CFI Workshop 6
Core Topic 12
Airworthiness
Limitations
Where do they Really
come from?
Presented to: CFI Workshops
By: The FAASTeam
Date: January 1, 2012
Federal Aviation
Administration
You can’t beat the laws of Physics
• 1 June 2010, 1705 hrs, Anchorage, AK
• Pilot
– age 33
– Commercial, single-engine land & sea
– 1718 hours TT, 81 hours make & model
• Phase of flight
– Takeoff / climb out
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You can’t beat the laws of Physics
• Aircraft
–
–
–
–
–
–
Cessna 1976 U206F
Souls on board – 5
Maximum allowable take off weight - 3,600 Lbs.
Empty weight – 2165.5
Useful load – 1434.5
Fuel, occupants, & cargo weight – 2092.7
• Pilot’s estimate – 1,400 – 1,450 Lbs
– Takeoff weight – 4258.2
• 658 over max & 3.95 – 8.22 In. aft of cg limit
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You can’t beat the laws of Physics
http://dms.ntsb.gov/aviation/AccidentReports/v233vt4542baswfpmqymx
q451/R07052011120000.pdf
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Where do limitations Come From?
• Physics
– Example: The maximum rate of climb that an airplane is
capable of is governed by the forces on it. Wing area, power,
and thrust all influence the rate of climb.
– Violating limitations imposed by physics typically results in bent
metal.
• Regulation
– Establishes legal limitations based on the rules that the
airplane was certified under.
– Regulatory limitations are based on physics, but usually have a
safety factor added.
– Example: 23.65 says “Each normal, utility … must have a
minimum climb gradient of at least 8.3 % for land planes or 6.7
% for seaplanes…… “ (at maximum gross weight)
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We’ll discuss:
–
–
–
–
–
–
–
Weight and c.g. limitations
Landing and Take off performance
Stall Speed
Airspeed limitations
Power Plant limitations
How Floats affect limits
How Skis affect limits
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Airspeed Limits
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Examples of Airspeed Limits
-
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Examples of Airspeed Limits
Flaps Down Stall Speed
(at gross weight)
-
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Examples of Airspeed Limits
Flaps Down Stall Speed
(at gross weight)
Flaps Up Stall Speed
(at gross weight)
-
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Examples of Airspeed Limits
Flaps Down Stall Speed
(at gross weight)
Flaps Up Stall Speed
(at gross weight)
Vne, Never Exceed
-
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VNE
What’s the consequence of operating above
VNE?
A.
Catastrophic airframe failure
B.
Unknown & untested
C.
Irreversable airframe stress
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Flutter testing
Tail Flutter Test.mov
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Examples of Airspeed Limits
Flaps Down Stall Speed
(at gross weight)
Flaps Up Stall Speed
(at gross weight)
Vne, Never Exceed
-
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Examples of Airspeed Limits
Flaps Down Stall Speed
(at gross weight)
Flaps Up Stall Speed
(at gross weight)
Vne, Never Exceed
Vf, Max Flap Extension Speed
-
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Examples of Airspeed Limits
Flaps Down Stall Speed
(at gross weight)
Flaps Up Stall Speed
(at gross weight)
Vne, Never Exceed
Vf, Max Flap Extension Speed
Vc, cruise speed
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The Operating Envelope (V-n Diagram)
Stall Line
10% safety margin
n, g
Va, Maneuvering
Speed
3.8 g (Normal
category
1
Speed, V
o
Gust
Lines
Vd (Dive Speed)
Vc (bottom of Yellow
arc)
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Vne (red line)
Vne (red line)
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The Operating Envelope (V-n Diagram)
n, g
10% safety margin
3.8 g (Normal
category)
1
Speed, V
o
Vd (Dive Speed)
Vne (red line)
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The Operating Envelope (V-n Diagram)
n, g
10% safety margin
3.8 g (Normal
category
1
Speed, V
o
Gust
Lines
Vd (Dive Speed)
Vne (red line)
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The Operating Envelope (V-n Diagram)
Stall Line
n, g
10% safety margin
3.8 g (Normal
category
1
Speed, V
o
Gust
Lines
Vd (Dive Speed)
Vc (bottom of Yellow
arc)
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Vne (red line)
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The Operating Envelope (V-n Diagram)
Stall Line
10% safety margin
n, g
Va, Maneuvering
Speed
3.8 g (Normal
category
1
Speed, V
o
Gust
Lines
Vd (Dive Speed)
Vc (bottom of Yellow
arc)
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Vne (red line)
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Airspeed Limits
• Va is the design maneuvering airspeed at
which the airplane will be able to do a limit
maneuver without stalling. (3.8 g for normal
category airplanes)
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Airspeed Limits - True or False
1. The bottom of the Yellow arc is the airspeed above which the
airplane is at risk of damage from a 50 fps gust.
True and gusts in excess of 25 fps are common.
2. If the Air is turbulent, Slow down to below the yellow arc.
Also true. Operating in the yellow arc with any turbulence is very
stressful to the aircraft.
3. If an airplane has been flown in severe turbulence above VC,
additional inspection should be conducted.
That’s true damage associated with severe turbulence is
common.
4. The installation of larger engines makes it less likely that a
pilot will be able to fly well into the yellow arc.
False – Larger engines make it easier to fly too fast for
conditions
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Airspeed Limits - True or False
5. Vne is set by structural considerations as well as flutter.
That’s true. Vne is determined with respect to structural
considerations as well as flutter.
6. Flutter is very sensitive to slop in control systems and to the
balance of the control surfaces. The airplane is certified to Vd
which is 10% over Vne.
This is also true. A light coating of frost was enough to cause
aileron flutter on a CE – 210 in Virginia. The aileron was torn
from the airframe but luckily the pilot was able to land
successfully. If it had been tail flutter the outcome would
have been much worse.
7. The ASI on most GA aircraft is accurate enough to operate
right up to Vne.
Maybe true maybe false. It depends on the health of your
pitot/static system & ASI. The question is though – are you
willing to bet your life on it?
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Extra Credit
As gross weight decreases Va will:
A. Decrease
B. Remain the same
C. Increase
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Weight & Balance Limitations
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Center of Gravity
The forward C.G. limit is critical for:
A. Nose wheel strength
B. Ability to flare
C. Stall recovery
C. Tail strength
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Examples of Weight and Balance Limits
Typically based on
climb, strength
Nose Gear limits, ability to
flare, trim
(weight)
Horizontal Tail
Strength,
Ability to flare,
Nose Gear
Tail gear structural
limit, stick forces
going to zero, spin
resistance,
longitudinal
stability, can’t push
fwd on balked
landing
(Center of Gravity)
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Weight Limits – True or False
1. Maximum gross weight is selected early in the design of most
airplanes and the rest of the airplane is designed around that
number.
Yes – that’s true.
2. Exceeding maximum gross weight routinely can result in
fatigue problems.
You bet – exceeding max gross weight – even by a little bit will
result in fatigue problems. As the fleet ages we’re seeing
more of this.
3. Exceeding maximum gross weight results in lower climb rates
and can result in structural failure.
Well duh – of course we’re going to climb slower but the
insidious thing is the possibility of structural failure.
4. When exceeding Max Gross Wt. Stall speed goes up,
controllability can be reduced, ability to maneuver without
entering an accelerated stall can be reduced.
Yes this is all true when you exceed weight limits.
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Weight Limits – True or False
5. Structural limits have a 1.5 margin of safety built into them for
unexpected conditions, and to minimize the chances of
having fatigue problems, not because you really wanted to
carry that much stuff.
See the second statement above (Exceeding maximum gross
weight routinely can result in fatigue problems). The safety
margin is there for a reason and the reason is not so you can
overload by 50%.
6. Contrary to rumors, airplanes are not generally capable of
taking a lot more than the required loads. (In many if not most
cases, the existing gross weight limit is set because of a
failure in the static test program.
This is sobering. In many cases the max gross weight limit was
set because the airframe came apart in static testing.
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Forward c.g. limit
• The forward center of gravity limit (and the
angled limit if present) are typically critical
for:
– Ability to flare during landing.
– Ability of the horizontal tail to take the structural
loads.
– Nose gear loads.
• The installation of heavier engines often
makes airplanes nose heavy and subject to
violating the forward limit.
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Aft C.G. Limit
• The aft center of gravity is usually critical
for:
–
–
–
–
–
–
Spin recovery
Stick forces
Balked landing
Longitudinal and directional stability
Nose down trim
Tail Wheel Loads
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Takeoff Performance
• Takeoff performance numbers are
generated by an experienced flight test pilot
with a lot of time in the airplane simulating
an average pilot with a new engine. They
are often optimistic with respect to what can
be expected in the field.
• There is no Margin of Safety incorporated
into the published takeoff numbers!
• AOPA recommends that pilots add 50% to
published takeoff distances.
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Can I get out of that strip with the moose???
Piano or other heavy object ……….
• Don’t Fly above Gross Weight!!
• Don’t guess – weigh it!
Al Hikes Photo
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Can I get out of that strip with the moose???
• Example: PA-18-150 with stock prop
– Flight manual says that the take off run is 200 ft (500 over 50’
obstacle) at 1750 lb.
– What is the take off distance at 2000 lb? (I assume you have the
one ton STC…..)
2
 2000 

  1.3
 1750 
200 feet x 1.3  260 feet
260 x 1.5 AOPA safety factor  390 feet to get 250 lb of moose out
Weight
Ground Run 50’ Obstacle
1750
200
500
2000
260
650
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Not including
AOPA 1.5 safety
factor
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Can I get out of that strip with the moose???
Weight
Ground Run 50’ Obstacle
1.5 Safety
Factor
1750
200
500
300/750
2000
260
650
390/975
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Take off wind issues
• Head winds decrease takeoff distances. For
a head wind of 10 % of the take off speed,
the take off distance will be reduced 19%.
(Roughly)
• A tail wind of 10 % of the take off speed will
increase your take off distance by 21%.
• A cross wind will increase your take off
distance. (More drag from control surfaces
and even a direct cross wind has a
headwind component in the crab)
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Tail Wind Example
• C-172 sea level 20 deg C short field, hard
surface ground roll 980 ft. 51 knot lift off
speed. Consider a 5 kt tail wind (10% of lift
off speed) 980 x 1.21 = 1186 ft.
• Cessna handbook calculation is 10% for
every 2 knots for the 172. That results in a
distance of 1225 ft. A little more
conservative than the Axioms of flight
estimate.
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Crosswind
The maximum demonstrated cross wind component is?
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Crosswind
The maximum demonstrated cross wind component is:
The highest cross wind component demonstrated during flight
testing.
– The ability to handle a cross wind is highly dependent on pilot
and runway conditions. (Especially in gusty conditions)
– There is a point at which the airplane runs out of available
aileron and/or rudder deflection.
– When the controls are at their stops, pilot ability no longer
matters.
– 14 CFR part 23.233 requires that all airplanes be able to land
in a cross wind up to .2 times flaps up stall speed.
– For a C-172 the minimum required is 44 kts x .2 = 8.8 knots
(The 172 exceeds the minimum required)
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Discussion:
• What minimums do you set for your
students?
• How do you teach them to evaluate their
performance and adjust personal minimums
to reflect their ability?
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Alaskan Off-airport Operations Guide
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My Short Field Performance
Aircraft ___________ Gross Weight ___________ Test Weight_________
Airfield ___________ Elevation ___________ Density Altitude ________
Wind Direction _______ Wind Speed _______ X Wind Component ______
Indicated Approach Speed ___________ Flap Setting ____________
Landing Distance _____________
Takeoff Flap Setting __________ Rotation Speed __________
Rotation Speed x .70 __________ Vx __________ Vy __________
Distance to Rotation __________ Distance to 50 feet AGL ___________
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Engine Limitations
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Engine Limitations
• RPM
– Engine RPM limits are established to ensure that the engine will
probably make TBO without catastrophic failure (Wear out
before fracture)
– Some flat pitch propellers are capable of exceeding the engine
red line rpm during takeoff or climb. Allowing this to occur
routinely can dramatically reduce the life of the engine or lead to
premature catastrophic engine failure.
– Yellow arc on Tachometer and “avoid continuous operation”
ranges are usually present because of a vibration problem in the
propeller engine combination. Poor TAC calibration can result in
inadvertent operation in these ranges resulting in propeller
failure or crankshaft failure.
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Engine Limitations
Temperature
– Temperature limits are established to avoid break
down of oil, excessive heat damage of internal parts
(like pistons) or cracking due to thermal stresses.
– There are often telltales on the engine that will
indicate that an engine has been over temped.
– While low temperature limits are not usually
established, operating at low oil temperatures can
result in poor oil flow through oil coolers, water
contamination in the oil and resulting internal
corrosion.
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Some notes on Professionalism
• Walk the talk.
• Don’t let your students see you do anything
you don’t want them to do in a week or so.
• Have your students brief on limitations
before flight – don’t just hop in and go.
• If it’s not important to you it’s
not important to your
students.
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• Questions?
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QUIZ
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Question 1
Flying above the red line is permissable:
A. As long as the 10% margin of safety
is not exceeded.
B. Turbulence is no greater than
moderate
C.
Neither A nor B
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Question 2
Flying within the yellow arc is permissable as
long as:
A. All control surfaces have been
balanced.
B. Turbulence is no greater than
moderate
C.
No turbulence is present
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Question 3
A forward center of gravity will:
A.
Compromise stall recovery.
B.
Lighten pitch control forces
C. Place greater stress on the nose
wheel.
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Question 4
The aft C.G. limit is critical for:
A. Tail wheel strength
B. Spin recovery
C. Nose wheel strength
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Question 5
Exceeding the max gross weight limit will:
A. Improve takeoff and climb
performance
B. Cause undue stress to the aircraft
C. Cause fatigue problems
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NOW THE
ANSWERS
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Question 1
Flying above the red line is OK:
A. As long as the 10% margin of safety
is not exceeded.
B. Turbulence is no greater than
moderate
C.
Neither A nor B
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Question 2
Flying within the yellow arc is OK as long as:
A. All control surfaces have been
balanced.
B. Turbulence is no greater than
moderate
C.
No turbulence is present
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Question 3
A forward center of gravity will:
A.
Compromise stall recovery.
B.
Lighten pitch control forces
C. Place greater stress on the nose
wheel.
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Question 4
The aft C.G. limit is critical for:
A. Tail wheel strength
B. Spin recovery
C. Nose wheel strength
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Question 5
Exceeding the max gross weight limit will:
A. Improve takeoff and climb
performance
B. Cause undue stress to the aircraft
C. Cause fatigue problems
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END OF CFI WORKSHOP MODULE 6
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