Stability Distribution

Stability Distribution

Stability Distribution




Photo courtesy of PAUL BOWEN PHOTOGRAPHY.

One of the unique aspects of flying twin-engine and turbine airplanes is the wide range of operating weights. Even more important, utilizing more or less of this loading capability means there is potentially a much wider range of centers of gravity compared to smaller aircraft. Changes in CG location create differences in control feel, airplane performance and handling – all reflecting changes in airplane stability. This aspect of flying larger airplanes is not always explored in-depth during transition and recurrent training. Let’s look at the differences in CG-related stability distribution.

Relative Characteristics

An airplane loaded toward the forward edge of its weight and balance envelope, compared to an airplane loaded further aft, will:

  • be very stable in pitch;
  • tend to fly at a lower angle of attack, developing less lift;
  • have higher breakout forces, requiring more control force to change its pitch attitude.

Even within the approved envelope, compared to a more forward CG condition for any given indicated airspeed an aft-loaded airplane will: 

  • tend to pitch upward more; 
  • tend to fly at a higher angle of attack, unless the pilot resists the pitch-up; 
  • be less stable in pitch (tend to nose up and down, and not maintain a given attitude); 
  • have lower breakout forces. It will take less force on the elevator to change the pitch attitude, or conversely, respond more dramatically to the same amount of control input.

The FAA’s Pilot’s Handbook of Aero–nautical Knowledge, page 5-43, tells us:

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 the 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.

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 that drives the gross weight upward are usually added to the aft part of the cabin, moving the center of gravity toward the aft end of the weight and balance envelope. In some airplanes, the CG moves rearward as fuel is burned; in some types, the CG change is negligible, while in others the CG may actually move forward under some conditions. This is the sort of type-specific knowledge you should have learned when you transitioned into the airplane. If you don’t know the characteristics of the airplane you fly, now’s the time to figure it out.

Taking Off

Center of gravity distribution can have a significant impact on takeoff handling and performance. If the CG is forward, it will take more control force to establish the liftoff and initial climb attitudes. Give the controls the input you’d do at farther aft loads, and the airplane won’t climb as it should. The increase in control deflection necessary to overcome the breakout forces, and the higher angle of attack for a given pitch attitude that results, increase drag, increase the ground roll distance and reduce the initial climb rate.

If the CG is toward the rear of the envelope, the controls will be lighter. The airplane will tend to over-rotate and pitch up excessively, increasing angle of attack and making a takeoff stall more likely. The airplane will lift off sooner and climb faster, but only if the pilot puts more subtle effort into flying the proper pitch attitudes.

Photo courtesy of Pilatus Aircraft

Landing

When landing, the forward-loaded airplane will require more control force to flare. Fail to give it the input it needs, and you risk landing hard on the nosewheel, making directional control on the runway difficult and possibly overloading the gear to the point of failure.

The more critical – and typical – situation is when the airplane is 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:

  1. 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.
  2. The stall occurs high enough above the ground that the nose drops far enough the nose gear hits the ground first. The nose gear collapses, and the pilot may or may not lose directional control.
  3. Either attempting to correct for the stall or after initial impact with the ground, the pilot enters a PIO (Pilot-Induced Oscillation) that is exacerbated by the stability effects of aft CG and quickly increases in amplitude until the nose gear collapses or the pilot loses directional control. 

To be sure, wing and power loading have an impact on airplane stability, performance, and control. But within the weight range even of twin and turbine-powered airplanes, the changes that result from the center of gravity location are even greater.

Stability Training

Most flight training occurs with two persons on board and somewhere between half-full and full fuel tanks. Unless you’re in a much smaller airplane, this is a fairly lightweight and the center of gravity is near the forward edge of the envelope. Consequently, your experience with takeoffs, landings, go-arounds, stalls and other high-angle of attack maneuvers is usually under the best conditions of stability and handling your aircraft can provide. If the airplane has a high useful load or a wide center of gravity range, the airplane’s stability and control response in common accident scenarios may be very different than what you encounter in training. 

Very frequently hard landings and airport-environment Loss of Control – In-flight (LOC-I) involve a heavily loaded airplane 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) takeoffs and landings in this condition. If your airplane is one that has a large rearward movement of the center of gravity with fuel burn, you might also practice these maneuvers at lighter weights but with the CG near the aft limit.

Performing this exercise does three things for you:

  1. It refreshes you on the process of computing aircraft weight and balance, whether manually or by use of a loading app or other automated system. 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.
  2. You’ll gain an appreciation for the changes in airplane stability, performance and handling across its entire range of approved loading. 
  3. You’ll be better practiced and ready for a high-performance takeoff and landing, go-arounds and stall recoveries in conditions more typical of the way you 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 takeoff and landing crashes, 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 training event. One of the greatest capabilities of Flight Training Devices and flight simulators is their ability to mimic the airplane’s performance at varying weights and center of gravity locations. If you’re not augmenting your flight training with simulation, you’re missing a great opportunity. If you do attend simulator training, ask your instructor to let you experience normal and emergency scenarios at a wide range of weights and CG locations.

Think of changes in center of gravity location as the airplane’s stability distribution. Learn and consider the characteristics of the airplanes you fly across their entire CG range.

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