FLYING HEAVY ......
Acknowledgements: Thomas P. Turner (Mastery Flight Training Inc.)
A heavily loaded airplane is usually loaded further toward the aft end of the envelope than many pilots are used to. Passengers and baggage that make up the extra weight which drives the gross weight upward are usually added to the aft part of the cabin, moving the centre of gravity (CG) toward the aft end of the weight and balance envelope.
Even within the approved envelope, compared to a more forward CG condition, for any given indicated airspeed the aft-loaded airplane will:
tend to pitch upward more
fly at a higher angle of attack
be less stable in pitch (tend to nose up and down, and not maintain a given attitude)
take less force on the elevator to change the pitch attitude, or conversely, respond more dramatically to the same amount of control input
The rearward CG limit of an aircraft is determined largely by considerations of stability. The original airworthiness requirements for a type certificate specify that an aircraft in flight at a certain speed dampens out vertical displacement of the nose within a certain number of oscillations.
An aircraft loaded too far rearward may not do this. Instead, when the nose is momentarily pulled up, it may alternately climb and dive, becoming steeper with each oscillation. This instability is not only uncomfortable to occupants, but it could even become dangerous by making the aircraft unmanageable under certain conditions.
When entering a landing flare in an airplane loaded toward the aft end of its envelope, if the pilot applies the same amount of aft elevator control he/she is conditioned to add, the result will be a greater nose-up pitch and a higher angle of attack. The airplane will tend to flare high; it may stall and “drop in” for a hard landing.
Such a flare often results in one of three outcomes:
The airplane hits hard on the main landing gear, possibly blowing the tires or damaging the gear and causing the airplane to go out of control on the runway.
The stall occurs high enough above the ground that the nose drops far enough for the nose gear to hit the ground first. The nose gear collapses and the pilot may or may not lose directional control.
Either attempting to correct for the stall or after initial impact with the ground, the pilot enters a Pilot-Induced Oscillation - that is exacerbated by the stability effects of aft CG and quickly increases in amplitude until the nose gear collapses and/or the pilot loses directional control.
Most flight training occurs with two persons on board and somewhere between half-full and completely full fuel tanks. Unless you’re in a Cessna 152 or a J-3 Cub or similar, this is a fairly light airplane weight with the centre of gravity near the forward edge of the envelope.
Your experience with take-offs, landings, go-arounds, stalls and other high angle of attack manoeuvres, then, is usually under the best conditions of stability and handling your aircraft can provide. If the airplane has a high useful load capability and/or a wide centre of gravity range, the airplane’s stability and control response in common accident scenarios may be very different from those which you encounter in training.
Very frequently, hard landings and airport-environment Loss of Control – Inflight (LOC-I) involve a heavily-loaded airplane and/or one in which the CG is toward the aft end of the envelope. Your conditioned response to these scenarios, reinforced in practice and instruction, may not be adequate for recovery from performance excursions under these conditions.
Perhaps we should all carefully load our aircraft near the aft end of its CG envelope at the airplane’s maximum weight, and with an instructor experienced and current in that type of airplane go up and practice slow flight, stalls, go-arounds, and high performance (short- and soft-field) take-offs and landings in this condition. If your airplane is one that has a large rearward movement of the centre of gravity with fuel burn, you might also practice these manoeuvres at lighter weights but with the CG near the aft limit.
Performing this exercise does three things for you:
It refreshes you on the process of computing aircraft weight and balance. My experience is that very few pilots feel confident in making a loading calculation. This suggests that they rarely do so and are less likely to know when their airplane is loaded at the edges of the envelope - or even outside it
You’ll gain an appreciation for the changes in airplane stability, performance and handling across its entire range of approved loading
You’ll be better practised and ready for a high-performance take-off and landing, go-arounds and stall recoveries in conditions more typical of the way you may routinely fly your airplane … conditions less represented by the way you’ve been trained and evaluated on these skills
Ask your instructor to help you train for common LOC-I scenarios, and to avoid hard landings, by experiencing them at weights and load distributions more typical of how you fly the airplane. It might be a good focus of your next Flight Review or other training event.
FLY SAFE!