Instrument Aircraft Performance and Flight Planning
Instrument Aircraft Performance and Flight Planning
This domain covers weight and balance limits, density altitude, climb gradients and the fuel figures that produce a performance-limited rather than structurally limited departure.
Core concepts
- Three structural limits: MTOW, MLW and MZFW. The lowest of structural and performance limits governs the takeoff weight.
- Density altitude raises takeoff and landing distances and reduces climb performance; high elevation plus high temperature plus high humidity is the worst combination.
- PANS-OPS design assumes a 3.3% minimum climb gradient for departure procedures and 2.5% for missed approach; published procedures requiring more state the gradient on the chart, and you must check it against the aircraft's net gradient.
- Contaminated runways (water, slush, snow, rubber) increase rolling resistance and reduce braking; use the contaminated-runway figures, not the dry ones.
- Driftdown: after engine failure en route, the aircraft descends to the single-engine ceiling - check that this clears the MORA along the route.
Worked example
A SID requires 4.5% climb to 6,000 ft. Convert to a rate: 4.5% × 6,076 ≈ 273 ft/NM. At 150 kt ground speed (2.5 NM/min) the required rate is 273 × 2.5 ≈ 680 ft/min. If the aircraft achieves only 550 ft/min at that weight and temperature, you cannot accept the SID as cleared - reduce weight, delay for cooler temperature, or request a different procedure.
Traps
- Using indicated rather than ground speed for the gradient-to-rate conversion (wind matters).
- Checking only MTOW while the runway or the second-segment gradient is the real limit.
- Forgetting that ice on the airframe adds drag and weight and erodes the gradient.