t u d y .
mission f o r extensive
s h a l l make i t f r e e l y
I f u r t h e r agree that
per-
copying of t h i s t h e s i s f o r s c h o l a r l y
p u r p o s e s may be g r a n t e d by t h e Head o f my D e p a r t m e n t o r by
his
representatives.
I t i s understood that
cation of t h i s thesis f o r f i n a n c i a l gain
w i t h o u t my w r i t t e n
Department
of f^\yyy)AfAAU <W
U//^IA^A-
ll,
s h a l l n o t be a l l o w e d
permission.
liuA^f^-^ y ^ ^ ^ ^ t ^ j ^ ^ X H ^ .
The U n i v e r s i t y o f B r i t i s h C o l u m b i a ,
V a n c o u v e r 8, Canada
Date
c o p y i n g or p u b l i -
/ftsS"
ii
ABSTRACT
The use o f expensive and h i g h l y s p e c i a l i z e d equipment
i n any i n d u s t r y i s o n l y a d v i s a b l e i f the eost o f the equipment
can be j u s t i f i e d by a s u f f i c i e n t l y
l a r g e output.
The g r e a t e r
the output, the s m a l l e r w i l l be the c o s t o f the equipment t o be
borne by each u n i t .
management endeavours
Thus, once such equipment has been purchased,
t o m a i n t a i n output at a maximum i n order
to reduce u n i t c o s t s or t o i n c r e a s e p r o f i t s .
It
i s f o r t h i s reason t h a t a i r l i n e managements c o n t i n -
u a l l y endeavour t o i n c r e a s e the u t i l i z a t i o n o f j e t a i r c r a f t .
i n c r e a s i n g the number o f revenue
week on one j e t a i r c r a f t ,
net
f l y i n g hours o n l y one hour
By
each
an a i r l i n e w i l l r e a l i z e an a d d i t i o n a l
c o n t r i b u t i o n t o overhead
(or p r o f i t s ) o f approximately $60,000
per y e a r .
One method o f i n c r e a s i n g u t i l i z a t i o n I s t o decrease the
downtime o f the a i r c r a f t
f o r maintenance purposes.
This requires
a r e d u c t i o n o f the t o t a l elapsed time o f the maintenance
check.
The c r i t i c a l path technique has found wide a p p l i c a t i o n i n s o l v i n g
the g e n e r a l problem o f r e d u c i n g the time r e q u i r e d to complete a
p r o j e c t which c o n s i s t s o f many i n t e r - r e l a t e d j o b s .
F o r example,
the technique has been used t o reduce the time r e q u i r e d f o r
c o n s t r u c t i n g a b u i l d i n g , f o r completing the p e r i o d i c o v e r h a u l o f
a c h e m i c a l p l a n t , and f o r completing the P o l a r i s M i s s i l e
ment Program.
I t was t h e r e f o r e f e l t
t h a t the c r i t i c a l
Develop-
path
iii
technique
might be o f use i n s o l v i n g t h i s problem o f i n c r e a s i n g
the u t i l i z a t i o n o f j e t a i r c r a f t .
T h i s t h e s i s , based on the r e s u l t s o f a study
carried
out at Canadian P a c i f i c A i r L i n e s d u r i n g the months o f May
through August, 1 9 6 3 , d e s c r i b e s the v a r i o u s ways i n which the
technique
can be o f use i n s o l v i n g t h i s problem.
I t was found t h a t the technique
cability.
d i d have a wide a p p l i -
I n the i n i t i a l p e r i o d o f a p p l i c a t i o n , i t would be o f
great value as a t o o l f o r a n a l y z i n g the problems o f the check.
I t can be used both to p o i n t out the jobs o r chains o f jobs which
p r o h i b i t the r e d u c t i o n o f the check time and a l s o t o d i r e c t the
r e v i s i o n o f the s c h e d u l i n g
elapsed
time i s reduced.
of these
jobs i n such a way t h a t the
T h i s r e d u c t i o n o f elapsed
time w i l l
have the e f f e c t o f i n c r e a s i n g the number o f jobs which must be
completed a t the e a r l i e s t p o s s i b l e time i f the check
completion
time i s t o be a minimum.
As a r e s u l t there w i l l be a g r e a t e r
need t o use the technique
both f o r s c h e d u l i n g and monitoring a l l
the jobs o f the check.
viii
Acknowledgements
The w r i t e r wishes to express
Mr.
I.A.
h i s s i n c e r e thanks to
Gray, D i r e c t o r of Maintenance and E n g i n e e r i n g ,
P a c i f i c A i r L i n e s , f o r h i s encouragement and
t h i s study.
Thanks are a l s o due
indebted
cooperation.
to P r o f e s s o r J.P.
Gigch under whose d i r e c t i o n the study was
Thanks are a l s o due
guidance throughout
to the maintenance s t a f f of
Canadian P a c i f i c A i r L i n e s f o r t h e i r f u l l
The w r i t e r i s deeply
Canadian
carried
to P r o f e s s o r T.D.
van
out.
Heaver f o r h i s
guidance and d i r e c t i o n throughout the p r e p a r a t i o n of t h i s
thesis.
lv.l
Table o f Contents
Chapter
I
Page
THE PROBLEM
1
Airline Profits
1
Reasons
3
The
f o r Low P r o f i t s
Importance o f M a x i m i z i n g U t i l i z a t i o n
7
The Need f o r I n c r e a s e d U t i l i z a t i o n
at
Canadian P a c i f i c
A i rLines
11
M e t h o d o f P r o c e d u r e and
Sources o f Data
14
Plan of the Thesis
II
III
CONDITIONS AT CANADIAN P A C I F I C
AIR L I N E S
17
Departmental Organization
17
C h a r a c t e r o f t h e M a i n t e n a n c e Work
18
The Work o f t h e M a i n t e n a n c e P l a n n e r
20
The
Schedugraph Board
23
METHODS OF SOLVING THE PROBLEM
25
The
IV
16
Schedugraph
Board
25
The K.L.M. A p p r o a c h
26
Plannet
29
C r i t i c a l Path Scheduling
30
THE C R I T I C A L PATH TECHNIQUE
31
The T e c h n i q u e
31
The
33
Charting of Relationships
Calculations
37
:v
Chapter
V
Page
THE METHODS OF APPLICATION OF THE CRITICAL
PATH TECHNIQUE TO AIRCRAFT MAINTENANCE
43
The Technique i n the A n a l y s i s
o f Completed Checks
43
The Technique i n A n a l y s i s Before
and During the Cheek
Other A p p l i c a t i o n s
VI
49
THE BENEFITS OF CRITICAL PATH ANALYSIS
TO AIRCRAFT MAINTENANCE
VII
46
53
Reductions i n E l a p s e d Time
53
Reductions i n Man-hours
56
Reductions i n Costs
59
Less T a n g i b l e B e n e f i t s
60
EVALUATIONS
AND CONCLUSIONS
64
The Post-Mortem A n a l y s i s
64
The Use as a Framework f o r
a l l Scheduling
64
A n a l y s i s of Check Segments
68
PERT and Man-Scheduling
68
BIBLIOGRAPHY
72
vi
L i s t o f Tables
Page
I
Net Income as a Percentage o f T o t a l Operating
Revenues f o r United S t a t e s Domestic Trunk A i r l i n e s
II
III
IV
V
Net P r o f i t of U.S. Domestic Trunk A i r l i n e s
4
Net P r o f i t of Canadian P a c i f i c A i r L i n e s
5
Sample C a l c u l a t i o n s
40
Comparative I n t e r e s t and D e p r e c i a t i o n C o s t s ,
54
the DC-8 and the DC-3.
VI
2
Breakdown of a l l Work on Check
Aircraft
#603,
According
#25,
to C r i t i c a l n e s s
66
vii
L i s t of F i g u r e s
Page
1.
Sample C r i t i c a l Path Network
41
2.
Engine Removal
42
3.
Chart of A l l Jobs Performed by Group I I D u r i n g
E q u a l i z e d Check #25,
4.
on A i r c r a f t #603.
Chart of A l l Jobs Common t o A l l Checks
70
71
1
Chapter I - The Problem
Airline
Profits
The
a i r t r a n s p o r t a t i o n i n d u s t r y from i t s b e g i n n i n g , has
been c h a r a c t e r i z e d by a comparatively
low l e v e l o f p r o f i t a b i l i t y .
Commercial a i r t r a n s p o r t a t i o n i n the United
foundation
i n the A i r M a i l Act o f 1925, was i n the e a r l y years
made p r o f i t a b l e by the subsidy
contracts.
S t a t e s , which had i t s
o f the government through a i r m a i l
R e f l e c t i n g continuing
i n d u s t r y , the United
financial d i f f i c u l t i e s i n this
S t a t e s C i v i l Aeronautics
Act o f 1938 d i r e c t e d
t h a t a c a r r i e r ' s m a i l r a t e would be s u f f i c i e n t , together
i t s other
with a l l
revenues, t o enable the c a r r i e r " t o m a i n t a i n and con-
t i n u e the development o f a i r t r a n s p o r t a t i o n t o the extent
the c h a r a c t e r
and o f
and q u a l i t y r e q u i r e d f o r the commerce o f the U n i t e d
S t a t e s , the P o s t a l S e r v i c e , and the n a t i o n a l defence."
The
postwar years have shown some, though no great
progress toward f i n a n c i a l s t a b i l i t y f o r the a i r l i n e i n d u s t r y .
first
The
three years o f t h i s p e r i o d were years o f f i n a n c i a l l o s s f o r
the United
S t a t e s domestic trunk l i n e s .
1 .
F o r the next e i g h t
years the earnings o f the i n d u s t r y i n g e n e r a l were b e t t e r ,
although those o f many companies were h i g h l y e r r a t i c .
Then, i n
1957» the f i n a n c i a l impact o f the i n t r o d u c t i o n o f _tet equipment
began t o be f e l t and i n d u s t r y p r o f i t s became markedly lower.
(See
Table I.)
1. See A i r T r a n s p o r t , Moody's T r a n s p o r t a t i o n Manual. I960,
p. A - 6 6 .
2
Table I
Net Income as a Percentage
of T o t a l Operating Revenues
For U n i t e d S t a t e s Domestic Trunk A i r l i n e s ,
Dec.
(Years
ending
31.)
Year
$ Income
I960
Year
.06
$ Income
1954
5.27
1959
3.43
1953
5.51
1958
2.96
1952
6.97
1957
1.90
1951
6.61
1956
4.57
1950
5.86
1955
5.56
1949
2.91
Source.
pp. _ * 0 7 - 6 8 and
ISopdy'iL^rjiAgppr^atto^ MEPttaJls, 1 9 6 1 ,
1954,
pp.
A:60-6l.
3
not u n t i l 1962
I t was
that the i n d u s t r y showed a
d e f i n i t e recovery
from the shock of the i n t r o d u c t i o n of the j e t .
This recovery
continued
present
time.
has
with increasing p r o f i t s to
(See Table I I ) .
P a c i f i c A i r Lines p a r a l l e l s
The
the
p r o f i t h i s t o r y o f Canadian
t h a t of the U.S.
domestic trunk
l i n e s as i s i l l u s t r a t e d i n Table I I I .
A s e a r c h f o r the reasons f o r t h i s low p r o f i t l e v e l i n
postwar years r e v e a l s two
underlying
which are excess c a p a c i t y and
capacity i n a competitive
and
i n d u s t r y - w i d e problems
r a p i d t e c h n o l o g i c a l change.
i n d u s t r y prevents p r o f i t s from
Excess
being
at a maximum because the equipment i s not being f u l l y u t i l i z e d .
There are three main reasons f o r excess c a p a c i t y i n the a i r l i n e
industry.
F i r s t , the i n d u s t r y has
been o v e r - o p t i m i s t i c i n i t s
f o r e c a s t i n g o f the demands f o r i t s services.^»
Second, the a i r l i n e s are o f t e n managed on the b a s i s
s o c i a l and
principles.
p o l i t i c a l expediency r a t h e r on sound economic
For example, A. Vandyk p o i n t s out t h a t many
n a t i o n s which want to use
abroad, not
their carriers
t o show t h e i r
o n l y r e g u l a r l y operate t h e i r own
than break-even load f a c t o r s and
new
flag
a i r c r a f t at
less
at u n p r o f i t a b l e l e v e l s , but
t h e i r presence f o r c e the a i r l i n e s o f the o l d e r n a t i o n s
also.
of
to do
by
so
3 #
2.
G.R. McGregor, F a r e s - C a p a c i t y - F l i g h t ,
T r a n s p o r t a t i o n . May, 1963,
p.
24.
3.
A. Vandyke, The World's A i r l i n e s , A l r l i f t t
p o r t a j i p j i , May, 1963,
p.
21.
A i r l i f t ; World A i r
World, A i r T r a n s -
4
Table
II
Net P r o f i t o f U . S . Domestic Trunk A i r l i n e s .
Year
Profit
1963
14,471,000
1962
10,378,000
1961
(34,566,000)
Brackets
Source:
($)
indicate a loss.
I n c r e a s e s i n A i r l i n e O p e r a t i n g Revenues and P r o f i t s ,
A v i a t i o n Week and Space T e c h n o l o g y . June 1 , 1964, p . 34; and
May 6 , 1963, p . 4 5 .
5
Table III
Net Profit of Canadian Pacific Air Lines •
Year
Profit ($)
Year
Profit ($)
1964
4,300,000
1954
969,000 *
1963
347,000
1953
366,000
1962
(1,200,000)
1952
364,000
1961
(7,600,000)
1951
1,084,000
I960
(4,700,000)
1950
205,000
1959
(3,900,000)
1949
115,000
1958
(1,900,000)
1948
(194,000)
1957
(113,000)
1947
(584,000)
1956
525,000
1955
275,000
Brackets indicate a net loss.
* Includes $593,000 profit from sale of aircraft.
Sources:
Canadian Pacific Annual Reports. The Financial
Post Survey of Industrials and The Financial Post Survey of
Corporate Securities.
6
The
t h i r d reason f o r t h i s excess c a p a c i t y i s r a p i d
t e c h n o l o g i c a l change.
Since 194-5 there have been three major
a i r c r a f t changes f o r most o f the world's a i r l i n e s . F i r s t
the changeover from the prewar and
came
immediate postwar a i r c r a f t
to
the f o u r engine, long range a i r c r a f t such as the C o n s t e l l a t i o n and
the DC-6.
Then i n the mid
f i f t i e s came the turbo-props and
the end o f the decade, the j e t s .
These changes were
not because the e x i s t i n g equipment was
at
necessary
p h y s i c a l l y depreciated
and
causing e x c e s s i v e l y h i g h maintenance c o s t s , but r a t h e r , they were
a d i r e c t r e s u l t o f the nature
and
the obsolescence
of the p u b l i c demand f o r a i r t r a v e l
o f the r e p l a c e d a i r c r a f t .
reduce o p e r a t i n g c o s t s and
Thus, i n order
to a t t r a c t passengers who
to
demanded
seats i n the f a s t e s t and most modern a i r c r a f t , the a i r l i n e s were
f o r c e d t o order and
to p l a c e i n s e r v i c e the new
c r a f t as soon as p o s s i b l e , p r e f e r a b l y before
so.
and
larger a i r -
the c o m p e t i t i o n
did
T h i s caused i n d u s t r y o v e r c a p a c i t y as the l a r g e r planes
f r e q u e n t l y r e p l a c e d the s m a l l e r on routes where the new
f a c t o r s would be below the break-even p o i n t .
planes
that had
load
In a d d i t i o n , the
been r e p l a c e d were e i t h e r s o l d ( u s u a l l y at a
p r i c e lower than t h e i r d e p r e c i a t e d
or p l a c e d on other r o u t e s .
book v a l u e ) , put i n t o s t o r a g e ,
In t h i s l a s t case, low load f a c t o r s
were again the r e s u l t as the a i r c r a f t were o f t e n l a r g e r than the
t r a f f i c warranted.
4. For some a i r l i n e s , the i n t r o d u c t i o n o f the DC-6 or SuperC o n s t e l l a t i o n s i n the e a r l y 1950's must be counted as a f o u r t h
major a i r c r a f t change.
7
Rapid t e c h n o l o g i c a l change a l s o had a more d i r e c t
e f f e c t on a i r l i n e p r o f i t .
New a i r c r a f t are expensive.
Because
o f the r a p i d i t y o f the t e c h n o l o g i c a l change, the i n i t i a l c o s t o f
the a i r c r a f t g e n e r a l l y had t o be d e p r e c i a t e d
greater
years.
had
over a p e r i o d o f no
than seven years and sometimes as few as three or f o u r
*
I n a d d i t i o n t o the c o s t o f the plane i t s e l f ,
the a i r l i n e
t o bear the h i g h c o s t s o f new s p e c i a l i z e d equipment and
exclusive s t a f f training.
Since
these c o s t s c o u l d 'only be
amortized over a very s h o r t p e r i o d o f time, the h i g h per u n i t
costs resulted i n large f i n a n c i a l
The
drains.
Importance o f Maximizing the U t i l i z a t i o n
As a r e s u l t o f these problems the a i r l i n e s have been
f o r c e d t o economize i n every p o s s i b l e way.
m i n i m i z i n g the l o s s due t o the retirement
yet o b s o l e t e
One method o f
of mechanically
good,
equipment i s t o i n c r e a s e i t s u t i l i z a t i o n i n the years
proceeding i t s o b s o l e s c e n c e .
The r a t i o n a l e here i s based
on the r e a l i z a t i o n that the a m o r t i z a t i o n
firstly
r a t e f o r the a i r c r a f t i s
a f u n c t i o n o f the number o f years from i t s date o f purchase t o
the date o f obsolescence r a t h e r than a f u n c t i o n o f the number o f
hours o f u s e f u l s e r v i c e i t r e n d e r s .
d e p r e c i a t i o n w i l l be a constant
number o f hours flown.
T h i s means that the c o s t o f
d a i l y f i g u r e regardless
But a m o r t i z a t i o n
i s o n l y one o f the many
f i x e d c o s t s i n v o l v e d i n the o p e r a t i o n o f an a i r l i n e .
5.
o f the
S. A l t s c h o e l , Why the S t r e t c h i n J e t D e p r e c i a t i o n ?
World A i r T r a n s p o r t a t i o n . Feb. 1 9 6 3 , p. 3 5 .
Since the
Airlift:
8
r a t i o o f f i x e d to v a r i a b l e c o s t s f o r an a i r l i n e i s h i g h ,
earnings
from i n c r e a s e d
The
u t i l i z a t i o n should
o f two
ways.
may
utilization
be d e s c r i b e d
I f the t e n hours o f o p e r a t i o n
load f a c t o r s (no p r o f i t earned) and
operation
be s u b s t a n t i a l .
e f f e c t of an i n c r e a s e i n the d a i l y
(say from t e n to e l e v e n hours per day)
the
in either
are at break-even
an a d d i t i o n a l hour o f
i s added at the same load f a c t o r , a p r o f i t w i l l
from the i n c r e a s e d
u t i l i z a t i o n and
w i l l be equal
t o the
from the a d d i t i o n a l hour of o p e r a t i o n minus the d i r e c t
c o s t s f o r t h a t hour.
annum f o r one
This p r o f i t i s greater
jet aircraft a l o n e . O n
a d d i t i o n a l hour of o p e r a t i o n
result
revenue
operating
than $ 4 0 0 , 0 0 0
per
the other hand, i f the
i s i n c l u d e d i n the
a n a l y s i s the break-even load f a c t o r w i l l be
break-even
lower.
The
reason
f o r t h i s i s t h a t the f i x e d c o s t s p r e v i o u s l y spread over t e n hours
of operation
can now
be spread over e l e v e n hours.
Since
the
rate
charged each passenger i s u s u a l l y f i x e d e i t h e r by a government
body such as the A i r Transport
Board, or an i n t e r n a t i o n a l
o r g a n i z a t i o n such as the I n t e r n a t i o n a l A i r Transport
and
Association,
s i n c e the c o s t s a s s o c i a t e d w i t h each hour o f o p e r a t i o n
are
reduced, the number of revenue passengers t h a t are n e c e s s a r y to
provide
break-even revenues i s reduced.
6 * T h i s f i g u r e i s a r r i v e d at by assuming a 3 6 5 day year and a
c o n t r i b u t i o n to f i x e d c o s t s as $ 1 , 1 0 0 per hour ( 3 6 5 days/year
x $l,100/day • $401,500/year).
In a c t u a l i t y the $ 1 , 1 0 0 f i g u r e
i s very low s i n c e Canadian P a c i f i c A i r L i n e s estimates f i x e d
c o s t s to be $ 1 , 1 5 0 per hour when a u t i l i z a t i o n o f twelve hours
per day was used i n the c a l c u l a t i o n .
9
T h i s l a t t e r l i n e o f reasoning
Continental
airlines.
has been f o l l o w e d by
During the e a r l y years o f the j e t t r a n s i -
t i o n which were g e n e r a l l y u n p r o f i t a b l e ones, t h i s
c o n s i s t e n t l y Increased
scheduling
its jet utilization.
airline
I t d i d so by
a d d i t i o n a l f l i g h t s w i t h load f a c t o r s lower than i t s
average and i n 1961 had an average load f a c t o r o f
lowest o f the U n i t e d
50.3#» the
States domestic trunk l i n e s .
However, i t s
break-even load f a c t o r s were a l s o lowered ( p a r t l y because o f a
very s t r i c t c o s t c o n t r o l ) t o a p o i n t wnere t h i s a i r l i n e was
earning
any
a p r o f i t o f 10% o f gross s a l e s , a p r o f i t g r e a t e r
than
o f i t s competing domestic trunk c a r r i e r s . ' ' *
How can an a i r l i n e i n c r e a s e
simple answer i s t o send an a i r c r a f t
its utilization?
that i s t o be s i t t i n g
f o r some hours on a revenue producing f l i g h t .
idle
I t i s unfortunate
that the s o l u t i o n t o the problem i s not that easy.
occurs t h a t a s e r v i c e a b l e a i r c r a f t
One
i s sitting Idle.
It rarely
When t h i s
does occur i t i s almost always a r e s u l t o f the peaking problem
and
i t i s i d l e simply
because there i s no demand f o r i t . The
r e a l s o l u t i o n seems t o be o r g a n i z a t i o n a l i n n a t u r e .
a i r l i n e s there a r e two departments i n v o l v e d ;
I n most
s c h e d u l i n g and
maintenance.
I t i s the aim o f the s c h e d u l i n g
organize
the f l i g h t plans
of a l l a i r c r a f t
department t o so
that the o v e r a l l a i r -
7. J e t U t i l i z a t i o n - C o s t C o n t r o l Increase C o n t i n e n t a l ' s P r o f i t s ,
A v i a t i o n Week and Space Technology. August 21, 1961, p. 36.
10
craft
u t i l i z a t i o n i s maximum.
T h i s would mean no waste o r i d l e
minutes i n the l i f e o f the a i r c r a f t .
o n l y aim o f the department.
However, t h i s i s not the
I t must a l s o attempt t o minimize
such c o s t s as crew expenses and the c o s t o f meeting peak demands
f o r s e r v i c e a t c e r t a i n a i r p o r t s a t the busy hours o f the day.
a d d i t i o n , other f a c t o r s such as the need t o leave c e r t a i n
time i n the a i r c r a f t
In
slack
schedule so that a minor d e l a y w i l l not
a f f e c t subsequent f l i g h t s o f the a i r c r a f t , o r connecting
of other a i r c r a f t , must a l s o be c o n s i d e r e d .
flights
A l l o f these aims
are
pursued i n the l i g h t o f the o v e r a l l a i r l i n e o b j e c t i v e o f long
run
p r o f i t m a x i m i z a t i o n . S i n c e the m a j o r i t y o f the d e c i s i o n s
made i n r e l a t i n g one o f the minor aims t o the others are s u b j e c t i v e l y a r r i v e d a t , i t i s very d i f f i c u l t f o r a i r l i n e s
t h e i r u t i l i z a t i o n by improving t h e i r s c h e d u l i n g .
t o improve
I t i s , however,
q u e s t i o n a b l e whether the s c h e d u l i n g departments o f the a i r l i n e s
in
g e n e r a l a r e t a k i n g f u l l advantage o f such f a c t o r s as the h i g h
percentage o f f i x e d c o s t s and the r e c e n t developments i n s t a t i s t i c a l and computerized methods o f c o r r e l a t i o n and o p t i m i z a t i o n
i n order t o Increase t h e i r
utilization.
When the maintenance department attempts t o i n c r e a s e
a i r c r a f t u t i l i z a t i o n i t s problem i s simply, how can the
w
maintenance work be so o r g a n i z e d as t o minimize the number o f
hours that the a i r c r a f t
i s out o f s e r v i c e f o r mechanical r e a s o n s ? "
I f the maintenance department can complete a l l i t s work i n say
ten
fewer hours each month, the s c h e d u l i n g department can attempt
11
t o use the e x t r a hours f o r a d d i t i o n a l revenue f l i g h t s .
t h i s maintenance
It i s
problem which prompted the study upon which
t h i s t h e s i s i s based.
The d i s c u s s i o n o f the above paragraphs does not a p p l y
merely t o a i r l i n e s i n g e n e r a l ; i t a p p l i e d s p e c i f i c a l l y t o
Canadian P a c i f i c A i r L i n e s .
Table I I I , above p o i n t e d out the
p r o f i t . d i f f i c u l t i e s o f the company and the e f f e c t s o f the excess
c a p a c i t y and r a p i d t e c h n o l o g i c a l change a r e evidenced i n t h i s
p r o f i t record.
This a i r l i n e also f e l t
a need f o r i n c r e a s i n g i t s
u t i l i z a t i o n by r e d u c i n g the e l a p s e d time on i t s maintenance
checks.
Thi'sr need was evidenced i n f o u r d i f f e r e n t ways.
F i r s t , comparisons w i t h maintenance
data from o t h e r
a i r l i n e s showed that c e r t a i n o f the a i r l i n e s recorded a s h o r t e r
elapsed time on s i m i l a r maintenance
checks.
In a d d i t i o n , other
a i r l i n e s were making r a p i d r e d u c t i o n s i n t h e i r own e l a p s e d times.
The second evidence was found through a study o f the
a i r c r a f t mechanics*
time c a r d s .
These i n d i c a t e both the j o b
worked oh;and the d u r a t i o n o f each j o b .
"standby"
The number o f men on
(men t o whom no work has been assigned) was e x c e s s i v e
e s p e c i a l l y d u r i n g the l a s t few s h i f t s o f each check.
Coupled w i t h
t h i s was the f a c t that many unreasonably l o n g d u r a t i o n s were
found l i s t e d a g a i n s t c e r t a i n j o b s .
These d u r a t i o n s were o f t e n
f i v e t o t e n times g r e a t e r than the " s t a n d a r d " or "expected"
durations.
Although i t was found t o be not uncommon f o r a
12
m e c h a n i c t o c h a r g e a g a i n s t one
w o r k i n g on a n o t h e r ,
found to vary
indicated
was
p a r t i c u l a r j o b t h e t i m e he
the frequency
of i n a c c u r a t e time l i s t i n g s
somewhat i n v e r s e l y w i t h t h e w o r k l o a d .
Law.
busy.
The
This, of course,
conclusions
m e c h a n i c was
not
was
This
t h a t t h e m e c h a n i c s w o u l d o f t e n s p e n d f a r more t i m e
needed w o r k i n g oh a p a r t i c u l a r
of being
spent
job j u s t t o g i v e the
i s i n agreement w i t h
than
impression
Parkinson's
r e a c h e d were t h a t the work l o a d f o r e a c h
adequately
balanced
t h a t much o f t h e w o r k c o u l d be
throughout the check,
completed i n a s h o r t e r
and
elapsed,
time.
T h i r d , a b r i e f a n a l y s i s of the type
performed d u r i n g
those
t h e l a s t h a l f o f t h e c h e c k and
w h i c h p r o h i b i t e d an e a r l y c o m p l e t i o n
t h a t i h most c a s e s t h e
much e a r l i e r and
o f j o b s w h i c h were
particularly
o f t h e c h e c k showed
j o b s c o u l d have been s c h e d u l e d
thereby
of
an e a r l i e r c o m p l e t i o n
to
begin
might have been
possible.
F o u r t h , management d e s i r e d b o t h more e f f i c i e n c y i n t h e
m a i n t e n a n c e c h e c k s and
reasons,
one
proceeding
a reduction of elapsed
o f w h i c h was
a poor p r o f i t
1 9 6 3 > management f e l t
time.
record
For
i n the
t h a t e v e n w i t h an
various
years
increased
work l o a d p o s s i b l e i n the near f u t u r e , the maintenance department
r e q u i r e d n o t more b u t
s l i g h t l y fewer mechanics.
In a d d i t i o n ,
the d e s i r e f o r i n c r e a s e d a i r c r a f t u t i l i z a t i o n r e s u l t e d i n a
tenative scheduling
i n the elapsed
approximately
of the
time of the
f i f t y hours.
jet aircraft requiring a
reduction
" e q u a l i z e d " check from seventy-two
T h u s , t h e r e was
not
only evidence
to
of
an area f o r p o s s i b l e improvement i n work procedures but a l s o the
l i k e l i h o o d of major problems i f these improvements were n o t f o r t h coming.
The problem,
t h e r e f o r e , was t w o f o l d .
F i r s t , how t o
reduce the elapsed time of the maintenance checks, and second,
although o f l e s s e r Importance and urgency, how t o improve the
work procedures?
Although these are two d i f f e r e n t
they are n o t completely independent.
problems,
F o r example, change i n
e l a p s e d time w i l l o f t e n be made p o s s i b l e o n l y by a change i n work
procedures, and a change i n work procedures almost always
i n d i f f e r e n t elapsed times.
are
results
S i n c e improved:work procedures are
o n l y o f secondary importance both t o Canadian P a c i f i c A i r
L i n e s and t o t h i s present paper, they w i l l be d i s c u s s e d , i n the
f o l l o w i n g c h a p t e r s , o n l y i n terms o f t h e i r r e l a t i o n s h i p s t o the
elapsed time o f the checks.
D i s c u s s i o n o f improvements i n work
procedures s o l e l y f o r the sake o f i n c r e a s e d e f f i c i e n c y w i l l be
omitted.
The purpose o f t h i s paper i s t o o u t l i n e one type o f
s o l u t i o n t o the problem o f r e d u c i n g the elapsed time o f the
maintenance checks.
T h i s s o l u t i o n i n v o l v e s the a p p l i c a t i o n o f a
s c h e d u l i n g technique to the many I n d i v i d u a l jobs to be performed
i n such a way that the p r o j e c t be completed
elapsed time.
i n the minimum o f
Such a s c h e d u l i n g technique i s the " c r i t i c a l
method", or the " c r i t i c a l path t e c h n i q u e " .
path
14
The
Method o f P r o c e d u r e
The
familiar
first
and S o u r c e s
of
s t e p i n a n y m e t h o d s a n a l y s i s i s t o become
w i t h the r e a l problem.
That a problem
e v i d e n t f r o m t r a n s p o r t a t i o n literature,®'
Canadian
Pacific
Pacific
A i r L i n e s p e r s o n n e l and
A i r L i n e s maintenance d a t a .
a s o l u t i o n t o the problem
though s t i l l
p o r t a t i o n and
The
solution.9»10«
industrial
an analysis of
Canadian
S i n c e t h e r e was
step,
a lesser,
procedures
a s o l u t i o n w h i c h w o u l d meet
Here l i t e r a t u r e
i n both the
a d m i n i s t r a t i o n f i e l d s was
c r i t i c a l p a t h method was
Two m a i n d i f f i c u l t i e s
t h e t e c h n i q u e had
discussion with
o f improving work
a s s o c i a t e d w i t h the main problem,
preferable.
e x i s t e d was
Following this f i r s t
i s sought.
important problem
b o t h n e e d s was
Data
trans-
searched.
conceived of as a possible
arose.
I n most i n s t a n c e s
b e e n a p p l i e d t o p r o j e c t s o f much l o n g e r
d u r a t i o n , u s u a l l y many m o n t h s . T h e
aircraft
maintenance
8.
See C o m p u t e r S p e e d s J e t T h r o u g h O v e r h a u l , A i r l i f t : W o r l d
T r a n s p o r t a t i o n , Volume 26, J a n . 1 9 6 3 .
9.
For a d e s c r i p t i o n o f the technique see:
J.E. K e l l y J r . , and
M.R. W a l k e r , C r i t i c a l P a t h P l a n n i n g and S c h e d u l i n g , P r o c e e d i n g s
E a s t e r n J o i n t C o m p u t e r C o n f e r e n c e 1 6 0 - 1 7 3 » B o s t o n , December 1 - 3 ,
1959; I n t e r n a t i o n a l Business Machines, General Information
M a n u a l , PERT ( P r o g r a m E v a l u a t i o n and R e v i e w T e c h n i q u e ) ; Ray N.
Sauer, I n t e r n a t i o n a l B u s i n e s s Machines, LESS, 1620 G e n e r a l
Programme L i b r a r y , 1 0 . 3 0 0 3 ; and J.W. P o c o c k , PERT a s a n a n a l y t i c a l a i d f o r P r o g r a m P l a n n i n g - I t s P a y o f f and P r o b l e m s ,
O p e r a t i o n s R e s e a r c h . V o l . 1 0 , # 6 , 1 9 6 2 , pp. 8 9 5 - 8 9 6 .
10.
F o r ways o f a d a p t i n g t h e t e c h n i q u e t o t h e a i r c r a f t m a i n t e n a n c e
problem see:
E.R. B o s s a n g e J r . , P l a n n e t ( P l a n n i n g N e t w o r k ) A
p a p e r p r e s e n t e d a t t h e 1 9 6 2 E n g i n e e r i n g and M a i n t e n a n c e C o n f e r ence, A i r Transport A s s o c i a t i o n , October 2 4 , 1962;
Computer
S p e e d s J e t T h r o u g h O v e r h a u l , op
c i t . : and W.R. M o r r i s o n and
E. R a y m o n d , A i r l i n e Management o f O v e r h a u l P r o c e s s e s W i t h
C r i t i c a l P a t h Methods, U n i t e d A i r L i n e s .
itt
11#
Air
See
J.W. P o c o c k , o p . c i t .
p r o j e c t s had d u r a t i o n s o f from one and o n e - h a l f t o two and oneh a l f days.
In a d d i t i o n , i n the a i r c r a f t maintenance
projects
approximately o n e - h a l f o f the jobs to be performed c o u l d not be
known u n t i l the f i r s t
e i g h t or more hours o f the check had
elapsed.
For
these reasons i t was f e l t
that the f e a s i b i l i t y o f
a p p l y i n g the technique c o u l d only p r o p e r l y be determined a f t e r a
p e r i o d o f e x p e r i m e n t a t i o n on a t r i a l and e r r o r b a s i s .
T h i s was
conducted by the w r i t e r under the d i r e c t i o n of P r o f e s s o r J.P. van
Gigch and w i t h the c o o p e r a t i o n of the maintenance
Canadian P a c i f i c A i r L i n e s .
company's maintenance
staff of
The p r o j e c t was c a r r i e d out at the
base a t Vancouver A i r p o r t , B.C., d u r i n g the
months o f May through August, 1 9 6 3 •
During these months attempts were made t o apply the
technique i n f o u r d i f f e r e n t ways.
of
The f i r s t was t o the a n a l y s i s
checks which had a l r e a d y been completed.
The o b j e c t i v e was t o
p o i n t out what mistakes were made and t o l a y plans f o r a v o i d i n g
them i n the f u t u r e .
Secondly, an attempt was made to make f u l l
use o f the technique before and d u r i n g each check.
T h i r d l y , from
evidence gained i n the e a r l y a n a l y s i s i t became obvious that i t
was not the check as a whole but c e r t a i n p o r t i o n s t h e r e o f which
were l e n g t h e n i n g the elapsed times o f a m a j o r i t y o f the checks.
T h e r e f o r e , attempts were made t o segment the checks i n such a way
that these problem areas could be analyzed on t h e i r own.
an attempt was made to apply both the P E R T
Scheduling -^* computer
1
programs
1 2 ,
and the Man-
to the e q u a l i z e d checks.
Finally
16
S i n c e t h i s e x p e r i m e n t a t i o n l e d the a n a l y s t and management t o conclude t h a t there was
c o n s i d e r e d value i n the use of
the c r i t i c a l path t e c h n i q u e , the f i n a l
step was
to f o r m a l i z e the
use of the technique and then to e x p l a i n i t s use both to those
who
to
would be mainly r e s p o n s i b l e f o r i t s implementation and
also
a l l those i n d i r e c t l y a f f e c t e d by i t s implementation.
P l a n of the T h e s i s
The f o l l o w i n g chapter d e s c r i b e s the maintenance d e p a r t -
ment at Canadian P a c i f i c A i r L i n e s and e x p l a i n s such
background
i n f o r m a t i o n as i s necessary f o r an understanding of both the
problem
of e x c e s s i v e e l a p s e d time and a l s o of the p o s s i b l e
s o l u t i o n to i t .
In Chapter I I I , v a r i o u s of these s o l u t i o n s are
presented along w i t h t h e i r major advantages
and disadvantages.
The remaining f o u r chapters d e a l w i t h the c r i t i c a l
method as a s o l u t i o n to the problem.
path
In Chapter IV the technique
and the method of c h a r t i n g a p r o j e c t are d e s c r i b e d i n d e t a i l .
Then, i n Chapter V, the v a r i o u s methods of a p p l y i n g t h i s
to
technique
a i r c r a f t maintenance are presented f o l l o w e d by a d i s c u s s i o n of
the b e n e f i t s to be d e r i v e d from them i n Chapter V I .
The
final
chapter c o n t a i n s a c r i t i c a l e v a l u a t i o n of the u s e f u l n e s s of the
technique and the c o n c l u s i o n s .
12.
As w i l l be p o i n t e d out i n Chapter V, the PERT (Program E v a l u a t i o n and Review Technique) program has the advantage of prov i d i n g some measure of the p r o b a b i l i t y of completing a g i v e n
p r o j e c t at a c e r t a i n hour or d a t e . See I n t e r n a t i o n a l Business
Machines, PERT, op. c i t .
13.
T h i s program i s designed to schedule a l l jobs i n such a way
t h a t the work load f o r each workman i s balanced w h i l e the
c r i t i c a l path i s not lengthened. See Chapter V I I and I n t e r n a t i o n a l Business Machines, Man-Scheduling Program f o r the
IBM, 1620, 1620 General Program L i b r a r y , 10.3.013.
17
Chapter I I - C o n d i t i o n s
a t Canadian
P a c i f i c A i r Lines
Departmental
Organization
Before proceeding t o a d e t a i l e d d i s c u s s i o n o f the
a p p l i c a t i o n o f the c r i t i c a l p a t h technique, the o r g a n i z a t i o n and
a c t i v i t i e s o f the maintenance department a t Canadian P a c i f i c A i r
L i n e s p r i o r t o t h i s study must be c o n s i d e r e d .
1 .
The department
i s headed by the D i r e c t o r o f Maintenance and E n g i n e e r i n g .
far
As
as the maintenance o f j e t a i r c r a f t i s concerned, h i s super-
v i s i o n extends over f o u r d i v i s i o n s :
v i z : engineering,
ance, q u a l i t y c o n t r o l , and maintenance p l a n n i n g .
b i l i t y o f the e n g i n e e r i n g
mainten-
The r e s p o n s i -
d i v i s i o n i s t o o u t l i n e i n d e t a i l the
work t h a t must be done on each j o b . Since
the vast m a j o r i t y o f
the jobs are o f a r o u t i n e n a t u r e , such as the replacement o f
components and the r e p a i r i n g o f f a u l t y p a r t s , t h i s
division
concerns i t s e l f w i t h the r e g u l a r maintenance checks o n l y when
major problems a r i s e o r when some a i r c r a f t m o d i f i c a t i o n i s
required.
The maintenance d i v i s i o n i s r e s p o n s i b l e
completion o f the work assigned
f o r the a c t u a l
to each check w h i l e the q u a l i t y
c o n t r o l d i v i s i o n must see t h a t a l l work performed complies w i t h
the s a f e t y r e g u l a t i o n s .
1. The c o n d i t i o n s have changed l i t t l e
s i n c e that time.
18
The m a i n t e n a n c e
the
planning d i v i s i o n
i s responsible f o r
assignment o f t h e work l o a d t o each maintenance
check.
This
i s done, o f c o u r s e , w i t h i n t h e l i m i t s imposed on i t by t h e o t h e r
three
divisions.
There a r e f o u r t y p e s o f maintenance
DC-8
aircraft.
The "M
1° a n d "M 2"
checks f o r the
checks are performed
after
e a c h f l i g h t , u n l e s s a more m a j o r c h e c k i s t o be p e r f o r m e d , a n d
have a t o t a l e l a p s e d time o f from s i x t y
t o ninety minutes.
The
" a n n u a l c h e c k " , o r major o v e r h a u l , i s performed once each y e a r
and r e q u i r e s f r o m f i v e
p e r f o r m e d e v e r y 300
t o 500
days f o r c o m p l e t i o n .
five
t o t e n days.
The " e q u a l i z e d c h e c k " i s
f l y i n g h o u r s a n d r e q u i r e s two t o t h r e e
Since Canadian P a c i f i c
A i r L i n e s has o n l y
j e t a i r c r a f t , i t p e r f o r m s o n l y one e q u a l i z e d c h e c k p e r week,
and o n l y f i v e m a j o r o v e r h a u l s p e r y e a r .
Because
f r e q u e n c y o f t h e a n n u a l c h e c k s and b e c a u s e
scheduled t o f a l l
Canadian P a c i f i c
of the lack of
they are purposely
i n t h a t p e r i o d o f t h e y e a r when t h e demand f o r
A i rLines'
j e t f l i g h t s i s n o t a t a peak, t h e
v a l u e o f d e c r e a s e d d o w n - t i m e f o r t h e s e c h e c k s was c o n s i d e r e d t o
be o f s e c o n d a r y i m p o r t a n c e .
the
d o w n t i m e o n t h e M 1 a n d M2 c h e c k s was n o t made b e c a u s e o f
their
short duration.
Ohapters w i l l
the
I n a d d i t i o n , an attempt t o reduce
T h e r e f o r e t h e emphasis
i n the remaining
be o n t h e r e d u c t i o n o f t h e t o t a l e l a p s e d t i m e o f
equalized checks.
19
Most o f the maintenance work i s e i t h e r o f a r o u t i n e or
an urgent n a t u r e .
The r o u t i n e work i s assigned r e g u l a r l y t o the
same type o f check each time i t becomes n e c e s s a r y , and the urgent
work must be performed whenever i t a r i s e s .
the
However, f o r many o f
j o b s , the maintenance p l a n n i n g d i v i s i o n has the freedom t o
a s s i g n the work t o one o f a number o f checks.
In t h i s way i t
attempts t o balance the work l o a d .
T h i s work load on the e q u a l i z e d check i s composed o f
jobs from seven job c a t e g o r i e s .
The f i r s t
i s the b a s i c category
c o n s i s t i n g o f jobs which must be performed every e q u a l i z e d check
(See
Figure 4).
Next are the p e r i o d i c i t y items.
These jobs
must be performed i n a r e g u l a r manner every second, t h i r d ,
- - - n t h check.
fourth,
Many o f the component p a r t s o f the a i r c r a f t
( u n i t s ) demand independent maintenance.
For each o f these u n i t s
a h i s t o r y i s kept on f i l e i n d i c a t i n g the l o c a t i o n (the a i r c r a f t
number and p o s i t i o n i n that a i r c r a f t ) o f the component and the
number o f f l y i n g hours u n t i l i t s removal f o r maintenance or
replacement.
I t i s the removal and maintenance jobs on these
component p a r t s that comprise the t h i r d c a t e g o r y .
The f o u r t h
c a t e g o r y c o n t a i n s the e x t r a work which the maintenance p l a n n i n g
d i v i s i o n deems a d v i s a b l e .
T h i s i s o f t e n rescheduled work which
a r i s e s when the work load on the proceeding check was e x t r a
heavy, or f l e e t campaign work which a r i s e s when i t i s f e l t
a l l a i r c r a f t should be examined
that
t o ensure that the m a l f u n c t i o n
found i n one a i r c r a f t does not e x i s t or occur i n the others a l s o .
In
the f i f t h c a t e g o r y are the a i r c r a f t m o d i f i c a t i o n s requested
20
by the e n g i n e e r i n g
division.
The jobs i n the above f i v e c a t e g o r i e s account f o r
about f i f t y percent o f those t o be performed on an average
e q u a l i z e d check.
The remaining jobs are r e f e r r e d to as "snags"
and are the unscheduled maintenance.
f l i g h t snags and i n s p e c t i o n snags.
the
They are o f two types;
The f l i g h t snags which are
s i x t h c a t e g o r y a r i s e because o f m a l f u n c t i o n o f the a i r c r a f t
and are r e p o r t e d by the f l i g h t crew t o the maintenance
division
a few hours before the check b e g i n s .
The
planning
inspection
snags, c o m p r i s i n g the seventh c a t e g o r y , are r e p o r t e d d u r i n g the
check i t s e l f e i t h e r by the i n s p e c t o r s or the mechanics.
The
vast
m a j o r i t y o f the snags are i n s p e c t i o n snags.
The Work o f the Maintenance P l a n n e r
In p l a n n i n g the work content o f an e q u a l i z e d check,
the
p l a n n e r , who
division,
first
i s a member o f the maintenance
lists
a l l jobs which under normal circumstances
would be performed d u r i n g the check.
the
first
planning
s i x c a t e g o r i e s above.
T h i s i n c l u d e s the jobs i n
He then attempts to balance the
work load so that the p r o d u c t i o n o f each mechanic and the t o t a l
work done on the a i r c r a f t are both maximized.
He must at the
same time attempt t o ensure t h a t the a i r c r a f t w i l l be s e r v i c e a b l e
at i t s scheduled departure time.
21
His next task i s t h e r e f o r e
to seek out those
jobs
which are of a non-urgent nature f o r the purpose o f adding them
to,
or s u b t r a c t i n g them from the check as the work load d i c t a t e s .
Jobs i n the a i r c r a f t
modifications
and
planned e x t r a work c a t e -
g o r i e s are f r e q u e n t l y non-urgent as are some of the snags.
In
a d d i t i o n , when the number o f hours between removals of components
i s s e v e r a l thousand ( t h i s i s f r e q u e n t l y the case) there
i s often
a n ^ o p p o r t u n i t y to schedule the removal ahead or back one
check.
There are many f a c t o r s which make the planners*
much more complicated
assembly-line
men
balancing
involved:
etc.;
than the s o l v i n g o f a
viz:
problem.
electrical;
upholstery;
c l e a n i n g ; and
o f t h e i r own
from one
group and
fuel;
and
woodwork and
inspection.
straight-forward
There are ten groups o f work-
engines and
h y d r a u l i c s , undercarriage
a i r c o n d i t i o n i n g , oxygen,
f l i g h t controls; radio;
p a i n t ; sheet metal;
These workmen are t r a i n e d i n the work
a workman cannot r e a d i l y be
group to another.
transferred
In a d d i t i o n , the number of
working w i t h each group i s not
stable.
men
General manpower problems
such as t r a n s f e r s , r e s i g n a t i o n s , e t c . produce c o n d i t i o n s
were found i n the a i r c o n d i t i o n i n g and
o f 1963.
s i x men
i n s h i f t #5.
i n s h i f t #4,
such as
oxygen group i n the
At t h i s time i n s t e a d of having e i g h t men
there were e i g h t men
task
seven i n s h i f t #6,
per
spring
shift,
and
only
In c e r t a i n groups, e s p e c i a l l y r a d i o ,
the
t e c h n i c i o n s are a v a i l a b l e o n l y when there are no p r e s s i n g needs
22
f o r them elsewhere on the maintenance base.
the manpower b a l a n c e .
i s p a r t i a l l y reduced
S i c k n e s s a l s o upsets
However, the s e r i o u s n e s s o f these problems
by the f a c t that i n cases o f urgent need,
a d d i t i o n a l manpower can be obtained from or s u p p l i e d t o the shops
and
servicing
divisions.
F u r t h e r c o m p l i c a t i o n s a r i s e when c e r t a i n o f the a i r c r a f t ' s systems are c o n s i d e r e d .
o f power i n the j e t a i r c r a f t :
hydraulic.
There are three major
v i z : e l e c t r i c a l , pneumatic and
Many jobs i n v o l v i n g workmen from almost
trade groupings
sources
a l l the
can o n l y be performed when one p a r t i c u l a r power
source i s a v a i l a b l e .
However, there are an even g r e a t e r number
of jobs which can be performed only when one or more o f the
power sources i s turned o f f .
When one c o n s i d e r s the v a r i o u s areas o f the a i r c r a f t ,
the problem o f c o n g e s t i o n comes t o the f o r e .
C e r t a i n areas,
e s p e c i a l l y the c o c k p i t and the engines, may r e q u i r e the work o f
many men and a number o f d i f f e r e n t trade groupings
the course o f the check.
throughout
However, there a r e o f t e n p h y s i c a l
r e s t r i c t i o n s as t o the number o f men t h a t can work i n the area
at any one time.
I n the c o c k p i t , f o r example, the maximum i s
three men working a t one time and even then they w i l l be i n one
another's way i f they are not working i n separate c o r n e r s o f t h i s
eonfined
area.
The p l a n n e r s ' major problems, however, are not those
d e s c r i b e d above.
They a r i s e out of two unknown q u a n t i t i e s ; the
number and nature o f the snags which w i l l be r e p o r t e d i n the
i n i t i a l hours o f the check, and the number o f manhours r e q u i r e d
23
to
complete the work on many of the complicated and n o n - r o u t i n e
repair
jobs.
The p l a n n e r ' s main means o f overcoming these problems
and keeping the work load balanced are h i s breadth o f
knowledge
and experience i n a i r c r a f t maintenance, as w e l l as h i s a b i l i t y
to
a d j u s t , w i t h i n l i m i t s , both the work load and the manpower
as the need
arises.
The a c t u a l c o m p l e t i o n o f the work i n the check i s the
r e s p o n s i b i l i t y of the maintenance d i v i s i o n .
On each s h i f t the
man
assuming t h i s r e s p o n s i b i l i t y i s the c h i e f mechanic.
the
a c t u a l assignment o f h i s men
all
work done on the a i r c r a f t .
He makes
t o d e f i n i t e jobs and s u p e r v i s e s
Working along w i t h him, though
i n an a d v i s o r y c a p a c i t y , i s a maintenance p l a n n e r from the
planning d i v i s i o n .
all
jobs completed, those s t i l l
be s t a r t e d .
to
ing
I t i s h i s r e s p o n s i b i l i t y to keep a r e c o r d o f
i n p r o g r e s s , and those s t i l l t o
With the h e l p o f a schedugraph board he attempts
c o o r d i n a t e the a c t i v i t i e s o f the t e n trade groupings, i n v o l v about f i f t y men
and some f i v e hundred j o b s , and a d v i s e s the
c h i e f mechanic as t o when each of the jobs should be performed.
The schedugraph board i s a mechanical a i d to job
organization.
I t has an area o f twenty square f e e t being
f e e t wide and f o u r f e e t i n depth.
five
V e r t i c a l l y , i t i s divided
i n t o e i g h t y rows, each o f which c o n t a i n s the work t o be performed
by one man.
the
first
I t i s divided
h o r i z o n t a l l y by a time s c a l e .
column r e p r e s e n t s the work t o be done d u r i n g the
Here,
24
f i r s t hour o f the check, and the subsequent columns, the work t o
be done In the f o l l o w i n g h o u r s .
The work load on a g i v e n check i s organized by
first
p r e p a r i n g a c a r d f o r each job c o n t a i n i n g an estimate o f the time
r e q u i r e d f o r the completion o f the j o b .
This estimate i s
i n d i c a t e d by p l a c i n g a mark on the base o f the card so t h a t i t s
d i s t a n c e from the l e f t - h a n d s i d e o f the c a r d , measured i n
i n c h e s , corresponds t o the elapsed time of the j o b .
Next each
c a r d i s p l a c e d on the schedugraph board i n the row which c o r responds t o the workman to be assigned t o the job w i t h i t s
l e f t - h a n d edge c o r r e s p o n d i n g t o the hour o f the check at which
time the work on the job i s expected to b e g i n .
In t h i s manner
both the p l a n n e r and the c h i e f mechanic can see at a glance a l l
the work that i s to be done by one man,
g i v e n time.
by one group, or a t any
As the check p r o g r e s s e s the board i s c o n t i n u a l l y
updated r e f l e c t i n g such changes as new d u r a t i o n s f o r c e r t a i n
j o b s , changes i n the a v a i l a b l e manpower and changes i n job s e quence and
scheduling.
2. The schedugraph system has s i n c e been r e p l a c e d by the
"Plannet" approach. See c h a p t e r I I I , and E.R. Bossange J r . ,
op. c i t .
25
Chapter I I I - Methods o f S o l v i n g the Problem
The Schedugraph Board
The problem as s t a t e d i n the p r o c e e d i n g c h a p t e r s was
t o reduce the t o t a l elapsed time o f the e q u a l i z e d checks.
are many ways of approaching t h i s problem.
schedugraph board i s one.
There
The use o f the
The main advantage of t h i s system i s
that i t b r i n g s order out o f c o n f u s i o n (the completion o f f i v e
hundred, and at times one thousand d i f f e r e n t
j o b s , many o f which
are i n d i v i d u a l l y i n t e r - r e l a t e d w i t h many o t h e r s , a l l t o be
completed i n the b a r e s t minimum o f time by one hundred
men working on three d i f f e r e n t
confusion).
fifty
s h i f t s can e a s i l y r e s u l t i n
The schedugraph board enables the c h i e f mechanic
and h i s a d v i s o r , the maintenance p l a n n e r , t o balance the work
load c o n t i n u a l l y throughout the check i n the l i g h t of the
latest
developments.
The major disadvantage o f t h i s system l i e s i n i t s
f a i l u r e t o draw a t t e n t i o n t o the important job i n t e r - r e l a t i o n ships.
I t i s true that f o r one man working on a c h a i n o f
r e l a t e d jobs each o f which f o l l o w s the other i n an e f f i c i e n t
o r d e r , the i n t e r - r e l a t i o n s h i p s are a d e q u a t e l y p o i n t e d out by a
glance at the a p p r o p r i a t e row.
S i m i l a r l y , the r e l a t i o n s h i p s
between a number of jobs a l l t o be performed at the same time
are r e v e a l e d by an a n a l y s i s o f the columns o f the board.
these are f r e q u e n t l y o n l y o f secondary importance.
But
Of primary
concern are the c a u s a l r e l a t i o n s h i p s s t a t i n g t h a t job B cannot
begin u n t i l job A i s completed.
The reason f o r t h i s i s t h a t i t
i s a c h a i n o f jobs which determines
time.
the maximum elapsed check
I f such c h a i n r e l a t i o n s h i p s d i d not e x i s t , the elapsed
time o f the check c o u l d be made t o equal the elapsed time o f the
longest job simply by i n c r e a s i n g the manpower and performing a l l
jobs s i m u l t a n e o u s l y .
and
Because o f the complexity
o f the a i r c r a f t
i t s systems, i t i s common f o r job B t o be performed by a
workman from a d i f f e r e n t group than the one who performed j o b A.
Thus w i t h a schedugraph board,
rows i n order t o f i n d
job B.
the planner must scan a l l the
The problem becomes f a r more
complicated when one f a c e s such common occurrences as t e n or
twenty jobs a l l dependent upon the completion o f job A.
The K.L.M. Approach
Another approach i s being u t i l i z e d by K.L.M. - Royal
Dutch A i r l i n e s , i n the major overhauls o f t h e i r j e t
f l e e t . I t
i n v o l v e s p l a n n i n g a s e r i e s o f d e a d l i n e completion p o i n t s
out the check.
F i r s t of a l l ,
by the s c h e d u l i n g department.
through-
the check completion p o i n t i s s e t
Next, d e a d l i n e s are s e t f o r each
system i n the a i r c r a f t such as r i g g i n g , pneumatics, and h y d r a u l i c s .
These d e a d l i n e s as s e t so as t o give adequate time f o r f u n c t i o n 1. J.M. Roos and G. H e r r i n g o f the P r o d u c t i o n , P l a n n i n g and
C o n t r o l , T e c h n i c a l D i v i s i o n a t K.L.M. d e s c r i b e d t h i s approach
t o the w r i t e r i n an i n f o r m a l d i s c u s s i o n a t the 10th Meeting
o f the I n t e r n a t i o n a l A i r Transport A s s o c i a t i o n ' s P r o d u c t i o n
P l a n n i n g and C o n t r o l group i n D u b l i n , I r e l a n d , A p r i l , 1964.
27
ing
2
each system before the a i r c r a f t departure
time.
s i m i l a r f a s h i o n , key d e a d l i n e s are s e t c l o s e r to the
o f the checks.
In a
beginning
Then each of the jobs throughout the whole
check are g i v e n a d e a d l i n e i n r e l a t i o n to these key p o i n t s .
Such a method overcomes many of the shortcomings of
the schedugraph system.
Here the many d e a d l i n e p o i n t s do draw
a t t e n t i o n to the important
job i n t e r - r e l a t i o n s h i p and
importance of job c h a i n s .
However, such a s c h e d u l i n g method i s
not designed
t o reduce the elapsed
guarantee completion
than minimum time.
to the
time to a minimum but
to
of the check w i t h i n a g i v e n , though g r e a t e r
T h i s i s because the job d e a d l i n e s are s e t
so as to g i v e the maximum amount o f time f o r the completion
the m a j o r i t y o f the j o b s .
The
system i s t h e r e f o r e best s u i t e d
t o an a i r l i n e whose s e r v i c e s are s u f f i c i e n t l y peaked that
great p r e s s u r e
of
no
i s p l a c e d on the maintenance department f o r a
more r a p i d check completion.
Such was
the case at K.L.M. d u r i n g
1963.
Another s e r i o u s disadvantage of t h i s method a r i s e s from
the l o c a t i o n o f the " s l a c k time" d u r i n g the check.
This s l a c k
time i s the d i f f e r e n c e between the minimum job time, or check
completion
time, and
the maximum.
I t i s unwise to schedule
check to be completed on the b a s i s o f the estimated
t i o n times s i n c e many time-consuming c o m p l i c a t i o n s
2.
a
job complemay
arise.
F u n c t i o n i n g i s the term used to d e s c r i b e the t e s t i n g of
v a r i o u s systems t o determine any m a l f u n c t i o n i n g .
the
Thus some s l a c k time i s added t o reduce the p o s s i b i l i t y o f a
delayed f l i g h t departure due t o these c o m p l i c a t i o n s .
Since
n e i t h e r the s e r i o u s n e s s o f the c o m p l i c a t i o n , nor i t s time o f
occurrence d u r i n g the check a r e p r e d i c t a b l e , the g r e a t e r the
s l a c k time and the c l o s e r i t f a l l s toward the end o f the check,
the s m a l l e r w i l l be the p r o b a b i l i t y o f a delayed d e p a r t u r e .
The K.L.M. approach t o the problem has the disadvantage that i t s
d e a d l i n e s are s e t on the b a s i s o f a minimum time t o check comp l e t i o n , l e a v i n g l i t t l e or no s l a c k i n the system once the f i r s t
d e a d l i n e i s reached.
The m a j o r i t y o f the s l a c k time f a l l s i n
the e a r l y hours, or days, o f the check and i s t h e r e f o r e , I n a
sense, wasted.
Plannet
Pan American World Airways have developed another
approach to t h i s problem c a l l e d P l a n n e t (Planning Network).3*
I t s use a t Pan American r e s u l t e d i n the r e d u c t i o n o f elapsed
time on the f o u r hundred hour DC 8 " E q u a l i z e d S e r v i c e s " check
from t h i r t y - f o u r t o twenty-two
hours.
There are two major reasons f o r i t s s u c c e s s .
First,
r e s p o n s i b i l i t y f o r the e a r l y completion o f the check was g i v e n
to the f i r s t
3.
line supervision.
T h i s i s composed o f the foremen
The m a t e r i a l presented i n t h i s s e c t i o n was gained from d i s c u s s i o n w i t h Mr. C.G. Rose, S u p e r v i s o r of Maintenance
P l a n n i n g , Canadian P a c i f i c A i r L i n e s , and from P l a n n e t . a
paper by E.R. Bossange J r . , op. c i t .
i n charge o f the ten trade groups.
mechanic.
They r e p o r t t o the c h i e f
S i n c e they are the c l o s e s t l e v e l of s u p e r v i s i o n t o
the workmen they have a wealth of knowledge and sphere of
i n f l u e n c e over t h e i r men
the plannet approach,
which no other s u p e r v i s o r s have.
i t i s these foremen who
draw up the
o v e r a l l schedule of work completion and set t h e i r own
standards f o r the work that i s to he done.
In
time
There i s t h e r e f o r e
more i n c e n t i v e to e q u a l or beat the estimated times.
Second, f u l l advantage
c e r t a i n of the jobs performed
that
i n a check are i n the " b a s i c "
c a t e g o r y and are thus performed
About 2%
i s taken o f the f a c t
on every e q u a l i z e d check.
of a l l jobs f a l l i n t o t h i s c a t e g o r y .
the m a j o r i t y of these jobs are performed
In p l a n n e t ,
i n a h i g h l y organized
manner approximating an a s s e m b l y - l i n e technique, and thus f a r
greater e f f i c i e n c y i s achieved.
One
major disadvantage of t h i s technique i s that work
that has not been pre-planned o f t e n upsets the a s s e m b l y - l i n e
system.
In a d d i t i o n , a l t h o u g h much of the r o u t i n e work and
the
o v e r - a l l check p l a n are scheduled w i t h c r i t i c a l path p r i n c i p l e s
i n mind, n o n - r o u t i n e work i s scheduled without the h e l p of a
f o r m a l c r i t i c a l path framework.
Another method of s h o r t e n i n g the elapsed time i s
simply t o schedule the a i r c r a f t departure time e a r l i e r .
Since,
w i t h i n c e r t a i n l i m i t s , the amount o f time r e q u i r e d to perform a
job
i s p r o p o r t i o n a l to the amount of time a v a i l a b l e , s c h e d u l i n g
30
a s h o r t e r check may of i t s e l f
This i s p a r t i c u l a r l y
s o l v e the elapsed time problem.
t r u e where there i s evidence
that the jobs
are being made t o l a s t i n order t o g i v e the appearance o f work
as was the case a t Canadian P a c i f i c
Air Lines.
A f i n a l method o f r e d u c i n g the elapsed time I s o f
course the c r i t i c a l path method
itself.
Only a few o f the methods o f s o l v i n g the problem have
been d e s c r i b e d above.
I t should be noted
are n o t m u t u a l l y e x c l u s i v e .
major s c h e d u l i n g procedure
may be added.
t h a t these methods
One method may be f o l l o w e d as the
t o which a d a p t a t i o n s from many others
F o r example, i n 1964 Canadian P a c i f i c A i r L i n e s
was u s i n g the schedugraph system coupled w i t h many a d a p t a t i o n s
from p l a n n e t .
I n a d d i t i o n , I t was making use o f many o f the
p r i n c i p l e s o f the c r i t i c a l path method and s c h e d u l i n g much
s h o r t e r checks.
The r e s u l t was a r e d u c t i o n o f elapsed
time
from seventy-two hours on a three hundred f i f t y hour check i n
1963 t o a time o f f o u r t y hours f o r a f i v e hundred hour check,
c o n t a i n i n g c o n s i d e r a b l y more j o b s , i n 1964.
31
Chapter IV - The C r i t i c a l Path Technique
The
Technique
The C r i t i c a l Path Technique i s a method o f s c h e d u l i n g
a c t i v i t i e s developed
by the United S t a t e s Navy d u r i n g the
P o l a r i s M i s s i l e Development Programme f o r the purpose o f reduci n g the elapsed time r e q u i r e d t o complete a p r o j e c t .
In t h i s technique
the t r a d i t i o n a l "Gantt" or"bar c h a r t "
i s r e p l a c e d by a network o f a c t i v i t i e s which r e p r e s e n t s a p r o j e c t .
(See F i g u r e 1)
Each arrow i n the network represents one a c t i v i t y
(job) t o be completed before the end o f the p r o j e c t .
Each
"node" i n the network r e p r e s e n t s the commencement o f and, o r
completion
o f one or more a c t i v i t i e s .
I t therefore represents a
p o i n t i n time.
The
a c t i v i t i e s are charted i n the network a c c o r d i n g
t o the order i n which they must be performed f o r s u c c e s s f u l
completion
o f the p r o j e c t .
Consider
f o r example, the a c t i v i t i e s
performed i n the changing o f an engine on a p r o p e l l e r d r i v e n
aircraft.
(See F i g u r e 2)
Node "A" r e p r e s e n t s the time a t
which work on the p r o j e c t , the engine change, may begin.
Three
j o b s , the removing o f the p r o p e l l e r , the p r e p a r i n g o f the new
engine,
and the d i s c o n n e c t i n g o f the engine t o be removed may
a l l commence a t t h i s time and, hence, are g i v e n the common
" t a i l node" &»
The t a i l node i n d i c a t e s the commencement o f the
job.
I n a d d i t i o n , each job i s g i v e n a "head node" (head o f the
arrow), i n d i c a t i n g the completion o f the j o b .
F o r the above
jobs these head nodes a r e G, E , and B r e s p e c t i v e l y .
The job o f
d e s l u d g i n g the p r o p e l l e r dome cannot be s t a r t e d u n t i l job A-G,
p r o p e l l e r removal, i s completed.
the
of
To i n d i c a t e t h i s
d e s l u d g i n g job i s g i v e n the t a i l node G.
removing the engine i s dependent
relationship
S i m i l a r l y , the j o b
upon the completion o f the
job
A-B, d i s c o n n e c t i n g o f the engine, and i s t h e r e f o r e g i v e n
the
t a i l node B.
However, t h i s job cannot be s t a r t e d u n t i l the
p r o p e l l e r has been removed.
T h i s would suggest that i t should
be g i v e n the t a i l node G, a l s o .
I t i s n o t p o s s i b l e t o have one
arrow (one job) emanating from two nodes.
job,
T h e r e f o r e , a dummy
G-B, i s drawn i n d i c a t i n g t h a t a l l jobs t e r m i n a t i n g a t
node G must be completed b e f o r e any work s t a r t i n g a t node
B can commence.
In g e n e r a l , a dummy i s i n s e r t e d t o avoid
ambiguity i n c h a r t i n g the dependent
r e l a t i o n s h i p s , and a l s o t o
make i t p o s s i b l e f o r each job t o have an unique p a i r o f node
numbers.
S i n c e a dummy does n o t r e p r e s e n t an a c t u a l j o b , but
merely a r e l a t i o n s h i p , i t s d u r a t i o n w i l l be z e r o .
In t h i s
manner, each job i s g i v e n an unique combination o f t a i l and
head
nodes.
The network must be c h a r t e d i n such a manner that f o r
each job there w i l l be i n d i c a t e d a t l e a s t one job which must
precede i t and a t l e a s t one which must f o l l o w i t .
Stating
this
another way, each node i n the network must be a head node and a
t a i l node.
One
e x c e p t i o n t o t h i s r u l e occurs f o r the node
i n d i c a t i n g the commencement o f the p r o j e c t and f o r the j o b ( s )
emanating from t h i s node.
I t i s a t a i l node but not a head
node and each job emanating from i t i s preceded
by no j o b .
The
o n l y o t h e r e x c e p t i o n t o t h i s r u l e occurs a t the node i n d i c a t i n g
the completion
o f the p r o j e c t where the node i s never a t a i l
node and each job t e r m i n a t i n g at i t i s f o l l o w e d by no j o b .
T h i s i d e n t i f i c a t i o n serves t o r e l a t e each job t o a l l other
and
t o the p r o j e c t as a whole.
jobs
In t h i s sense a dummy I s con-
s i d e r e d t o be a j o b .
The C h a r t i n g o f R e l a t i o n s h i p s
In c h a r t i n g , two main questions a r i s e .
should Interdependent
First,
how
r e l a t i o n s h i p s between jobs be charted
and
second, how should l i m i t e d resources a v a i l a b l e f o r perform-
ing
jobs be t r e a t e d ?
I n order f o r a job t o be g i v e n a p o s i t i o n on the
c h a r t or network, some dependent r e l a t i o n s h i p s between t h a t job
and other jobs i n the p r o j e c t must be r e c o g n i z e d .
job
must be g i v e n a t a i l node and a head node.
That i s , a
However, merely
i d e n t i f y i n g the r e l a t i o n s h i p s r e q u i r e d t o f a c i l i t a t e the c h a r t ing
o f the job i s i n s u f f i c i e n t .
The c r i t i c a l path t h a t w i l l be
c a l c u l a t e d from the c h a r t w i l l be m i s l e a d i n g i f a l l the i n t e r job
r e l a t i o n s h i p s f o r each j o b In the p r o j e c t are not i d e n t i f i e d
and
charted.
The f i r s t
type of dependency
e x i s t s when a g i v e n job
(X) cannot be s t a r t e d u n t i l c e r t a i n jobs are completed.
there i s o n l y one preeeeding a c t i v i t y r e q u i r e d
c h a r t i n g w i l l be -<_)
^—^O
— * 0
( j o b Y) the
6
are m u l t i p l e dependencies the use o f dummies may
For example,
If
W
n
e
r
e
there
be r e q u i r e d .
i f job X cannot be s t a r t e d u n t i l A, B and Y are
a l l completed the c h a r t i n g might
—o
be:
y
xy'
I t i s obvious that i f job D proceeds job A i n the n e t work diagram, s t a t i n g that job A must be completed before job X
can commence i m p l i e s t h a t job D must a l s o be completed.
s i m i l a r manner more than one job may
be dependent
commencement upon the completion of job X.
c h a r t i n g may
In a
fori t s
I f such i s the c a s e ,
be as f o l l o w s :
Another type o f dependency
e x i s t s where one job (or
a c h a i n of j o b s ) must be completed w h i l e another job i s i n
progress.
S i n c e each job i n the network must have an unique
n o d a l i d e n t i f i c a t i o n (the node numbers are omitted i n these
examples)
a job which must be completed w h i l e job X i s i n
progress w i l l be c h a r t e d as f o l l o w s :
Another dependency
e x i s t s when one job cannot be
worked on w h i l e a p a r t i c u l a r job i s i n p r o g r e s s .
That i s ,
jobs K and H cannot both be performed at the same time.
l o g i c a l reason can be found why
H should precede,
K the dependency i s simply t h a t of our f i r s t
for
If a
or f o l l o w ,
type.
However,
c e r t a i n jobs there i s no b e t t e r reason f o r performing
before K than there i s f o r K before H.
In such cases a p u r e l y
a r b i t r a r y d e c i s i o n w i l l have to be made and
accordingly.
H
the c h a r t drawn
However, once the c r i t i c a l path and
the
total
f l o a t s f o r a l l jobs have been c a l c u l a t e d , the s c h e d u l i n g o f
jobs H and K must be re-examined and rescheduled
( i f this i s
r e q u i r e d ) to provide f i r s t
the s h o r t e s t c r i t i c a l path
second, the g r e a t e s t t o t a l
float.
I t may
be t h a t a g i v e n job X can commence when job L
i s only p a r t i a l l y completed.
should be d i v i d e d i n t o two
job
and
For c h a r t i n g purposes,
jobs ( _ i , _2,)
job L
i n such a way
that
X w i l l commence as soon as L\ i s completed.
The
limitations.
second problem area i n c h a r t i n g a r i s e s from p h y s i c a l
Examples are a r e s t r i c t e d number of workers i n a
given t r a d e , a l i m i t a t i o n i n the equipment a v a i l a b l e or a
l i m i t a t i o n as to the number o f men
who
can work on a g i v e n
area, e.g. the c o c k p i t .
The problem mentioned above of how
to c h a r t two
which cannot be worked on s i m u l t a n e o u s l y i s e x a c t l y t h i s
problem, and,
t h e r e f o r e , the s o l u t i o n s are the same.
For
example, i f there are f o u r jobs which c o u l d be preformed
jobs
simultaneously
three men
but, because the area i s so s m a l l t h a t o n l y
can work there at one
time, only three jobs can
be
performed, an a r b i t r a r y d e c i s i o n might be made to d e l a y one
the f o u r .
This decision w i l l
when the c r i t i c a l path and
Experience
of
be re-examined by the a n a l y s t
t o t a l f l o a t s are known.
shows t h a t on an e q u a l i z e d check (other
checks are s i m i l a r ) there are a r g r e a t many more jobs which can
be performed at one
to
time than there are men
available.
Thus,
f o l l o w the above s o l u t i o n would r e s u l t i n a great many
a r b i t r a r y d e c i s i o n s as to which job f o l l o w s which and
the
development of many c r i t i c a l paths which are r e a l l y not
s i n c e the jobs do not n e c e s s a r i l y f o l l o w one
another.
critical
To
determine the t r u e c r i t i c a l path from such a network would
most
be
difficult.
The
second method o f s o l u t i o n i s to assume, f o r
purposes of c h a r t i n g , t h a t manpower i s u n l i m i t e d ( t h a t a l l
these
jobs may
be performed s i m u l t a n e o u s l y ) .
d i t i o n s the t r u e c r i t i c a l path w i l l
Under these
con-
be obvious from the network.
However, the work load i n each trade must then be examined t o
determine whether, w i t h the manpower a v a i l a b l e , a l l the work
for
t h a t trade can be completed w i t h i n the time l i m i t s
by the c r i t i c a l
The
imposed
path.
g e n e r a l c o n c l u s i o n i s t h a t where r e l a t i v e l y
few
a r b i t r a r y d e c i s i o n s are r e q u i r e d , the f i r s t s o l u t i o n i s the
best.
However, i f there are many jobs r e q u i r i n g such treatment,
37
the second s o l u t i o n i s p r e f e r a b l e
The
f o l l o w i n g r u l e s should be kept i n mind as
charting
proceeds.
1. Each job must have an unique p a i r of node numbers.
2.
The
node numbering need not n e c e s s a r i l y be
order.
That i s , i t i s p o s s i b l e to c h a r t a job as
from t a i l node 8 to head node
3.
The
i n increasing
4.
running
1,
c h a r t i n g f o r each job should i n d i c a t e :
a. What jobs must precede t h i s
one,
b. What jobs must f o l l o w t h i s
one,
c. What jobs can be performed
concurrently,
d. What jobs cannot be performed s i m u l t a n e o u s l y w i t h
t h i s job, but must not n e c e s s a r i l y precede i t or
follow i t .
Calculations
The
p o i n t out
o b j e c t i v e of t h i s c r i t i c a l p a t h technique i s to
the minimum elapsed
time f o r a p r o j e c t and
to i n d i c a t e
which jobs must be performed on schedule i f the p r o j e c t as a
whole i s not
again,
t o be d e l a y e d .
( F i g u r e 2).
The
Consider the engine change p r o j e c t
numbers p r i n t e d between the nodes
above the arrows i n d i c a t e the most probable time r e q u i r e d
complete the i n d i c a t e d a c t i v i t y .
1. T h i s r u l e may
For example, job
A-G,
vary w i t h the computer program used.
and
to
38
removing
complete.
of the p r o p e l l e r , i s expected to r e q u i r e one hour t o
I t can be seen t h a t i f a l l jobs were to be
completed
as soon as the time e s t i m a t e s i n d i c a t e , the p r o j e c t would not
be completed
i n l e s s than e l e v e n hours.
T h i s time l i m i t i s
e q u a l t o the l o n g e s t path, i n terms o f hours, between the f i r s t
and l a s t nodes i n the network.
T h i s path, i n d i c a t e d i n F i g u r e
2 by the double l i n e i s the " c r i t i c a l p a t h " .
to be completed
i n the s h o r t e s t p o s s i b l e time, each
i n the " c r i t i c a l p a t h " must be completed
w i l l permit.
I f the p r o j e c t i s
activity/
as soon as the schedule
However, any o f the other jobs i n the network
be delayed f o r a l i m i t e d amount of time.
may
Thus, there i s an
urgency a s s o c i a t e d w i t h the completion of c r i t i c a l
jobs which i s
not a s s o c i a t e d w i t h any of the o t h e r s .
Each a c t i v i t y i n the network i s c h a r a c t e r i z e d by a
p a r t i c u l a r value of each of the f o l l o w i n g :
a. E.S. - E a r l i e s t
start,
the e a r l i e s t
time (measured from
the commencement of the p r o j e c t ) that an
can
activity
start.
b. E.F. - E a r l i e s t
f i n i s h - the e a r l i e s t
time that an a c t i v i t y
can f i n i s h ; EF » ES +- d u r a t i o n .
c. L.S. - L a t e s t s t a r t ,
the l a t e s t
time at which an
activity
can s t a r t without j e o p a r d i z i n g the completion of
the whole p r o j e c t i n the minimum e l a p s e d time.
d. L.F. « L a t e s t f i n i s h , the l a t e s t
time at which an
activity
can f i n i s h without j e o p a r d i z i n g the completion of
LF
s
LS +• d u r a t i o n .
e. T.F. - T o t a l f l o a t , the time a v a i l a b l e between
latest
f i n i s h and e a r l i e s t
finish.
TF = LP - EF.
When
the t o t a l f l o a t i s z e r o , the maximum time a v a i l a b l e
equals the time r e q u i r e d f o r completion o f the job
and the job i s c o n s i d e r e d " c r i t i c a l " ,
f . F.F. - Free f l o a t , the maximum time an a c t i v i t y can be
delayed without d e l a y i n g the e a r l i e s t
following
s t a r t of the
activity.
In c a l c u l a t i n g these values f o r job H-I, p u t t i n g t h e
p r o p e l l e r back on, the a n a l y s t would proceed as f o l l o w s .
earliest
time a t which the j o b can s t a r t
completion o f a l l the p r o c e e d i n g j o b s .
The
i s dependent upon the
S i n c e the longest path
from node A t o node H i s the path A-B-C-D-E-F-H, the e a r l i e s t
start
i s l i m i t e d by the time r e q u i r e d t o complete
t h i s p a t h , 6 hours.
earliest
start
(the
f i n i s h f o r the j o b equals the
p l u s the d u r a t i o n , or 7 h o u r s .
i s t o be completed
latest
The e a r l i e s t
the jobs on
I f the p r o j e c t
i n the s h o r t e s t p o s s i b l e time, 11 hours, the
f i n i s h f o r the job H-I i s e q u a l t o 11 hours minus 3 hours
d u r a t i o n o f the l o n g e s t p a t h from node I t o node K the
end o f the p r o j e c t ) , 8 hours.
latest
The l a t e s t
start
f i n i s h minus the d u r a t i o n , or 7 h o a r s .
equals the l a t e s t
f i n i s h minus the e a r l i e s t
equals the
The t o t a l
float
f i n i s h (8-7), one
hour;
I n a s i m i l a r manner, c a l c u l a t i o n s f o r a l l jobs can be
made.
(See Table I V ) .
Table IV
E a r l i e s t E a r l i e s t L a t e s t L a t e s t T o t a l Free
Start F i n i s h Float Float
Finish
Start
Tall
Node
Head
Node
Duration
A
G
1
Prop. O f f
0
1
1
2
1
G
B
0
Dummy
1
1
2
2
1
1
G
H
3
Prop. Dome Desludge
1
4
4
7
3
2
A
E
3
Prepare New Engine
0
3
2
5
2
2
E
F
1
Engine In
5
6
5
6
0*
F
H
0
Dummy
6
6
7
7
1
H
I
1
Prop. On
6
7
7
8
1
F
I
2
Connect
6
8
6
8
0 *
I
J
1
F i n a l Inspection
8
9
8
9
0 *
J
K
2
Run-Up & F u n c t i o n
9
11
9
11
0 *
A
B
2
Disconnect
0
2
0
2
0 *
B
C
1
Remove Engine
2
3
2
3
0 *
C
D
1
Clean F i r e w a l l
3
4
3
4
0 *
D
E
1
Inspec tIFirewa11
4
5
4
5
0 *
Description
Indicates C r i t i c a l
jobs
1
F I G U R E ^ J.
SAMPLE C R I T I C A L PATH NETWORK
FIGURE
2
ENGINE REMOVAL
FINAL \ _ y RUN-UP
INSP.
AND •
FUNCTION
CHECKS
REMOVE
ENGINE
CLEAN
FIREWALL
FIGURE
DJJJl E: Y
2
t
t
^
—>
C RI T I C A L
A C T I V I T Y
,
-J
ro
43
Chapter V - The Methods o f A p p l i c a t i o n o f the C r i t i c a l
P a t h Technique to A i r c r a f t Maintenance
The Technique i n the A n a l y s i s o f Completed
The c r i t i c a l
preceding chapter.
Checks
path technique was e x p l a i n e d i n the
The q u e s t i o n that now
a r i s e s i s , "How
can
t h i s technique be a p p l i e d to a i r c r a f t maintenance?"
Let
the
us f i r s t
c o n s i d e r the use of the technique i n
a n a l y s i s of a completed e q u a l i z e d check o f a DC-8.
The
e q u a l i z e d check i s c o n s i d e r e d because i t i s a time consuming
p r o j e c t performed weekly by the maintenance crews.
The DC-8
is
chosen because of the maintenance p l a n n i n g problems r e s u l t i n g
from the great complexity of t h i s a i r c r a f t .
chosen f o r the f i r s t
A completed check i s
a p p l i c a t i o n because a l l o f the jobs performed
i n a completed check are known.
I f a check i s yet to be completed
many j o b s , such as those a r i s i n g from the i n i t i a l
of
inspection
the a i r c r a f t are not known and t h e r e f o r e cannot be p l o t t e d
i n the network.
In
first
a p p l y i n g the technique i n t h i s a p p l i c a t i o n , the
task i s t o p l o t the network of the jobs that were p e r -
formed on the check a c c o r d i n g to the sequence that the a n a l y s t
f e e l s t o be most e f f i c i e n t , not n e c e s s a r i l y i n the sequence.in
which the jobs were performed.
F o l l o w i n g t h i s , the
critical
44
p a t h w i l l be determined.
Because of the great number of jobs
i n v o l v e d i n each check, use i s made of an e l e c t r o n i c computer
i n determining t h i s path.
In g e n e r a l , i f the network c o n s i s t s
o f one hundred or more a c t i v i t i e s , i t i s more economical
to
employ a computer.
There are many computer programs a v a i l a b l e .
Sauer's
LESS program, Least Cost E s t i m a t i n g and S c h e d u l i n g , i s
the one t h a t was
The
Ray
chosen f o r use at Canadian P a c i f i c A i r L i n e s . *
1
i n p u t data c o n s i s t s of a separate card f o r each a c t i v i t y
i n the p r o j e c t c o n t a i n i n g :
a. T a i l node
b. Head node
c. D u r a t i o n
d. D e s c r i p t i o n *
2
The output, cards and/or a p r i n t o u t , c o n s i s t s o f a l l
the i n p u t i n f o r m a t i o n p l u s a c a l c u l a t i o n f o r each job of the
earliest start, earliest finish, latest start, latest
t o t a l f l o a t and f r e e f l o a t ;
finish,
t h a t i s , a t a b l e s i m i l a r t o Table
The next step i s the a n a l y s i s of t h i s output.
i s performed
w i t h two major o b j e c t i v e s i n mind.
most important, every attempt
1. Ray.
First,
w i l l be made t o reduce
IV.
This
and
the elapsed
N. Sauer, oj__ c i t .
2. As the name, L e a s t Cost E s t i m a t i n g and S c h e d u l i n g suggests,
the program w i l l a l s o accept input data on the c o s t of each
a c t i v i t y and c a l c u l a t e from i t the t o t a l p r o j e c t c o s t . However, i n the present example, the b e n e f i t s t o be d e r i v e d
from the a d d i t i o n a l knowledge gained are not great when
compared t o the a d d i t i o n a l c o s t of p r e p a r i n g the input d a t a .
time o f the p r o j e c t by r e d u c i n g the l e n g t h , i n hours, o f the
c r i t i c a l path.
critical"
Second, the t o t a l f l o a t o f c r i t i c a l and "near-
jobs w i l l be i n c r e a s e d .
e v e r , o f o n l y secondary
be pursued
importance
T h i s l a t t e r o b j e c t i v e i s , howand must, under no circumstances,
i f i t w i l l p r o h i b i t the attainment
reason f o r t h i s
o f the former.
l a t t e r o b j e c t i v e i s t h a t the t o t a l f l o a t
on e s t i m a t e d , not a c t u a l job d u r a t i o n s .
The
i s based
I t can be seen that i f a
snag causes a job w i t h one hour t o t a l f l o a t
to be delayed two
hours, the whole p r o j e c t w i l l be delayed one hour.
There i s an
i n v e r s e r e l a t i o n s h i p between the l e n g t h of the t o t a l f l o a t and
the p r o b a b i l i t y o f a d e l a y .
The a n a l y s i s begins w i t h an examination
path.
o f the c r i t i c a l
Those a c t i v i t i e s l y i n g on t h i s path which can be resched-
uled t o another p o r t i o n o f the.check where they w i l l n e i t h e r be
c r i t i c a l nor cause other a c t i v i t i e s t o become o v e r l y "nearc r i t i c a l " w i l l be r e s c h e d u l e d .
Revised time estimates w i l l be
g i v e n f o r a l l c r i t i c a l jobs which can be performed
time than the o r i g i n a l
estimates i n d i c a t e .
i n a shorter
T h i s w i l l be p o s s i b l e
where an i n c r e a s e i n manpower assigned t o a g i v e n j o b , an i n c r e a s e
i n equipment, or a change i n work procedures, e t c . , w i l l cause a
r e d u c t i o n i n the r e q u i r e d time.^*
F o l l o w i n g t h i s , whole groups o f
a c t i v i t i e s c o n t a i n i n g some c r i t i c a l o r n e a r - c r i t i c a l
jobs w i l l be
3. I t should be noted t h a t i n g e n e r a l , the d u r a t i o n s assigned t o
a p a r t i c u l a r job when the c h a r t i s f i r s t p l o t t e d w i l l be the
best estimate o f the time r e q u i r e d assuming t h a t the a l l o c a t i o n o f r e s o u r c e s w i l l be the most e f f i c i e n t and t h a t the
manpower assumed t o be assigned t o the job w i l l be t h a t which
would perform the j o b most e c o n o m i c a l l y , even i f t h i s means a
l o n g e r d u r a t i o n than might otherwise be p o s s i b l e .
46
examined f o r p o s s i b l e time r e d u c t i o n s .
completed,
When t h i s r e v i s i o n i s
c a l c u l a t i o n s w i l l be made t o determine
the, new c r i t i c a l
path which w i l l , i n a l l p r o b a b i l i t y , be comprised
o f more c r i t i c a l
jobs (on p a r a l l e l c r i t i c a l paths) y e t have a s m a l l e r e l a p s e d time.
T h i s process o f a n a l y s i s , r e v i s i o n , and p r o d u c t i o n o f r e v i s e d
c r i t i c a l paths w i l l continue u n t i l the a n a l y s t i s s a t i s f i e d
that
he has reduced the elapsed time t o a workable minimum.
In
t h i s manner the a n a l y s t determines what the schedule
should have been, or the " i d e a l " s c h e d u l e .
Since h i s r e c o r d s
tell
him what d i d a c t u a l l y happen i n the check, the a n a l y s t i s i n a
p o s i t i o n t o compare the a c t u a l w i t h the i d e a l w i t h a view t o
improving the performance
The Technique
The
on f u t u r e checks.
i n A n a l y s i s Before and During the Check
c r i t i c a l p a t h technique can a l s o be used t o p r o v i d e
the maintenance planner w i t h v a l u a b l e i n f o r m a t i o n before and
d u r i n g the check.
As was p o i n t e d out above, the major d i f f i c u l t y
w i t h t h i s approach
i s t h a t approximately f i f t y per cent o f the
jobs t h a t w i l l be performed
on any given e q u a l i z e d check cannot
be preplanned because they are not known u n t i l the check i s i n
progress.
A p o s s i b l e s o l u t i o n t o t h i s problem
c a l c u l a t i o n s o f the c r i t i c a l path throughout
Is t o make p e r i o d i c
the check.
For
example, a r e c a l c u l a t i o n o f the c r i t i c a l path c o u l d be made every
f o u r or f i v e hours on the b a s i s o f the l a t e s t a v a i l a b l e
t i o n on work load and job d u r a t i o n s .
informa-
The c r i t i c a l path f o r each
s u c c e s s i v e c a l c u l a t i o n would be expected t o change r e f l e c t i n g
both
the a d d i t i o n a l jobs i n c l u d e d i n the network and
c e r t a i n time e s t i m a t e s .
the r e v i s i o n of
Thus, as more r e c e n t c a l c u l a t i o n s
of
the c r i t i c a l p a t h are made a v a i l a b l e , the planner can reschedule
the jobs t h a t are s t i l l
t o be performed
i n such a way
that the
t o t a l e l a p s e d time f o r the check i s reduced.
One
g r e a t value of a p e r i o d i c c a l c u l a t i o n of the
c r i t i c a l p a t h d u r i n g the p r o j e c t i s t h a t r e v i s e d estimates of
t o t a l f l o a t are made a v a i l a b l e .
I n g e n e r a l , as the check p r o -
gresses the t o t a l f l o a t f o r a l l jobs s t i l l
approaches z e r o .
or
t o be
completed
The reasons f o r t h i s are t h a t the job
some preceding jobs i n the n e t w o r k ,
4,
itself,
were not s t a r t e d at
the e a r l i e s t p o s s i b l e s t a r t , some jobs took longer t o complete
than was
work.
e s t i m a t e d , o r , a d d i t i o n a l jobs were added t o the n e t -
T h i s p e r i o d i c c a l c u l a t i o n p r o v i d e s the planner w i t h i n -
f o r m a t i o n designed
to ensure
t h a t the n o n - c r i t i c a l jobs are not
ignored t o the p o i n t that they become c r i t i c a l .
For example, i f
t h e r e are f o u r hours f l o a t f o r a c h a i n of jobs but the s t a r t of
one
job i s delayed s i x hours, a most s e r i o u s problem has
arisen.
When computers are employed i n the c a l c u l a t i o n of the
c r i t i c a l path, e x t e n s i v e use may
category, " d e s c r i p t i o n " .
category i s reproduced
cards may
be made of the input data
A l l the input i n f o r m a t i o n f o r t h i s
on the output c a r d s .
Thus, the
output
be s o r t e d a c c o r d i n g to the d e s c r i p t i o n of the i n p u t .
For example, there may
be ten or more d i f f e r e n t trade groups a l l
working on the same check; p a i n t e r s , c l e a n e r s , e l e c t r i c i a n s ,
4. Because of the i n t e r d e p e n d e n c i e s of the jobs i n the network,
a d e l a y i n one job w i l l i n most c a s e s cause a r e d u c t i o n i n
the t o t a l f l o a t of the jobs f o l l o w i n g i t .
inspectors, etc.
I f the a n a l y s t i s i n t e r e s t e d i n examining i n
more d e t a i l the work performed by any p a r t i c u l a r group, he may
i n d i c a t e i n the d e s c r i p t i o n o f the job which group i s employed
and
then s o r t the output data a c c o r d i n g t o groups.
In t h i s way,
a foreman provided w i t h such a l i s t i n g may see at a glance a l l
jobs under h i s j u r i s d i c t i o n , both c r i t i c a l and n o n - c r i t i c a l .
It
may be t h a t the planner i s having d i f f i c u l t y i n c o - o r d i n a t i n g the
work i n one p a r t i c u l a r area
( e . g . the c o c k p i t or the f l a p s ) .
If
the d e s c r i p t i o n of each job i n c l u d e s the area i n which the job i s
to
be performed, a l l jobs i n the area may be s o r t e d from the out-
put deck f o r ease o f a n a l y s i s .
the manpower requirement
planner's
The d e s c r i p t i o n might a l s o c o n t a i n
f o r each job, thus f a c i l i t a t i n g the
job o f b a l a n c i n g the work l o a d .
F u r t h e r s o r t i n g may f a c i l i t a t e a n a l y s i s even more.
For
example, the jobs performed by a p a r t i c u l a r trade may be s o r t e d
in
order o f t o t a l f l o a t
t o i n d i c a t e , by p o s i t i o n on the l i s t ,
the r e l a t i v e c r i t i c a l n e s s o f v a r i o u s j o b s .
A different
a c c o r d i n g to e a r l i e s t s t a r t time may a l s o be h e l p f u l .
sort
Sorting
the output deck three times, f i r s t by t r a d e , and f o r each trade
s e p a r a t e l y by t o t a l f l o a t and next, e a r l i e s t s t a r t time, might
a l s o be d e s i r a b l e .
The r e s u l t i n g l i s t w i l l show f o r each t r a d e ,
jobs i n order o f i n c r e a s i n g e a r l i e s t s t a r t time, and f o r a l l
jobs w i t h i d e n t i c a l e a r l i e s t s t a r t i n the same t r a d e , jobs i n
o r d e r of i n c r e a s i n g t o t a l f l o a t .
Thus, at any time d u r i n g the
check the planner w i l l know f o r each trade which jobs c o u l d be
s t a r t e d and the urgency a s s o c i a t e d w i t h each.
49
Other a p p l i c a t i o n s
The
use o f the technique i s not
u t i l i z i n g a computer.
is preferable.
i n t e n s e l y one
l i m i t e d to
operations
In many i n s t a n c e s a manual c a l c u l a t i o n i s
For example, i t i s o f t e n d e s i r a b l e to study
s m a l l area t h a t has
been p r e s e n t i n g
c e r t a i n problems.
In such cases there may
not be enough jobs i n v o l v e d to warrant
the use of a computer.
T h i s "manual" technique i s of value
the "post mortem" a n a l y s i s , the a n a l y s i s of what was
the check.
aircraft
I t may
be t h a t one
(say h y d r a u l i c s ) has
number of d e l a y s .
wrong d u r i n g
p a r t i c u l a r system or area
may
be
may
i n the
been r e s p o n s i b l e , i n p a r t , f o r a
A manual c r i t i c a l path a n a l y s i s of
the
a c t i v i t i e s o f the system i n each of the checks where the
occurred
in
i n d i c a t e the reasons f o r them, and
future
delays
delays
avoided.
Experience shows t h a t c e r t a i n areas are the major
problem areas r e p e a t e d l y .
When t h i s i s the case a d e t a i l e d manual
c r i t i c a l path a n a l y s i s of the problem-area i s r e q u i r e d , but a
c r i t i c a l path a n a l y s i s of the whole check would mean wasting
a n a l y s t ' s time.
before
and
In a d d i t i o n , the manual technique i s u s e f u l
d u r i n g a check.
For example, the most recent
r e p o r t of a check t h a t i s not yet completed may
d e t a i l e d manual a n a l y s i s of the problem may
avoiding t h i s delay.
In a s i m i l a r way,
computer
i n d i c a t e a prob-
able d e l a y r e s u l t i n g from problems In a c e r t a i n system.
s c h e d u l i n g may
the
A
p o i n t out ways of
a d e t a i l e d study of work
be performed before work on a m o d i f i c a t i o n to the
a i r c r a f t or i t s system i s begun.
50
The
technique,
e i t h e r manual or w i t h the a i d o f a com-
p u t e r , w i l l be of value i n the study o f a wide v a r i e t y o f p r o j e c t s
i n the f i e l d
o f maintenance p l a n n i n g .
e q u a l i z e d check has been d e s c r i b e d
great value
I t s a p p l i c a t i o n t o the
above.
i n the a n a l y s i s o f overhauls
the t o t a l elapsed
I t w i l l a l s o be o f
and annual checks where
time f o r the check i s f i v e or more days compared
w i t h only two days f o r the e q u a l i z e d check.
On these
checks the t o t a l work load i s much g r e a t e r .
T h i s makes
i n g more complex.
schedul-
The a d d i t i o n a l time r e q u i r e d f o r the longer
checks, and the r e s u l t i n g complexity
i n the s c h e d u l i n g
jobs caused by the g r e a t e r volume of work both serve
the u s e f u l n e s s
longer
o f the many
to increase
o f the c r i t i c a l path technique.^°
For checks o f t h i s d u r a t i o n the a n a l y s t may p r e f e r to
make use o f the PERT program (Program E v a l u a t i o n and Review
Technique) r a t h e r than the LESS program to determine the c r i t i c a l
path. •
three
The value o f PERT l i e s i n the f a c t
separate
data.
estimates
that i t requires
o f the d u r a t i o n o f each a c t i v i t y as input
These a r e :
1. 0.
O p t i m i s t i c time; the s h o r t e s t p o s s i b l e time i n which
a job might be completed.
2. M.
Most probable time.
3. P.
P e s s i m i s t i c time; the longest p o s s i b l e time t o complete
the j o b .
5.
I t i s because the annual check i s performed so seldom at
Canadian P a c i f i c A i r L i n e s , (only f i v e times per year) t h a t
emphasis i s l a i d on the a p p l i c a t i o n t o the e q u a l i z e d checks.
6. See I n t e r n a t i o n a l Business Machines, PERT, op. c i t .
51
Time 0 and P would be chosen i n such a way
of
that the p r o b a b i l i t y
the a c t u a l time being e i t h e r l e s s than 0, or g r e a t e r than P
would be one i n one hundred.
The PERT output i s a measure of the p r o b a b i l i t y of
completing the p r o j e c t i n the time a v a i l a b l e , or c o n v e r s e l y , the
p r o b a b i l i t y of a d e l a y In the d e l i v e r y of the a i r c r a f t .
This
p r o b a b i l i t y measurement i s not to be mistaken f o r an accurate
measurement w i t h mathematical precision.''*
i s determined
I t i s t r u e that i t
on the b a s i s of mathematically sound formulae, but
the parameters
0 and P which are p a r t of these formulae are
s u b j e c t i v e l y r a t h e r than o b j e c t i v e l y determined.
However, even
w i t h t h i s l i m i t a t i o n the p r o b a b i l i t y measurement i s of c o n s i d e r able v a l u e .
Consider the example of a check which because
scheduled f l i g h t must be completed
i n 40 hours.
of a
Suppose that the
c r i t i c a l path a n a l y s i s i n d i c a t e s a check completion of 38
hours.
I f the p r o b a b i l i t y measurement as c a l c u l a t e d by the PERT method
for
the 38 hours completion were . 8 ,
l i t t l e concern.
the planners would show
However, i f the p r o b a b i l i t y were .1, the
planners would take immediate steps to reschedule some work, or
to reduce the d u r a t i o n s of some of the c r i t i c a l j o b s .
another p o s s i b l e area of a p p l i c a t i o n i s i n the a n a l y s i s
of new
or unique p r o j e c t s .
For example, the technique would be
most h e l p f u l i n p l a n n i n g the maintenance work f o r a new
type of
aircraft.
7. See F.E. Grubbs, "attempts t o v a l i d a t e c e r t a i n PERT s t a t i s t i c s , "
O p e r a t i o n Research. V o l . 10, No. 6, November-December, 1962.
L e t t e r s t o the E d i t o r , p. 912.
52
In a d d i t i o n , the t o o l i s v a l u a b l e
c e r t a i n sections
f o r the a n a l y s i s of
or segments of the p r o j e c t s mentioned above.
For
example, such segmentations may be made i n terms o f trade groupings
(e.g. e l e c t r i c i a n s ) ,
a i r c r a f t areas (e.g. c o c k p i t ) , a i r c r a f t
systems ( e . g . pneumatics) or s h i f t s .
53
C h a p t e r V I - The
B e n e f i t s of C r i t i c a l
Analysis to A i r c r a f t
The
critical
The
b e n e f i t s t o be d e r i v e d
project, reductions
i n the
The
out
reductions
The
used t o a t t a i n t h i s
various
It
be
cost of a p r o j e c t .
p a t h m e t h o d , as
i s to reduce the
ways i n w h i c h t h e
importance of reducing
hours.
productively.
Non-productive hours are
greater
the
cost
more i m p o r t a n t i t becomes t o make f u l l
(maximize the u t i l i z a t i o n ) of the
be
illustrated
T a b l e V shows t h a t
times greater
elapsed
technique
out.
this
i s a w e l l known f a c t t h a t c a p i t a l i n v e s t e d
I n a d d i t i o n , the
t i o n s can
was
In
time
presented.
ment e a r n s a r e t u r n f o r i t s o w n e r s o n l y when t h e
operating
a
for
o b j e c t i v e were a l s o p o i n t e d
the paragraphs t h a t f o l l o w the
t o a minimum w i l l
time of
hours r e q u i r e d
critical
proceeding chapters,
time of a given p r o j e c t .
numerous.
elapsed
i n the
the
Time
main o b j e c t i v e of the
i n the
i n the
number o f man
Reductions i n Elapsed
be
f r o m an a p p l i c a t i o n o f
reductions
c o m p l e t i o n o f a p r o j e c t and
can
Maintenance
path technique to a i r c r a f t maintenance are
tangible benefits are;
pointed
Path
i f the
by
use
of the
equipment.
considering
cost
of the
o f one
two
piece
t h a n t h a t o f a n o t h e r , and
i n equip-
equipment i s
non-revenue
equipment
output
These
the
capacity
generaliza-
different
aircraft.
of equipment i s
i f a l l other
factors
100
are
54
TABLE V
COMPARATIVE INTEREST AND DEPRECIATION COSTS;
THE DC-8 AND THE DC-3
DC-3
DC-8
T o t a l Cost
$7,000,000
$70,000
Annual D e p r e c i a t i o n (10%)
700,000
7,000
Annual I n t e r e s t Expense
of t o t a l Cost)
420,000
4,200
1,120,000
11,200
4,000
4,000
280.00
2.80
T o t a l Annual Expense
Revenue F l y i n g Hours Per Year
T o t a l I n t e r e s t and D e p r e c i a t i o n
Expense per Revenue F l y i n g Hour
Source:
airline
S i n c e each o f these f i g u r e s vary c o n s i d e r a b l y from
to a i r l i n e
those chosen are rough approximates o f
1963 data and are rounded f o r ease o f p r e s e n t a t i o n .
55
e q u a l , the a d d i t i o n a l e a r n i n g s r e s u l t i n g from i n c r e a s e d u t i l i z a t i o n o f the equipment w i l l be IGO times as great a l s o .
the
maintenance
Thus,
department w i l l e x e r t great e f f o r t s t o reduce the
check time of the DC-8
time f o r the DC-3
but w i l l not be too concerned i f the check
i s not at a minimum.
Consider a g a i n the example of a DC-8.
average revenue per seat m i l e of 6 . 3 ^ ,
I f we
assume an
an average number o f
passengers o f 65 (a,50$ load f a c t o r ) and an average speed o f 500
m i l e s per hour, the r e t u r n per a i r c r a f t per revenue f l y i n g hour
i s approximately $ 2 , 0 5 0 per hour.
( 6 . 3 ^ / s e a t m i l e x 65 seat x
500 miles/hour s $2,047.50/hour.)
I f we a l s o assume the o u t - o f - p o c k e t c o s t o f f l y i n g
the
DC-8
one e x t r a hour t o be $900, i t i s obvious that the net
c o n t r i b u t i o n t o overhead
•
$1,150).
(and p r o f i t s ) i s $ 1 , 1 5 0 ,
($2,050 -
$900
(The assumed f i g u r e s are v e r y c l o s e approximations to
a c t u a l operating experience.)
Thus, i f an a p p l i c a t i o n o f the
c r i t i c a l p a t h a n a l y s i s to the e q u a l i z e d check can r e s u l t i n a
r e d u c t i o n o f elapsed time of f i v e hours per check, and i f 50
e q u a l i z e d checks are performed each y e a r , the r e s u l t a n t
t i o n to overhead would be $287,500 ( $ 1 , 1 5 0
contribu-
x 5 hours/check x 50
checks/year B $287,500 per y e a r ) .
The u n d e r l y i n g assumption here i s that the f i v e
addi-
t i o n a l hours of f l y i n g time made a v a i l a b l e w i l l be used by the
s c h e d u l i n g department f o r average revenue producing f l i g h t s .
If
a l l o f these a d d i t i o n a l hours cannot be used, or i f the revenues
56
received
from these a d d i t i o n a l f l y i n g hours are l e s s than
average, the net c o n t r i b u t i o n w i l l be reduced.
It w i l l
be
remembered from the example of C o n t i n e n t a l A i r l i n e s g i v e n i n
chapter I that the break-even load f a c t o r must not o f n e c e s s i t y
be a t t a i n e d on the a d d i t i o n a l f l i g h t .
each a d d i t i o n a l f l i g h t
for
i s greater
So long as the revenue o f
than the out-of-pocket
costs
t h a t f l i g h t , the a i r l i n e w i l l be i n c r e a s i n g i t s p r o f i t s .
the otherhand, i t may
be
On
that by making the a i r c r a f t r e g u l a r l y
a v a i l a b l e say f i v e hours e a r l i e r , an e x t r a f l i g h t of ten hours
may
be scheduled where no f l i g h t
at a l l had
been p o s s i b l e
before.
I f t h i s i s the case, the y e a r l y c o n t r i b u t i o n to overhead w i l l
t w i c e $ 2 8 7 , 5 0 0 or
be
$575,000.
Reductions i n Manhours
In g e n e r a l , use
to i n c r e a s e d
man
efficiency.
o f the c r i t i c a l path a n a l y s i s w i l l
There w i l l be marked e f f e c t s on
hours r e q u i r e d f o r c r i t i c a l and
lead
the
n o n - c r i t i c a l jobs as w e l l
as
a r e d u c t i o n i n standby time.
A l l c r i t i c a l and
n e a r - c r i t i c a l jobs w i l l be performed i n
the s h o r t e s t p o s s i b l e elapsed
time.
w i l l be shortened and
hour requirements f o r these
the man
w i l l be a f f e c t e d i n one
The
d u r a t i o n of c e r t a i n jobs
of f o u r ways, as d e s c r i b e d
hereunder.
1. Some jobs w i l l be performed more q u i c k l y because o f
increased
e f f i c i e n c y of the mechanic who
job on which he
i s working i s c r i t i c a l .
jobs
the
r e a l i z e s that
the
2. Some jobs w i l l have a shortened d u r a t i o n because the a p p l i c a t i o n o f the technique
l e d to a more e f f i c i e n t work procedure„
3. Some jobs w i l l experience a decrease
i n d u r a t i o n o n l y by
s u f f e r i n g an i n c r e a s e i n the number o f man hours r e q u i r e d .
For example, one mechanic may be a b l e t o complete a g i v e n job
i n three hours.
However, i f two men are assigned t o the j o b ,
completion may be p o s s i b l e i n two hours,
requirement
the t o t a l manhour
being f o u r manhours.
4 . Some Jobs w i l l experience a decrease
s u f f e r i n g an Increase
i n costs.
i n d u r a t i o n only by
F o r example, more equipment
may be r e q u i r e d t o reduce the d u r a t i o n o f c e r t a i n j o b s .
I t can be seen t h a t f o r c r i t i c a l and n e a r - c r i t i c a l
whose d u r a t i o n s are shortened,
those which f a l l i n t o
1, 2, o r 4 , experience a r e d u c t i o n l n manpower
jobs
groupings
requirements.
In a d d i t i o n , n o n - c r i t i c a l jobs w i l l a l s o experience a
r e d u c t i o n i n manpower requirements.
this.
There are two reasons f o r
F i r s t , the c r i t i c a l path technique w i l l l e a d t o an
a n a l y s i s of. n o n - c r i t i c a l as w e l l as c r i t i c a l
r e s u l t i n more e f f i c i e n t work procedures.
a p r o b a b i l i t y o f a d e l a y and the c r i t i c a l
jobs which w i l l
Second, when there i s
jobs are not e a s i l y
d i s t i n g u i s h a b l e from the n o n - c r i t i c a l , there i s a p o s s i b i l i t y o f
attempting
t o complete the n o n - c r i t i c a l jobs i n the s h o r t e s t
p o s s i b l e time a l s o .
T h i s may r e s u l t i n an i n e f f i c i e n t use o f
manpower (See the example of group #3 above).
path technique
I f the c r i t i c a l
i s employed the n o n - c r i t i c a l jobs w i l l be c l e a r l y
i n d i c a t e d and t h i s manpower waste w i l l be avoided.
58
The
third
power r e q u i r e m e n t s
time
i s present
inability
effect
that the technique
concerns
i n aircraft
standby
c o u l d make a v a i l a b l e
i n each group.
to the planner
o f e a c h g r o u p and e a c h w o r k e r .
tion
i s a computer output
ing
and o v e r t i m e .
balanced
load
given
necessitates
start
listing
systematized
time
path
data
on t h e work
An example o f t h i s
a l l jobs
o f each
technique
informa-
t o be p e r f o r m e d
by a
o f each j o b , o r , accord-
job.
Because the
technique
scheduling of a l l jobs, the planner
i sin
a b e t t e r p o s i t i o n t o f o r e c a s t work l o a d s a t any g i v e n t i m e .
Act-
ing
on t h i s
a detailed
Standby
work l o a d f o r a l l t r a d e
The c r i t i c a l
group a c c o r d i n g t o the c r i t i c a l n e s s
to the e a r l i e s t
have on man-
maintenance because o f the p l a n n e r ' s
to provide a p e r f e c t l y
groups and a l l workers
time
will
k n o w l e d g e , he may
jobs t o balance
t h e work l o a d .
reschedule
certain
I f r e s c h e d u l i n g does n o t s o l v e
1 .
the problem, a complete deferment o f c e r t a i n
m a i n t e n a n c e c h e c k may be p o s s i b l e .
advance w a r n i n g w i l l
non-critical
also result
jobs t o a f u t u r e
I t c a n be s e e n t h a t
i h a reduction of
such
overtime
expenses.
Thus t h e o v e r a l l e f f e c t
is
an i n c r e a s e i n e f f i c i e n c y .
completion
the
o n manhours o f t h e t e c h n i q u e
T h i s may
o f more work i n a c o n s t a n t
result
i n either a
number o f manhours o r o f
same amount o f work i n f e w e r manhours, t h u s
freeing
manpower.
1. I t s h o u l d be n o t e d t h a t i n a c h e c k l a s t i n g 60 h o u r s , a p p r o x i m a t e l y Q0% o f t h e j o b s w o u l d h a v e a t o t a l f l o a t i n e x c e s s o f
10 h o u r s a n d , i n more g e n e r a l t e r m s , t h e v a s t m a j o r i t y o f j o b s ,
f o r most p r o j e c t s , a r e n o n - c r i t i c a l .
T h e r e i s t h e r e f o r e much
opportunity f o r rescheduling at this point.
See T a b l e 1.1.
59
However, i t may be a d v i s a b l e t o permit
to
the t o t a l manpower c o s t s
i n c r e a s e i f the a d d i t i o n a l c o s t s a r e more than compensated f o r
by the i n c r e a s e d revenue from g r e a t e r a i r c r a f t u t i l i z a t i o n .
i n c r e a s e d manpower costs may a r i s e f o r two reasons.
First,
may r e s u l t from an i n e f f i c i e n t use o f manpower or other
These
they
resour-
s e s , caused by the r e d u c t i o n o f the d u r a t i o n o f c e r t a i n j o b s .
Second, i f i t i s l i m i t e d manpower and not job sequence that i s
p r o h i b i t i n g a c o n t r a c t i o n o f the elapsed
time f o r the c h e c k ,
2 ,
h i g h c o s t s might r e s u l t i f the a d d i t i o n a l manpower r e q u i r e d t o
overcome t h i s l i m i t a t i o n cannot be g i v e n employment elsewhere i n
the maintenance o p e r a t i o n s
once the check has been completed.
If
such i s the case, t h e r e w i l l be an i n c r e a s e i n standby time
chargeable
to the check.
Reductions In Costs
The
expenditures
c r i t i c a l path technique
w i l l effect
income and
i n f o u r ways.
1. Where r e d u c t i o n s i n elapsed
time f o r a check are p o s s i b l e , and
where the a d d i t i o n a l f l y i n g hours made a v a i l a b l e can be turned
i n t o revenue f l y i n g hours, great c o n t r i b u t i o n s t o overhead
w i l l be o b t a i n a b l e .
2. In order to reduce the elapsed time on a check to a minimum,
i t may be a d v i s a b l e to i n c r e a s e the c o s t o f performing
critical
certain
jobs i n order t h a t t h e i r d u r a t i o n s may be reduced.
2. An example o f t h i s s i t u a t i o n would be where the t o t a l p r o j e c t
d u r a t i o n based on the l e n g t h of the c r i t i c a l path i s 30 hours
but the t o t a l estimated work load f o r a given trade o f only
f o u r men i s l60 hours. The c r i t i c a l f a c t o r here i s not job
sequence, 30 hours, but the work load per man, 40 hours.
6 0
This
increase
materials,
3.
ks
be f o u n d
i s a cost
which
i s p r i m a r i l y the cost
following
3.
regarding
a r e o f t h e same o r d e r
jobs
savings
from
the cost
above i n t h a t
tion
i n reducing
procedures.
of the analyst's
evaluation.
planning,
time.
from those
mentioned
themselves t o o b j e c t i v e , but
In general,
scheduling
the technique
facili-
and c o n t r o l and b e t t e r
alloca-
In the paragraphs that
b e n e f i t s w i l l be m e n t i o n e d a l o n g
how e a c h i s o b t a i n e d
magnitude t o
Benefits
t h e y do n o t l e n d
o f manpower.
the d u r a t i o n o f
a s , and l a r g e l y o f f s e t by
less tangible benefits d i f f e r
subjective
tangible
t h e above may be made.
b e n e f i t s , may be o f s u f f i c i e n t
Less Tangible
The
incurred
improved
The l e s s t a n g i b l e
tates better
time.
costs.
critical
rather
analysis
m a g n i t u d e t h a n t h e above
costs
offset
affected.
a d d i t i o n a l revenues r e s u l t i n g from the a d d i t i o n a l
The i n c r e a s e d
the
f o r the jobs
o f the a n a l y s t ' s
hours are of a f a r g r e a t e r
mentioned
job procedures
o f c a r r y i n g out the c r i t i c a l path
generalizations
The p o s s i b l e
flying
2.
r e s u l t i n g i n lower c o s t
There
The
1.
e q u i p m e n t , manpower, e t c .
a r e s u l t o f t h e a n a l y s i s , c e r t a i n improved
will
4.
w o u l d r e s u l t f r o m a d d i t i o n a l e x p e n d i t u r e s on
f o l l o w some o f t h e l e s s
with a description of
by t h e a p p l i c a t i o n o f t h e c r i t i c a l p a t h
method.
1.
The t e c h n i q u e p e r m i t s and e n c o u r a g e s management t o e x e r c i s e
more c o n t r o l o v e r t h e p r o j e c t
analyzed.
The
systematized
61
i n f o r m a t i o n t h a t i s made a v a i l a b l e
ing
the check i n c r e a s e s t h e i r
the
various a c t i v i t i e s
this
of
ability
spotted.
i n the check.
T h e r e a r e two ways i n w h i c h t h i s
collected
will
( f o r example, t h e time
become more a c c u r a t e
c a u s e management w i l l
collected,
the data
examining
standards,
Is encountered.
will
standards
produce expected
d e t a i l s and
i n an attempt t o
management w i l l ,
to deviations
elapsed
of h i s analysis
f o r d e v i a t i o n s from
I n an
these
be e v a l u a t i n g t h e
and, i n d i r e c t l y ,
From t h i s
be p o i n t e d
as a r e s u l t
i n actuality,
m e c h a n i c s , t h e work p r o c e d u r e s
abilities.
be more a l e r t
o f work p e r f o r m a n c e .
t o determine the reasons
weaknesses w i l l
the data
and d u r i n g t h e p r o j e c t o r i n
the planner
management
i s be-
o f t h e manner i n w h i c h t h e t e c h -
post-mortem a n a l y s i s ,
standards,
being
e s p e c i a l l y where an e x c e s s i v e
Regardless
i s used, e i t h e r before
attempt
will
times.
from the expected
nique
This
and i r r e g u l a r
f o r the i r r e g u l a r i t i e s
S e c o n d , management w i l l
time
performance.
spends on a p a r t i c u l a r j o b )
and h e n c e , more u s e f u l .
i t f o r unexpected
study
increased control
be s y s t e m a t i c a l l y r e v i e w i n g
demanding e x p l a n a t i o n s
reduce elapsed
past
that i s at present
a man
a
be d o n e , and t h e r e f o r e ,
i s f o r c e d t o examine and t o e v a l u a t e
First,
organize
Because
In a d d i t i o n the a n a l y s i s i n v o l v e s both
b e n e f i t management.
and d a r -
d e v i a t i o n s f r o m i t a r e more
what was done as w e l l a s what s h o u l d
management
before
t o c o - o r d i n a t e and
t o be p e r f o r m e d
p l a n i s much more d e t a i l e d ,
easily
t o the planners
i t s own
e v a l u a t i o n , s t r e n g t h s and
o u t , and c o r r e c t i v e
a c t i o n , where
62
required,
2.
will
be
I n a d d i t i o n , the
project
and
carried
out.
analysis
technique forces
provides
a t o o l by
Without
or
irregular
intervals.
The
each analyzed
outcome o f
analysis
i s more o b j e c t i v e and
t i o n and
conclusions.
collects.
d a t a may
analyst
For
r e c u r r i n g problems,
This
t o the
i t be
not
able
be
benefit, that
aware t h a t
the
jobs, those
critical
for delays,
loss of
From t h a t
systematic
systematic
use
of the
of pointing
to o b j e c t i v e l y evaluate
to point
etc.
more
information
to
how
information
a r e a s most
trends,
out
the
that
the
be
r e q u i r e more o r
l a c k o f equipment or
acquired
i n gaining
the
and
the
seriousness
l o s s to the
nor
used
frequently
a n a l y s i s , each o f t h e s e problem a r e a s can
Once t h i s
comparable
critical
the
tained.
framework
followed,
for
and
informa-
p a t h technique can
those trades
t i m e due
at
a c e r t a i n problem e x i s t s ,
critical
manpower, t h e
specific
a n a l y s i s Is not
However, t h e
responsible
study of
for indicating
problem i s .
out
an
etc.
I f a systematic
management may
will
third
the
lack
e x a m p l e , r e g u l a r l y c o l l e c t e d and
value
be
tends to
t o more r e l i a b l e
t o make g r e a t e r
frequently
problem areas.
the
leads
technique,
a
i n d e p e n d e n t l y and
In a d d i t i o n , t h i s
have some s t a t i s t i c a l
leads
9
s y s t e m , r e s u l t i n g i n the
segments t h e r e o f
he
at a l l
a n a l y s i s of
a n a l y s i s may
a goad s i m i l a r t o t h i s
checks or
encourages the
a systematic
w h i c h s u c h an
of a check i f performed
o v e r a l l plan
3.
taken.
company f r o m e a c h
i s known, d e f i n i t e a c t i o n t o w a r d s
materials,
basic
be
less
data
evaluated,
ascerthe
63
s o l u t i o n o f the major problems can be taken,
4 . The f o u r t h b e n e f i t , a r e d u c t i o n i n the number and s e r i o u s n e s s
of
d e l a y s , has both t a n g i b l e and i n t a n g i b l e a s p e c t s .
the c r i t i c a l path technique throughout
the check w i l l
Use o f
i n many
cases p o i n t out the l i k e l i h o o d of a d e l a y e a r l i e r than would
otherwise be p o s s i b l e .
F o r the c a l c u l a t i o n o f the c r i t i c a l
path, time estimates must be made, and as the check progresses
and s u c c e s s i v e c r i t i c a l paths are c a l c u l a t e d , estimates must
a l s o by updated
f o r a l l jobs which appear t o be c r i t i c a l .
T h i s p r o v i d e s the planner w i t h complete
i n f o r m a t i o n , based on
the best estimates a v a i l a b l e , on the l i k e l i h o o d o f d e l a y s .
In
a d d i t i o n , the technique w i l l
p r o v i d e him w i t h more o f the
i n f o r m a t i o n h e l p f u l i n e i t h e r a v o i d i n g , o r r e d u c i n g the
s e r i o u s n e s s o f the expected d e l a y s .
The t a n g i b l e and l e s s
t a n g i b l e b e n e f i t s a r e a s a v i n g o f a l l the c o s t o f the d e l a y
that was avoided such as passenger
will,
inconvenience, l o s s o f good-
and c o s t o f r e s c h e d u l i n g a i r c r a f t .
64
Chapter
In
of
V I I - E v a l u a t i o n s and
the f i r s t
four different
chapter
attempts
of this
Conclusions
t h e s i s m e n t i o n was
to apply the
critical
maintenance.
of
a p p l i c a t i o n s was
g i v e n , f o l l o w e d by
expected
a p p l i c a t i o n s were made.
could
to
be
c e r t a i n circumstances,
Pacific
The
to
i f such
evaluate
analysis
first
o f checks
little
q u e s t i o n as
listed
i n chapter
should
such
u s e f u l and
than
attempt
to the
an a n a l y s i s
powerful
tool
be
once e v e r y
source
performed
two
of data
perform
and
be
due
to Canadian
applications
technique
a l r e a d y been completed.
not
be
performed?
man
here.
was
There
a n a l y s i s and
repeated
for closer
an
Is
the b e n e f i t s
How
frequently
B e c a u s e i t s e r v e s as
a
c o n t r o l , y e t r e q u i r e s no
t o c o m p l e t e and
more
analyze, i t
intervals
at
least
T h e n a g a i n , b e c a u s e o f i t s v a l u e as
a guide
and
t o p r o b l e m a r e a s , i t may
be
a
advisable
i t much more f r e q u e n t l y .
At t h e
the technique
during
o f the
at a r e g u l a r or i r r e g u l a r
months.
However,
each.
value of such
six will
form
a p p l i c a t i o n s were n o t a d v i s a b l e .
t o make use
t h a t had
to
the b e n e f i t s which
i s t o examine t h e s e
ten hours f o r a t r a i n e d
should
to
chapter
the u s e f u l n e s s of
The
the d e t a i l e d
some o f w h i c h w e r e p e c u l i a r
A i r L i n e s , some o f t h e s e
purpose of t h i s
VI,
p a t h method
aircraft
these
I n c h a p t e r s V and
made
as
the check.
beginning
of the
study,
the b a s i s o f a l l the
i t seemed w i s e t o
scheduling both
use
before
T h i s w o u l d have r e q u i r e d e x t e n s i v e use
and
of a
computer.
For many reasons t h i s p a r t i c u l a r a p p l i c a t i o n proved to
be i n a d v i s a b l e .
F i r s t of a l l , a computer was
the maintenance base.
not a v a i l a b l e at
Because of t h i s , even i f the data
be c o l l e c t e d and prepared
could
f o r the computer i n a minimum of time,
say one-half hour, the planner would r e c e i v e a r u n - o f f c o n t a i n i n g
i n f o r m a t i o n t h a t was
at l e a s t three hours o l d .
When the
majority
o f the checks are o n l y f o u r t y to f i f t y hours l o n g , d e c i s i o n s
must not be delayed
f o r three hours.
However, a computer c o u l d be provided
on the maintenance
base r e d u c i n g t h i s time l a g to one-half hour, which i s an
able time.
accept-
T h i s would be p o s s i b l e o n l y i f the department under-
went e x t e n s i v e changes i n work o r g a n i z a t i o n , and was
willing
bear the c o s t of the a d d i t i o n a l data p r o c e s s i n g i n v o l v e d .
was
deemed i n a d v i s a b l e at the time the study was
because Canadian P a c i f i c A i r L i n e s was
the plannet
carried
to
This
out
at t h a t time i n t r o d u c i n g
approach to the problem of reducing the elapsed
time.
N e i t h e r the c o s t , nor the presence o f plannet would
have prevented
however, had
two
the i n t r o d u c t i o n o f the c r i t i c a l path
p r e l i m i n a r y experimentation
discoveries.
One
was
reason
f o r a n a l y z i n g these
path p r i n c i p l e s .
of the
jobs
a t o t a l f l o a t of e i g h t hours
or more (see Table VI and F i g u r e 3 ) .
little
and a n a l y s i s not l e d to
t h a t e i g h t y - n i n e percent
performed on the average check had
technique,
T h i s meant that there
jobs i n the l i g h t of
Rather, f o r these
was
critical
j o b s , a method of assembly-
66
Table VI
Breakdown o f a l l Work on Check
Aircraft
#25,
#603, A c c o r d i n g t o C r i t i c a l n e s s
No. of Jobs
% of T o t a l
33
4
34
4
22
3
Jobs w i t h 8 to 16 hours f l o a t
159
19
Jobs w i t h 16 to 24 hours f l o a t
346
44
Jobs w i t h f l o a t over 24 hours
231
26
Critical
jobs (no f l o a t )
Jobs w i t h l e s s
than 4 hours f l o a t
Jobs w i t h 4 t o 8 hours f l o a t
Total
825 Jobs
100%
l i n e b a l a n c i n g would be more e f f i c i e n t .
This conclusion i s ,
f u r t h e r s u b s t a n t i a t e d by the f a c t t h a t f o r c e r t a i n checks, the
c r i t i c a l path i s so short t h a t i t i s l a c k of manpower alone whihh
p r o h i b i t s an e a r l i e r check
The
second d i s c o v e r y was
work of the same one
and was
completion.
causing
or two
that i t was
groups which was
the d e l a y s .
frequently
the
the c r i t i c a l work
T h i s meant t h a t i t was
a detailed
c r i t i c a l path a n a l y s i s of the work^ of these problem groups t h a t
was
r e q u i r e d , not an a n a l y s i s of the whole check.- "
Since
1
work i n these groups was
not
great, t h i s analysis could
the
be
performed manually.
The
g e n e r a l c o n c l u s i o n i s , t h e r e f o r e , t h a t when the
c o n d i t i o n s are s i m i l a r to those that e x i s t e d at Canadian P a c i f i c
Air
L i n e s , a c o n t i n u a l use o f a computerized c r i t i c a l
a n a l y s i s of each check as i t progresses
but may
be very w a s t e f u l .
other techniques
path
i s not only unwarranted
However, these c r i t i c a l p a t h and
w i l l be used to reduce the elapsed
time and
as
t h i s happens, a l a r g e r percentage of the jobs w i l l become
c r i t i c a l or near c r i t i c a l as w i l l more groups.
The
result
be an i n c r e a s i n g need f o r the computerized "in-check"
as the elapsed
time of the check decreases.
analysis
Thus i t i s to be
expected t h a t the computer w i l l see i n c r e a s e d use i n t h i s
analysis i n future
1. See,
will
years.
Computer Speeds Jet Through Overhaul, op. c i t .
The
technique w i l l f i n d
i t s most frequent
i n the a n a l y s i s of v a r i o u s segments of the cheek.
need a r i s e s f o r a reduced
of
application
Whenever the
elapsed time, be i t of a j o b , a c h a i n
j o b s , or the check as a whole, the technique
I t w i l l be used b e f o r e , d u r i n g , and
should be used.
a f t e r a check as the b a s i c
framework f o r the a n a l y s i s of t r o u b l e s p o t s .
Most f r e q u e n t l y the work w i l l
important
i n v o l v e c h a r t i n g the
elements of the network on paper.
On c e r t a i n i n s t a n c e s
the c h a r t i n g of the r e l a t i o n s h i p s w i l l be a mental process
only.
In other c a s e s , where p a r t i c u l a r segments of the check are cont i n u a l l y p r e s e n t i n g the planner w i t h problems, a c r i t i c a l
board may
be made use o f .
T h i s w i l l have the appearance of a
c h a r t but w i l l have the advantage that the j o b s , t h e i r
r e l a t i o n s h i p s , and
path
t h e i r d u r a t i o n s may
moving the job cards on the board.
inter-
be c o n t i n u a l l y updated by
Because of the complexity
the job i n t e r - r e l a t i o n s h i p s , such a board
of
i s only o f value i n
the a n a l y s i s of a f a i r l y s m a l l number of jobs such as those i n
one
group, one area of the plane, e t c .
As was
s t a t e d i n chapter I, the f o u r t h attempt was
to
2
apply the PERT and
the Man-Scheduling programs. *
u s e f u l n e s s of PERT was
d e s c r i b e d i n chapter s i x .
u l i n g program i s designed
to schedule
The
limited
The Man-Sched-
each job i n such a manner
2. I n t e r n a t i o n a l Business Machines, PERT, op c i t . , and I n t e r n a t i o n a l Business Machines, Man-Scheduling Program, op. c i t .
t
69
that the work load f o r each workman i s balanced, y e t each job i s
completed
before i t s t o t a l f l o a t expires.
i m p o s s i b l e to apply t h i s program because
of
I t was found to be
the storage c a p a c i t i e s
the computers a v a i l a b l e were too s m a l l , and the computer time
r e q u i r e d proved t o be too c o s t l y .
The study performed
a t Canadian P a c i f i c A i r L i n e s showed
that the c r i t i c a l path technique had, a t the time o f the study,
a wide a p p l i c a b i l i t y
analytical
tool.
i n a i r c r a f t maintenance p a r t i c u l a r l y
This a p p l i c a b i l i t y
as an
i s not o f a temporal nature
but should be made use o f i n c r e a s i n g l y u n t i l the p o i n t i s
reached when a l l the o r g a n i z a t i o n
will
and s c h e d u l i n g o f the check
be based upon and monitored by the c r i t i c a l path technique.
72
BIBLIOGRAPHY
1. A i r T r a n s p o r t , Moody's T r a n s p o r t a t i o n Manual,, 1954, pp. A 6 0 A65;
I960, pp. A66-A71; and 1961, pp. A67-A71.
2. A l t s c h o e l , S., Why
the S t r e t c h In J e t D e p r e c i a t i o n ?
World A i r T r a n s p o r t a t i o n . Feb.. 1963,
3.
Bossange, E.R.,
p.
Airlift:
35.
J r . , Maintenance Dept., Overseas D i v i s i o n , Pan
American World Airways, I n c . , " P l a n n e t " (Planning Network), A
paper presented at the 1962 E n g i n e e r i n g and Maintenance Conf e r e n c e , A i r Transport A s s o c i a t i o n , October 24,
1962.
4. Canadian P a c i f i c Railway Company, Annual Report, y e a r s ,
1950-
1964.
5.
"Computer Speeds J e t Through Overhaul", A i r l i f t
T r a n s p o r t a t i o n . V o l . 26, January,
6.
E i s n e r , H.,
1963.
A G e n e r a l i z e d Network Approach to the P l a n n i n g and
Scheduling o f a Research P r o j e c t .
Feb.,
and World A i r
Operations Research. Jan.-
1962, V o l . 10, #1, pp. 115-130.
7. The F i n a n c i a l Post Survey o f Corporate S e c u r i t i e s . The Maclean
P u b l i s h i n g Company L i m i t e d , Toronto.
8. The F i n a n c i a l Post Survey o f I n d u s t r i a l s . The
Maclean-Hunter
P u b l i s h i n g Company L i m i t e d , Toronto.
9.
F o r d , L.R. J r . , and R.D.
F u l k e r s o n , Flows i n Networks.
Princeton University Press, Princeton,
10. Freeman, R.,
1962.
A G e n e r a l i z e d Network Approach to P r o j e c t
Activity
Sequencing, I n s t i t u t e of Radio E n g i n e e r s . T r a n s a c t i o n s on
E n g i n e e r i n g Management. EM-7,
11. F u l k e r s o n , D.R.,
Sept., i 9 6 0 .
A Network Flow Computation f o r P r o j e c t Cost
Curves, Rand Paper P - 1947, March 18, i 9 6 0 , Management
S c i e n c e . 7,
No.
2.
12. G e n e r a l E l e c t r i c , Computer Department, Phoenix,
Critical-Path
13. Grubbs, F.E.,
Method, An A p p l i c a t i o n to the G.E.
Attempts
Arizona
2 2 5 Computer.
to Volidate Certain Pert S t a t i s t i c s ,
Operations Research. Nov.-Dec, 1 9 6 2 , V o l . 10, # 6 ,
L e t t e r s to
the E d i t o r , pp. 912-914-.
14. I n t e r n a t i o n a l Business Machines, PERT (Programme E v a l u a t i o n
and Review Technique), General I n f o r m a t i o n Manual,
15.
I n t e r n a t i o n a l Business Machines, Man-Scheduling
the IBM,
1 6 2 0 , 1 6 2 0 General Programme L i b r a r y ,
E20-8067.
Programme f o r
10.3.013.
16. I n t e r n a t i o n a l Business Machines, MISS LESS, 1 6 2 0 General
Programme L i b r a r y , 10-J-011-C, 1 0 . 3 . 0 1 2 . T .
17. J e t U t i l i z a t i o n - Cost C o n t r o l Increase C o n t i n e n t a l ' s P r o f i t s ,
August 21, 1 9 6 1 ,
A v i a t i o n Week and Space Technology.
18. K e l l y , J . E . J r . , and M.R.
pp.36-39.
Walker, " C r i t i c a l Path P l a n n i n g and
S c h e d u l i n g , " Proceedings E a s t e r n J o i n t Computer
160-173, Boston, December 1 - 3 ,
Conference
1959.
19. K e l l y , J . E . J r . , " C r i t i c a l Path P l a n n i n g and S c h e d u l i n g :
Mathematical B a s i s , " Operations Research. May-June,
20. McGregor, G.R.,
F a r e s - C a p a c i t y - F l i g h t , A i r l i f t : World A i r
T r a n s p o r t a t i o n . May,
21. Malcolm, D.G.,
1 9 6 3 , pp. 24-28.
J.H. Roseboom, C.E.
A p p l i c a t i o n o f a Technique
C l a r k and
f o r Research
Frazer,
and Development
Program E v a l u a t i o n , Operations Research. V o l . 7 ,
pp.
1961.
1959,
646-669.
22. M o r r i s o n , W.R.,
and Raymond,E., A i r l i n e Management o f
Overhaul
Processes With C r i t i c a l Path Methods, U n i t e d A i r L i n e s .
23.
PERT Guide f o r Management Use, PERT C o - o r d i n a t i n g Group,
U n i t e d S t a t e s Department
o f Defence, O f f i c e S e c r e t a r y o f
Defense, June, 1963.
24. Pocock, J.W., PERT as an A n a l y t i c a l A i d f o r Program P l a n n i n g
- I t s P a y o f f and Problems, Operations Research. V o l . 10, #6,
Nov.-Dec, 1962, pp. 893-897.
25.
RAMPS$ Resources A l l o c a t i o n and M u l t i - P r o j e c t S c h e d u l i n g ,
S t e e l . August 6. 1962.
26.
Sauer, Ray N., LESS - Least Cost E s t i m a t i n g and Scheduling,
IBM Program L i b r a r y ,
27.
10.3.003.
Vandyke, A., The World's A i r l i n e s , A i r l i f t ; World A i r
T r a n s p o r t a t i o n . May 1963, p. 21.
Download
Related flashcards
Management

42 Cards

Create flashcards