FAA Aircraft Dispatcher (ADX)Flight Planning and PerformanceMedium
A dispatcher is planning an approach into an airport reporting a surface temperature of -30°C, well below the ISA standard. With the altimeter set to the current local setting, the aircraft's true altitude at a given indicated altitude will be:
- AHigher than indicated altitude
- BLower than indicated altitude
- CUnpredictable and cannot be estimated
- DEqual to indicated altitude, since the altimeter setting compensates for temperature
Show answer & explanationAnswer & explanation
Correct answer: B. Lower than indicated altitude
Pressure altimeters are calibrated for a standard atmosphere and do not compensate for non-standard temperature. In air colder than standard, air density is greater, so the aircraft is actually lower than the altimeter indicates, requiring a cold-temperature correction to published minimum altitudes.
Why the other options are wrong
- A. Opposite of the actual effect in cold air.
- C. The effect is well-defined and quantifiable using cold temperature correction tables.
- D. The altimeter setting corrects for pressure only, not temperature.
Cold Temperature Altimetry Error
In air colder than standard, a pressure altimeter will indicate a higher altitude than the aircraft's true altitude, requiring correction on approach.
- 'High to Low, Look Out Below' also applies to temperature
- Corrections are applied to MDA/DA and stepdown altitudes
- FAA cold temperature correction tables give exact adjustments
Memory trick: Cold air shrinks the atmosphere — you're lower than you think.