A dispatcher notes that the enroute temperature at the planned cruise altitude is significantly warmer than the ISA standard for that altitude. Compared to a standard-day at the same Mach number, this warmer-than-standard air will generally result in:
- ADecreased true airspeed and decreased specific range
- BIncreased true airspeed and improved specific range
- CNo change in true airspeed or specific range since Mach number is held constant
- DIncreased true airspeed but decreased specific range due to reduced engine efficiency and increased fuel flow
Show answer & explanationAnswer & explanation
Correct answer: D. Increased true airspeed but decreased specific range due to reduced engine efficiency and increased fuel flow
Since the speed of sound increases with temperature, flying at a constant Mach number in warmer-than-standard air produces a higher true airspeed. However, turbine engines lose thermal efficiency in warmer air and require more fuel flow to produce the same thrust, so the increase in fuel flow outweighs the increase in TAS, resulting in a net decrease in specific range (nautical miles per pound of fuel).
Why the other options are wrong
- A. TAS actually increases with warmer temperatures at constant Mach, not decreases.
- B. TAS does increase, but specific range typically decreases, not improves.
- C. TAS varies directly with the local speed of sound (temperature), so it is not unchanged.
Temperature Effect on Specific Range
Flying in air warmer than ISA standard increases true airspeed at a constant Mach number, but reduced engine efficiency and higher fuel flow generally cause specific range to decrease.
- Speed of sound increases with temperature
- TAS rises for a constant Mach in warmer air
- Fuel flow increase typically outweighs TAS gain, lowering specific range
Memory trick: Warmer air: faster but thirstier — range suffers.