FAA Aircraft Dispatcher (ADX)MeteorologyEasy
A dispatcher is planning a departure from a high-elevation airport on a hot afternoon. Pressure altitude at the field is 6,000 feet and the reported OAT is 30°C, well above the standard temperature for that altitude. The resulting high density altitude will primarily affect aircraft performance by:
- ADecreasing engine power output and lift, resulting in a longer takeoff roll and reduced climb performance
- BHaving no measurable effect on takeoff performance below 10,000 feet pressure altitude
- CIncreasing engine and aerodynamic performance because the less dense air reduces drag
- DIncreasing propeller efficiency while decreasing wing lift by an equal amount
Show answer & explanationAnswer & explanation
Correct answer: A. Decreasing engine power output and lift, resulting in a longer takeoff roll and reduced climb performance
High density altitude means the air is less dense than standard, which reduces engine power (normally aspirated), propeller thrust, and wing lift, all of which increase takeoff distance and reduce climb performance. This is a fundamental performance-planning concept for dispatchers at hot, high-elevation airports.
Why the other options are wrong
- B. Density altitude effects are significant well below 10,000 feet, especially at high elevations in hot weather.
- C. Backwards — less dense air reduces, not increases, performance.
- D. Both effects are negative, not offsetting; lift and thrust both decrease.
Density Altitude
Pressure altitude corrected for nonstandard temperature; represents the altitude the aircraft 'feels' it is performing at.
- High temperature and/or high elevation increase density altitude
- Higher density altitude = degraded engine, propeller, and aerodynamic performance
- Critical factor in takeoff/landing performance planning at hot, high airports
Memory trick: Hot and High means the plane can't Fly (as well)