Weight and balance

An aircraft has to be light enough to fly and balanced so it can be controlled. The center of gravity (CG) must stay between forward and aft limits, which come from the Type Certificate Data Sheet (TCDS) or the flight manual. Too far forward and the tail can't raise the nose; too far aft and the aircraft can become unstable and hard to recover from a stall.

Everything is measured from a datum, a reference point the manufacturer picks (often the firewall or the wing's leading edge). The arm is the distance from the datum in inches, plus if it's behind (aft) and minus if in front. The moment is weight × arm, in pound-inches.

Finding the CG

Add up the weights, add up the moments, and divide: CG = total moment ÷ total weight.

Item           Weight   Arm    Moment
Empty aircraft  1,800   35.0   63,000
Pilot             200   44.0    8,800
Total           2,000          71,800
CG = 71,800 ÷ 2,000 = 35.9 inches aft of the datum

Standard weights help: aviation gasoline is about 6 pounds a gallon, jet fuel about 6.7, and engine oil about 7.5.

Weighing an aircraft

Weigh it indoors with the aircraft level, fuel drained to the unusable amount and fluids at the levels the manual gives. Put a scale under each wheel, read all three, and subtract the tare (the chocks or blocks that sat on the scales). The empty weight and its CG go into the weight and balance record, and whenever a repair or alteration adds or removes equipment, the mechanic works out and records the new empty weight and CG.

A quick check: adding weight behind the CG moves it aft; removing weight in front of the CG also moves it aft.