Hot-day derate at Queenstown: the derate that does not exist
Queenstown (NZQN), runway 05, 1,891 m TORA, elevation 1,171 ft, QNH 1013 hPa, OAT 26 °C. You command a twin turbofan business jet (MTOW 15,900 kg, two 3,600 lbf engines) with six passengers and a slot in 25 minutes. The AFM assumed-temperature table for 1,171 ft pressure altitude gives a maximum assumed temperature of 47 °C. Company standard is a 15 % assumed-temperature derate; today that would require an assumed temperature of 52 °C. At full takeoff thrust the AFM shows 1,480 m takeoff distance and 38 °C EGT margin. Takeoff weight is 15,400 kg and the field-length-limited weight at these conditions is 15,600 kg; the net second-segment climb gradient required is 2.4 %. The cabin is hot. The first officer proposes keeping both air-conditioning packs on and using the full 15 % derate because 'the book has plenty of margin'.
1. The first officer wants the full 15 % derate. Is it available today, and what actually limits it? Explain the assumed-temperature method as part of your answer.
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No - the 15 % derate does not exist at these conditions.
The assumed-temperature (ATM) method reduces takeoff thrust by telling the FADEC that the air is hotter than it really is. The FADEC then schedules fuel flow and N1 to the takeoff rating for that higher temperature, which is a lower thrust. Three conditions must all be satisfied:
- the assumed temperature must be at or above the actual OAT, otherwise the FADEC would command more thrust, not less;
- it must not exceed the certified maximum assumed temperature for the pressure altitude - 47 °C at 1,171 ft;
- the resulting performance must still satisfy field length, obstacle clearance and the net takeoff flight path.
Here the 15 % derate needs 52 °C, which is outside the certified envelope, so the entry is not available and the corresponding chart does not exist. The underlying reason is flat rating: at 26 °C and 1,171 ft the air is already about ISA +13 °C, so the engine sits near the top of its flat-rated band and most of the thermal margin a derate would 'spend' has already been taken by the day itself. Pushing for the company-standard derate is arguing with the temperature, not with the book.
2. What derate can actually be used, and what must you check before accepting it?
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Use the largest derate the AFM data supports, not the largest figure the FADEC will accept.
- Take the maximum assumed temperature of 47 °C and interpolate the ATM takeoff chart for 15,400 kg, 1,171 ft, 26 °C, QNH 1013, the actual wind and slope. Read the takeoff distance, the net flight path and the expected EGT. That gives the maximum available derate at this weight - likely single figures, not 15 %.
- Compare with the field-length-limited weight of 15,600 kg. You are only 200 kg below it, so any further loss of performance - packs, anti-ice, a wet runway, a lower assumed temperature - makes the takeoff field-length-limited rather than weight-limited.
- Confirm the chart is entered for the correct configuration: packs on or off, bleed configuration, actual wind and slope. A packs-on chart is not interchangeable with a packs-off chart.
- Check the EGT margin the derate actually buys. A setting near the top of the band on a hot day protects the engine far less than the company figure implies.
If the numbers are marginal at the derate you wanted, take less derate - or none at all if the charts allow - rather than inventing a temperature the certification does not cover.
3. Packs on or off for the takeoff, and what will you cross-check after rotation to know the takeoff was made as computed?
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Packs. Bleed-air extraction for the packs reduces available thrust and raises EGT, so the takeoff data must match the configuration actually used. With only 200 kg of weight margin, use packs-off data and switch the packs off for takeoff, or use a packs-on chart with a derate that still clears every requirement. Cabin comfort and pressurization scheduling matter, but obstacle clearance and field length come first.
After rotation:
- cross-check N1 against the target takeoff N1 for the assumed temperature;
- check EGT against the takeoff limit and against the value the ATM chart predicted;
- assess acceleration and the achieved flight path against the planned profile.
If N1 is clearly below target, the aircraft is not performing as computed. Below V1 the answer is to reject. Airborne, fly the flight path first; the derated setting is the minimum you planned to use, and the levers can be advanced to maximum takeoff thrust if the actual flight path requires it. Any N1 or EGT exceedance goes in the technical log with peak, duration and phase of flight.