Weight and balance questions on the General test are mostly arithmetic built on a few definitions from chapter 6 of the FAA-H-8083-30B handbook.
The definitions
- Datum: an imaginary vertical plane from which horizontal measurements are taken.
- Moment: weight multiplied by its arm (its distance from the datum).
- Center of gravity (CG): total moment divided by total weight.
Weighing the aircraft
Weigh inside a hangar, since wind over the surfaces makes scale readings fluctuate. Never weigh with partly full tanks, because the fuel cannot be accounted for. Subtract tare, such as chocks and jack fittings resting on the scales, from each reading.
Worked example: an equipment change
An airplane weighs 1,600 lb with its empty weight CG at +40.0 in, a moment of 64,000 lb-in. A 5 lb transponder is installed at +90 in, adding 450 lb-in, and an 8 lb radio at -30 in is removed, which subtracts -240 lb-in and so adds 240 lb-in. The new weight is 1,597 lb and the new moment 64,690 lb-in, so the new CG is about +40.51 in.
Adverse-loading checks
A forward adverse-loaded check loads every item ahead of the forward CG limit and leaves items behind it empty; the aft check does the reverse. A piston airplane is checked with minimum fuel, one-half pound per METO horsepower. If the empty weight CG falls within the range the type certificate data sheet gives, no adverse-loading check is needed.
Percent of MAC
Larger aircraft give CG as a percentage of the mean aerodynamic chord: subtract the position of its leading edge (LEMAC) from the CG and divide by the MAC length. A CG at 458 in with LEMAC at 410 in and a 120 in MAC is at 40 percent of MAC.
Practice FAA Aviation Mechanic General questions with explanations and timed mock tests.