FAA Airline Transport Pilot (ATM)Transport Aerodynamics and High-Altitude OperationsEasy
A flight crew departs from a high-elevation airport on a hot day. Compared to a sea-level standard day departure, which statement correctly describes the effect on takeoff performance?
- AThe reduced air density increases wing lift at a given airspeed, shortening the takeoff roll
- BAir density has no effect on turbojet engine thrust, only on propeller aircraft performance
- CReduced air density decreases engine thrust and requires a higher true airspeed for the same indicated/lift airspeed, lengthening the takeoff roll
- DThe aircraft needs a lower true airspeed to become airborne because indicated airspeed requirements decrease
Show answer & explanationAnswer & explanation
Correct answer: C. Reduced air density decreases engine thrust and requires a higher true airspeed for the same indicated/lift airspeed, lengthening the takeoff roll
Lower air density at high density altitude reduces both engine thrust output and lift produced at a given true airspeed. Since the wing still needs the same indicated airspeed to generate sufficient lift, a higher true airspeed is required, and reduced thrust further lengthens the takeoff roll.
Why the other options are wrong
- A. Reduced density decreases lift at a given airspeed, not increases it.
- B. Turbojet thrust is also density-dependent; it decreases as air density decreases.
- D. Indicated airspeed requirements for takeoff remain essentially the same; it's true airspeed that increases.
Density Altitude Effect on Takeoff
Higher density altitude (from high elevation and/or high temperature) reduces air density, decreasing both engine thrust and lift efficiency, requiring higher true airspeed and longer takeoff distance.
- High elevation + high temp = high density altitude
- Reduces thrust for both turbine and piston engines
- Requires higher TAS at same IAS, lengthening takeoff roll
Memory trick: Thin hot air = weak engines, long runway needed