Weight, Balance and Aircraft Performance
Weight, balance and performance tests mass limits, centre of gravity, loading calculations and the use of POH performance data.
Weight and balance
- Datum, arm and moment: moment = weight × arm; total moment divided by total weight gives the CG.
- Limits are forward and aft. %MAC = (CG − LEMAC) ÷ MAC length × 100.
- Know MTOW, MLW, MZFW, basic empty weight, useful load and payload.
- Weight shift: ΔCG = (weight moved × distance moved) ÷ total weight.
Performance
- Pressure altitude = elevation + 30 ft × (1013 − QNH), with QNH in hPa.
- Density altitude = PA + 120 × (OAT − ISA temperature), where ISA = 15 °C − 2 °C per 1,000 ft.
- POH charts apply to a specified weight and configuration; apply wind, slope and surface corrections, and the 50 ft obstacle factor.
- Crosswind component = wind speed × sin(angle between wind and runway).
Worked example
Aerodrome elevation 1,500 ft, QNH 1000 hPa, OAT 25 °C. PA = 1,500 + 30 × 13 = 1,890 ft. ISA temperature = 15 − 2 × 1.89 = 11.2 °C. DA = 1,890 + 120 × (25 − 11.2) = 1,890 + 1,656 = 3,546 ft. At that density altitude the take-off distance may be around 40% longer than at sea level, so compare the chart figure with the runway available and the required margin.
Traps
- Using MTOW instead of actual weight in the charts.
- Forgetting that a forward CG increases take-off distance and reduces elevator authority, while an aft CG reduces stability.
- Mixing metres and inches for arms.
- Omitting grass, wet or slope corrections.