Flight Performance and Planning
Flight Performance and Planning tests mass and balance, takeoff and landing performance, density altitude, climb, fuel planning and the practical use of charts. Mauritius' warm, humid air makes density-altitude questions especially relevant.
Altitudes and ISA
Indicated altitude is shown on the altimeter; pressure altitude is indicated altitude with 1013.25 hPa set; density altitude is pressure altitude corrected for temperature. ISA temperature at sea level is 15 C, lapsing 1.98 C per 1,000 ft. A useful approximation is density altitude = pressure altitude + 120 x (OAT - ISA temperature), with temperatures in Celsius.
Mass and balance
MTOW, MLW and MZFW must not be exceeded. Calculate moments from the datum and confirm the centre of gravity stays inside the forward and aft limits for takeoff, in flight and landing. An aft CG reduces longitudinal stability and can increase cruise speed; a forward CG increases takeoff distance and stick force.
Takeoff and landing
Takeoff distance increases with mass, altitude, temperature, tailwind, upslope and a wet or contaminated runway. Climb gradient is affected by excess thrust; obstacle clearance is a gradient requirement, not just a rate. Landing distance increases with mass, temperature, tailwind, downslope and poor braking action.
Worked example
Aerodrome elevation 150 ft, QNH 1003 hPa, OAT 30 C. Pressure altitude = 150 + 30 x (1013 - 1003) = 450 ft. ISA temperature at 450 ft = 15 - 1.98 x 0.45 = 14.1 C. Temperature excess = 30 - 14.1 = 15.9 C. Density altitude = 450 + 120 x 15.9 = about 2,360 ft. The aircraft performs as if at 2,360 ft, so takeoff distance is greater than the pressure altitude suggests.
Traps
Using QFE instead of QNH for pressure altitude; forgetting wind, slope and runway surface; ignoring fuel temperature and density; leaving taxi fuel out of the flight plan.