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Introduction
First introduced on the series 200 in Feb 1979 as the Performance Data Computer System (PDCS), the Flight
Management Computer (FMC) was a huge technological step forward. Smiths Industries (formerly Lear Seigler)
has supplied all FMCs installed on the 737.
The PDCS was developed jointly by Boeing and Lear Seigler in the late 1970's. It enabled EPR and ASI bugs to be
set by the computer and advise on the optimum flight level, all for best fuel economy. It was trialed on two inservice aircraft, a Continental 727-200 and a Lufthansa 737-200 for nine months in 1978 with regular line crews
and a flight data observer. The 737-200 showed average fuel savings of 2.95% with a 2 minute increase in trip time
over an average 71 minute flight. The 727 gave a 3.94% fuel saving because of its longer sector lengths. The
PDCS quickly became standard fit and many were also retrofitted. By 1982 the autothrottle had been devised and
thrust levers could be automatically driven to the values specified by the PDCS.
The true FMC was introduced with the 737-300 in 1984 this kept the performance database and functions but also
added a navigation database which interacts with the autopilot & flight director, autothrottle and IRSs. The
integrated system is known as the Flight Management System (FMS) of which the FMC is just one component.
Most aircraft have just one FMC, but there is an option to have two this is usually only taken by operators into
MNPS airspace eg Oceanic areas. The FMS can be defined as being capable of four dimensional area navigation
(latitude, longitude, altitude & time) while optimising performance to achieve the most economical flight possible.
The photograph above is of the Control Display Unit (CDU), which is the pilot interface to the FMC. There are
normally 2 CDUs but only one FMC. Think of it as having two keyboards connected to the one PC. The CDU in the
photograph has a DIR INTC key at the beginning of the second row but some have a MENU key. This key gives
access to the subsystems such as FMC, ACARS, DFDMU, etc.
In its most basic form, the FMC has a 96k word navigation database, where one word is two bytes (ie a 16 bit
processor). This was increased to 192k words in 1988, 288k in 1990, 1 million in 1992 and is now at 4 Mega words
for the 737-NG with Update 10.7. The navigation database is used to store route information which the autopilot will
fly when in LNAV mode. When given data such as ZFW & MACTOW, it takes inputs from the fuel summation unit to
give a gross weight and best speeds for climb, cruise, descent, holding, approach, driftdown etc. These speeds can
all be flown directly by the autopilot & autothrottle in VNAV mode. It will also compute the aircrafts position based
upon inputs from the IRSs, GPS and radio position updating.
The latest FMC – Model 2907C1, has a Motorola 68040 processor running at 60MHz (30Mhz bus clock speed),
with 4Mb static RAM and 32Mb for program & database.
In 2023 the CDU was changed to a touchscreen CDU. More details here.
FMC Databases
An FMC has three databases: Software options (OP PROGRAM), Model/Engine data base (MEDB) and Navigation
data base (NDB), all of which are stored on an EEPROM memory card. These databases can all be updated via
the data loader.
The Software options database includes the operational program and its update, plus any company specific
differences. For a full list of all FMC software updates and their features, please refer to the book.
The MEDB holds all the performance data for V speeds, min & max speeds in climb, cruise & descent, fuel
consumptions, altitude capability etc.
The NDB is comprised of Permanent, Supplemental (SUPP) and Temporary (REF). The Permanent database
cannot be modified by crew. There are four types of data: Waypoint, Navaid, Airport and Runway. Runway data is
only held in the permanent database.
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There is capacity in the SUPP and REF databases for up to 40 waypoints, 40 navaids and 6 airports. SUPP data
can only be entered on the ground. It is then stored indefinitely but crew may delete individual data or the whole
database. Any existing SUPP data should be checked for accuracy before flight using the SUMMARY option (U6+
only) or DELeted and re-entered, cross-checking any Lat & Longs between both crew members. All Temporary
(REF) data is automatically deleted after flight completion.
When entering navaids into either the REF NAV DATA or SUPP NAV DATA database, you will be box prompted for
a four letter “CLASS” classification code. The following table should be used:
Navaid Classification Codes
Box Numbers
VHF Navaids
1
VOR
V
2
TACAN Ch 17-59, 70-117
T
MILITARY TACAN Ch 1-16, 60-69
M
DME
D
ILS/DME
I
3
Terminal
T
Low Altitude
L
High Altitude
H
Use unrestricted by range or altitude
U
4
Scheduled Weather Broadcast
B
No Voice on Navaid Frequency
W
Automatic Trans Weather Broadcast
A
Eg. To create a new en-route VOR/DME, the Class code would be VDHW.
FMC Pages
Green: CRT, White: LCD
Pre-Flight Preparation Pages
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The contents page. Pages required for the departure
are listed on the left hand side.
This INIT/REF INDEX page (shown right) is from an
U6.2 aircraft equipped with ACARS and IRS navigation.
Selecting MSG RECALL allows the recall of deleted
CDU scratchpad messages whose set logic is still valid.
ALTN DEST allows the entry of selected alternate
airports.
The NG (U7+) has some extra options available.
The OFFSET function is used in-flight to fly parallel to a
portion of the route. This may be used to avoid the
turbulent wake of the aircraft ahead or to increase
separation with opposite direction traffic.
5R will offer MAINT when on the ground, see below for
some of these pages.
The contents page.
IDENT: Check aircraft model and engine rating is what
you are flying, especially if your airline operates a mixed
fleet. Note that if a model is appended “.2” it has the
SFP package and two position tailskid. Also check the
database effectivity and expiry dates. Software update
number is given in brackets, here U10.5A. U10 was
specifically designed for the 737-NG but can be used on
classics.
The prompt at 6R leads onto POS INIT
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POS INIT is used to enter the aircraft position into the
IRS's for alignment. The Lat & Longs of REF
AIRPORTS & GATES are held in the database and do
not need typing in but should be cross-checked against
published data.
Routes are usually entered by inputting the CO ROUTE.
Eg AMSLPLRPL = Amsterdam to Liverpool Repetitive
Flight Plan. If the company route is not recognised the
route can be entered manually by filling in the VIA & TO
columns.
In-flight the OFFSET option will be available on U7
onwards. See below to use this to access the hidden
NEAREST AIRPORTS and ALTERNATE DESTS pages.
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After the route is entered, press the DEP ARR button on
the CDU to access the departures and arrivals for the
ends of the route.
This is a typical arrivals page and allows selection of the
required SID, Transition and approach. These are
denoted by <ACT> when they have been selected.
The PERF INIT page is completed before fight and
gives the FMC the data to calculate leg times & fuels
and the optimum & max altitudes.
Cost Index is the ratio of the time-related operating
costs of the aircraft vs. the cost of fuel. If CI is 0 the
FMC gives maximum range airspeed and minimum trip
fuel. If CI is max the FMC gives Vmo/Mmo for climb &
cruise; descent is restricted to 330kts to give an
overspeed margin. The range of CI is 0-200 (Classics)
and 0-500 (NGs).
CRZ WIND is actually used for both climb wind and
cruise wind. If you enter the forecast top-of-climb wind
before departure the FMC will recalculate your climb
speed accordingly. When in the cruise if you enter the
average cruise wind, the time and fuel calculations will
be updated. If you do not enter anything here the FMC
will assume still air.
In-Flight Pages
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Standard take-off page for PF. Speeds & assumed
temperature must be entered manually from
performance tables. TO SHIFT should be entered if
departing from an intersection as this is used to update
the FMC position when TOGA is pressed at the start of
the take-off roll.
Notice that the reduced takeoff N1's (90.5/89.6) are
different for each engine. This is because when the
photo was taken only one pack (the right) was running.
Used for entry of de-rates, where allowed.
This is the N1 Limit page pre-U10. Provides thrust limit
and reduced climb thrust selection. This is usually
automatic but manual selections can be made here.
The most common use is either to select a reduced
climb thrust (1 or 2) after a full power take-off to reduce
engine wear or to delete the reduced climb thrust to get
a high rate of climb.
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One of the most useful pages in the Boeing FMC and
has just been updated to six pages in update 10.6. The
fix can be anything in the database ie airfield, beacon or
waypoint. An abeam point can be constructed (as
illustrated) or a radial or range circle can be displayed
on the EHSI. Surprisingly, there is no equivalent page
on the Airbus.
Climb Pages
Standard ECON CLB page. 302 (Kts IAS) is highlighted
indicating that this is the target speed, this will
automatically change over to mach (0.597 in this case)
during the climb. Other climb modes are available with
keys 5 & 6, L & R.
CLB-1 indicates that the autothrottle is commanding a
reduced thrust climb power (reduces N1 by approx 3% =
10% thrust reduction). CLB-2 is a reduction of a further
10% ie 20% total. The reduced climb thrust setting
gradually increases to full climb power by 15,000ft.
Selecting MAX RATE from a climb page will show this
page with the target speed not highlighted and the
ERASE option available until the EXEC button is
pressed. If EXEC is pressed you can return to ECON by
line selecting it.
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Selecting MAX ANGLE from a climb page will show this
page with the target speed not highlighted and the
ERASE option available until the EXEC button is
pressed. If EXEC is pressed you can return to ECON by
line selecting it.
Selecting ENG OUT on a climb page gives the following
choice of LT or RT engines.
When the LT or RT engine has been selected this page
shows your MAX ALT (climb or driftdown alt) TGT SPD
and max cont N1. This is a useful page to check if flying
over ground above 15,000ft. Remember the altitude
penalties for anti-ice.
The EXEC light will now be illuminated, if you press it
you will lose all VNAV info. Select ERASE to return to
the two engine CLB/CRZ pages.
Cruise Pages
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Standard PF in-flight cruise page. The target speed
(highlighted) is the ECON speed which is derived from
the cost index and winds.
The fuel at EGLL figure is blank because either it is
being recalculated or there is a route discontinuity.
Selecting ENG OUT on the cruise page will give this
page which asks you to select which engine is inop.
After selecting the appropriate engine, the FMC will
calculate your driftdown target speed and stabilisation
altitude. It is worthwhile making this check before
crossing over areas of high terrain. Do not press EXEC
at any stage otherwise you will lose VNAV information,
these actions can be undone by selecting ERASE.
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Selecting LRC gives Long Range Cruise speed. This is
calculated as 99% of the maximum range speed for a
given weight & altitude in still air conditions. It is used in
preference to MRC because it is a more stable speed
and hence gives less autothrottle movement.
LRC takes no account of winds so it may give a higher
fuel burn than ECON. It also takes no account of
operating costs; hence it has little practical value.
Descent Pages
FPA is the actual flight path angle of the aircraft, it is
zero in this example because the aircraft is in level
flight. It is typically 3 to 4 degrees in a descent.
V/B is the required vertical bearing to reach the WPT/
ALT ie TIGER/16000. This would be an altitude
restriction in the LEGS page that you could enter (or
delete) manually or with the ALT INTV button on the
MCP if fitted.
V/S is the required vertical speed to make good the V/B.
As you approach your ToD the FPA will remain at zero
(because the aircraft is in level flight), but the V/B and V/
S will both increase until the V/B is at about 3 to 4
degrees, or whatever the FMC has calculated is the
optimum value. If VNAV is engaged the aircraft will
descend with a FPA equal to the V/B. The actual V/S will
however be slightly different to the computed V/S
because V/S changes during the descent.
FMC position can be forced to any of the other positions
at this page.
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You can either select a waypoint from the legs page or
use PPOS (Present POSition) on which to base the
hold. The database will give the turn direction & inbound
course. Leg time will be that appropriate to your altitude
. Target speed will default to the best speed which will
be that for max endurance. Hold avail is calculated from
the present fuel, fuel flow and reserves figure entered
into the PERF INIT page.
Very rarely used. Sets min & max speed limits for climb,
cruise & descent.
Very useful howgozit page, but don't trust the fuel
summation unit! it always overreads by approx
1-200kgs. For an accurate arrival fuel subtract 2-300kgs
to allow for this and the drag of flying slower than the
optimum speed whilst on the approach.
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Very rarely used but can be used to try to reach a
waypoint at a specific time.
PROGRESS 3/3: Standard landing page for PF.
Especially useful for giving crosswind component and
SAT in icing conditions. Confusingly this page has now
been moved to 2/4 on U10.7, so instead of selecting
Progress-Prev Page you now have to select ProgressNext Page.
APPROACH REF: Standard landing page for PNF. Vref
is calculated from the current gross weight, pedants
may wish to overwrite the GROSS WT with the
predicted GW for landing.
Vref is "hardened" by line-selecting it over itself which
will cause it to be displayed on the speed tape.
IRS Navigation Pages (If installed)
The following pages are only available on ANS equipped aircraft.
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Similar to ACT RTE LEGS.
Similar to PROGRESS.
Similar to ACT RTE DATA.
Maintenance Pages
Caution do not access these pages without engineering supervision.
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Hidden Pages
These are various pages hidden away in most FMC's that are not always immediately available to the pilots, I
suspect because there would be a surcharge to their airlines to use them. There are some ingenious ways of
getting to these pages by careful use of simultaneous CDU entries.
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This is not a hidden page, but it is the way into them.
You can bring up this OFFSET page either from RTE or
INIT/REF INDEX.
1. Have this page displayed on both CDU's.
2. Enter any offset but do not execute.
3. Simultaneously press ERASE on both CDU's.
These steps will bring ALTERNATE DESTS. (See
below)
You can enter up to 5 alternates here, selecting 1R to
5R against any entered alternate will show the info
below...
All the diversion data is now shown based on you flying
direct to this alternate from present position (VIA
DIRECT). Selecting MISSED APP will show the same
data but calculated from the missed approach point.
Selecting nearest airports will give...
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This extremely useful page takes a couple of minutes to
calculate but will list the nearest airports in the database
in order of DTG.
Once again, line selection of 1R to 5R will give more
useful diversion information as shown below.
All the diversion data is now shown based on you flying
direct to this airport from present position (VIA DIRECT).
Selecting MISSED APP will show the same data but
calculated from the missed approach point.
Selecting INDEX to return to the normal pages.
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