FAA Airline Transport Pilot (ATM)Aircraft Performance and Weight and BalanceEasy
A dispatcher notes that the departure airport's density altitude is significantly higher than the field elevation due to high temperature. What is the primary performance effect on the jet's takeoff?
- ANo change in performance since jet engines are unaffected by density altitude
- BIncreased engine thrust output and decreased takeoff distance required
- CReduced stall speed, allowing an earlier rotation
- DReduced engine thrust output and increased takeoff distance required
Show answer & explanationAnswer & explanation
Correct answer: D. Reduced engine thrust output and increased takeoff distance required
High density altitude means lower air density, which reduces engine thrust, decreases lift for a given true airspeed, and reduces propeller/rotor efficiency, all of which increase the required takeoff distance.
Why the other options are wrong
- A. Jet engines are significantly affected by air density since it changes mass airflow.
- B. This is the opposite effect; thrust decreases, not increases.
- C. Stall speed (indicated) does not decrease with density altitude; true airspeed for stall increases.
Density Altitude Effect on Takeoff
High density altitude (from heat, humidity, or elevation) reduces air density, decreasing engine thrust and aerodynamic lift, thereby increasing takeoff distance.
- Hot, humid, high-elevation airports have higher density altitude
- Reduced thrust and lift both increase required runway length
- Affects both piston and turbine aircraft
Memory trick: 'Hot and high, thrust says goodbye'