Flight Performance and Planning (Mass and Balance, Performance)
What the exam tests
This domain tests whether you can compute mass, centre of gravity and aircraft performance, and whether you can recognise when density altitude, wind, slope or obstacle clearance makes a planned flight unsafe.
Core rules
- Mass and balance uses mass, arm and moment. Moment equals mass multiplied by arm. Total moment divided by total mass gives the CG position. The CG must remain inside the forward and aft limits for take-off, in flight and landing.
- Maximum take-off mass, maximum landing mass and maximum zero-fuel mass are separate limits. Fuel, baggage and crew must be added at their correct arms.
- Performance depends on pressure altitude, temperature, mass, wind, runway slope and surface. High density altitude reduces lift, thrust and climb performance and increases take-off and landing distance.
- Pressure altitude is indicated altitude with the subscale set to 1013.2 hPa. About 1 hPa equals 30 ft at low altitude.
- Obstacle clearance must be checked after calculating take-off distance and climb gradient, not before.
Worked example
An aircraft has basic mass 1,000 kg at arm 1.20 m, giving moment 1,200 kg·m. Add 100 kg at arm 2.00 m, moment 200 kg·m. New mass is 1,100 kg and total moment is 1,400 kg·m, so CG is 1,400 divided by 1,100, or 1.27 m. If the aft limit is 1.25 m, the loading is outside limits even though the mass may be legal.
Traps
Using QFE instead of QNH for pressure altitude; mixing kilograms and pounds; forgetting fuel burn changes mass and CG; checking CG only at take-off; and ignoring runway slope or a tailwind.