Flight Performance and Planning: Mass, Balance and Take-off Data
What you must know
Performance questions test the relationship between mass, density altitude, wind, runway slope and surface, and the resulting take-off and landing distances, climb gradients and cruise figures. Candidates must be able to compute mass and balance: basic empty mass, crew, passengers, baggage and fuel; centre of gravity position as a percentage of mean aerodynamic chord; and the forward and aft CG limits. Understand density altitude as pressure altitude corrected for temperature, and how it degrades engine power, propeller thrust and lift. Know the definitions of take-off distance available, take-off run available, landing distance available and the significance of obstacles and clearways.
Traps in questions
- Confusing pressure altitude with density altitude; only temperature correction gives density altitude.
- Forgetting that aft CG reduces stability but may shorten take-off distance; forward CG increases stability but lengthens it.
- Using still-air figures when a headwind or tailwind is stated.
- Mixing units: litres versus kilograms of fuel, pounds versus kilograms.
Worked rule of thumb
Compute density altitude: pressure altitude plus 120 ft per °C above ISA temperature at that level. Example: pressure altitude 2,000 ft, ISA temperature 11 °C, actual 31 °C, difference 20 °C, so density altitude ≈ 2,000 + (20 × 120) = 4,400 ft. Then enter the performance chart at 4,400 ft, not 2,000 ft.
Study approach
Practise one full mass and balance calculation and one take-off chart reading per study session, always writing units.