Flight Performance and Planning: Mass, Balance and Performance
Mass and balance
Every performance question rests on mass and centre of gravity. Know the definitions: basic empty mass, dry operating mass, take-off mass, landing mass, and the difference between maximum structural masses and performance-limited masses. The CG must lie within the certified forward and aft limits; forward of the limit, elevator authority and rotation may be insufficient, while aft of the limit, stability and stall recovery degrade.
Performance
Take-off and landing performance depends on pressure altitude, temperature, mass, wind, runway slope and surface condition. Density altitude is the key concept: high altitude, high temperature and high humidity all reduce air density, lengthening the take-off run and reducing climb performance. Know the difference between the balanced field length concept and the actual runway available, and how V1, VR and V2 are selected.
Planning
Be able to compute fuel: trip fuel, contingency, alternate, final reserve and extra fuel. Know that the alternate may be omitted under specified conditions, and that the final reserve is a fixed time at holding consumption.
Traps
- Mixing up pressure altitude and density altitude.
- Forgetting that a tailwind increases both take-off and landing distances.
- Assuming a lower mass always improves landing performance - it does, but check the CG effect.
- Using the wrong chart axis or interpolating outside the chart's validity.
Worked example
A runway at 4000 ft pressure altitude with an outside air temperature of ISA+15 requires a take-off distance factor of about 1.4 compared with sea level ISA. If the sea-level ISA distance is 800 m, expect roughly 1120 m before applying wind and slope corrections. Always apply corrections in the order the chart specifies.