Mass and Balance
What this domain tests
Mass and Balance tests the load sheet, CG position, maximum structural weights and how loading changes handling. Expect moment calculations, %MAC conversions and questions on MZFW, MTOW and MLW limits.
Core rules
- Mass x arm = moment. Total mass and total moment give CG: CG = total moment / total mass.
- Limits: MTOW is structural and performance-limited; MLW limits landing; MZFW limits payload before fuel. The most restrictive limit governs.
- CG envelope: the forward limit is often set by elevator authority and landing flare; the aft limit by stability and stall recovery. An aft CG reduces longitudinal stability.
- %MAC = (CG - LEMAC) / MAC x 100.
- Floor loading and cumulative load limits can override the CG calculation.
Worked example
Basic mass 4,000 kg at arm 6.0 m; payload 1,000 kg at 8.0 m. Moments: 24,000 + 8,000 = 32,000 kg m; mass 5,000 kg; CG = 6.4 m. If LEMAC = 5.5 m and MAC = 1.5 m, %MAC = (6.4 - 5.5)/1.5 x 100 = 60%. That is inside a typical envelope, but check the actual aircraft limit.
Traps
Using takeoff mass for a landing CG check; forgetting that fuel burn moves CG; mixing inches and metres; assuming a forward CG is always safe, since it increases stall speed and can reduce elevator authority. A last-minute change must produce a revised load sheet, not just a verbal note.
Key habit
Build a table: item, mass, arm, moment. Then check every limit explicitly, including ZFW and floor limits.