CG limits and controllability margin
POH/AFM fwd/aft CG limits: max CG positions meeting longitudinal stability, set by certification flight testing. Outside them, handling is unsatisfactory or not investigated (TC AIM RAC 3.4.3).
| Limit | Set by |
|---|---|
| Aft | Spin recovery |
| Forward | Trim and power-off landing |
Aft limit (PHAK ch.5):
- Aft CG: less controllable, especially at slow speed. Stall recovery progressively harder.
- Far enough aft: flat spin. Centrifugal force through rearward CG pulls tail out from spin axis. Nose cannot be lowered to recover.
- Limit: most aft CG allowing normal spin recovery.
- Utility category (intentional spins): aft limit several inches forward of normal-category limit.
- TP 1102: careless aft loading, even within max gross weight, "can lead to very hazardous balance and control problems."
- Stall characteristics can change drastically.
- Spin recovery can be "extremely difficult if not impossible."
- Example: reduced fuel, light people up front, very heavy people in rear seats, aft baggage at max weight.
Forward limit (PHAK ch.5):
- Trim must hold a normal power-off glide.
- Conventional aircraft must make a full-stall power-off landing.
- Forward CG needs more back-elevator pressure; elevator control must stay adequate down to the stall.
Trap: aft CG is also a control problem: can make a spin unrecoverable.
References
- TC AIM RAC 3.4.3
- TP 1102 Weight and Balance, Centre of Gravity
- PHAK ch.5, Effect of Load Distribution