An airplane departing from a high-elevation airport on a hot day experiences a significant increase in density altitude. Compared to a standard-day sea-level departure at the same indicated airspeed and gross weight, the airplane will require a longer takeoff roll primarily because:
- Athe critical angle of attack increases at higher density altitudes, delaying lift-off
- Bhigher density altitude increases air density, which requires less power to accelerate the airplane
- Cthe wing must reach the same lift-off indicated airspeed at a higher true airspeed, while the engine and propeller produce less thrust in the thinner air
- Dindicated airspeed required for lift-off decreases as density altitude increases
Show answer & explanationAnswer & explanation
Correct answer: C. the wing must reach the same lift-off indicated airspeed at a higher true airspeed, while the engine and propeller produce less thrust in the thinner air
Indicated airspeed for lift-off stays essentially the same because it directly reflects dynamic pressure on the wing, but at high density altitude the true airspeed needed to reach that same indicated airspeed is higher, meaning the airplane must accelerate to a greater groundspeed. Simultaneously, the thinner air reduces engine power output and propeller efficiency, both factors combining to lengthen the takeoff roll.
Why the other options are wrong
- A. Critical angle of attack is a fixed airfoil characteristic and does not change with density altitude.
- B. High density altitude actually means LOWER air density, not higher, and thrust decreases, not power requirement decreasing.
- D. Indicated airspeed for lift-off is essentially unchanged by density altitude.
Density Altitude and Takeoff Performance
High density altitude reduces air density, requiring higher true airspeed to achieve the same indicated airspeed for lift-off, while simultaneously reducing engine power and propeller efficiency, both lengthening takeoff distance.
- IAS for lift-off stays roughly constant
- TAS needed increases with density altitude
- Engine power and prop efficiency both decrease in thin air
Memory trick: Hot and high, thrust says goodbye, TAS climbs high.