Aircraft weight and balance profiles

The 180 hp, four-seat Skyhawk SP. With four 170 lb adults aboard, the 2,550 lb maximum weight - not the aft CG limit - is what limits the fuel: full tanks will not fit. The 47.3 in aft limit comes into play when the load sits far back - a full rear seat and a lot of baggage, most of all in baggage area 2. It is approved for spins only in the utility category, which is a different envelope and a different maximum weight from the one most pilots calculate.

The 160 hp Skyhawk of 1977 to 1980, certificated to 2,300 lb - 250 lb less than the 172S, on the same datum, seat and baggage arms. With four adults aboard, the weight limit - not the CG - is what limits the fuel: on the handbook's sample airframe about two thirds of the standard tanks fit. It is approved for spins only in the utility category, at 2,000 lb with the rear seat and baggage empty.

A two-seat trainer with a genuinely small useful load, and the one type where the FORWARD CG limit is the one that catches people out: two heavy occupants with the seats run forward and the tanks low can put the CG ahead of the limit even though the aeroplane is nowhere near its 1,670 lb maximum. The 152 is certificated in the utility category only - there is no normal-category envelope.

Piper’s four-seat trainer, and geometrically a very different aeroplane from a Cessna: the datum sits 78.4 in ahead of the wing, so every arm is a number in the eighties and nineties rather than the thirties and forties. Its aft CG limit is a flat 93.0 in and the fuel sits at 95.0 in, just aft of it, so burning fuel always walks the CG forward.

The 180 hp Archer shares the Warrior’s airframe geometry and its flat 93.0 in aft limit, but carries 110 lb more and has a forward limit that moves aft much faster - from 82.0 in at 2,050 lb to 88.6 in at 2,550 lb. A heavy Archer with nothing in the back can therefore fail on the FORWARD limit, which is the opposite of the failure mode most pilots are trained to expect.

A metric aeroplane with an imperial second opinion: the AFM works in kilograms and metres and prints pounds and inches beside them. Two things make its weight-and-balance different from a Cessna or a Piper - the maximum LANDING mass is 128 lb below the maximum takeoff mass on the original main-gear strut, so a full-fuel departure can be a legal takeoff and an illegal landing; and the rearward CG limit tightens by 1.6 in if the long-range tank is fitted.