A flight is planned over mountainous terrain at an indicated altitude of 8,000 feet using the current local altimeter setting. The reported temperature at that level is -21°C, while the ISA temperature for 8,000 feet is -1°C. What effect does this have on the aircraft's true altitude, and what action should the dispatcher take?
- ATrue altitude equals indicated altitude because the altimeter setting already compensates for temperature
- BTrue altitude will be significantly lower than indicated; a cold temperature correction should be applied to terrain clearance
- CTrue altitude will be slightly lower than indicated, but the deviation is too small to require correction
- DTrue altitude will be higher than indicated; no correction is necessary
Show answer & explanationAnswer & explanation
Correct answer: B. True altitude will be significantly lower than indicated; a cold temperature correction should be applied to terrain clearance
Altimeters are calibrated to standard atmosphere; when the actual air column is much colder than standard, air is denser and the true altitude is lower than the indicated altitude ('cold, low, look out below'). With a 20°C deviation from ISA, the error is substantial and a cold-temperature correction is required, especially over mountainous terrain, per FAA cold-weather altimetry guidance.
Why the other options are wrong
- A. The altimeter setting corrects only for surface pressure, not for non-standard temperature aloft.
- C. A 20°C deviation is large enough to produce hundreds of feet of error, which is not negligible.
- D. Cold air causes true altitude to be lower, not higher, than indicated.
Cold Temperature Altimeter Error
When actual air temperature is colder than standard, true altitude is lower than indicated altitude because cold air is denser and pressure levels are compressed closer together.
- Rule: 'hot high, cold low, look out below'
- Altimeter settings correct for pressure, not temperature
- Large temperature deviations require published cold-weather corrections, especially near terrain
Memory trick: 'Cold, low, look out below' — cold air shrinks your safety cushion.