FAA Sport Pilot Airplane (SPA)WeatherHard
A pilot departs an airport where the altimeter is set correctly but the outside air temperature is significantly colder than standard. While flying to a distant airport at a constant indicated altitude, how will the aircraft's true altitude compare to the altimeter indication?
- ATrue altitude will exactly match indicated altitude since the altimeter setting was correct
- BTrue altitude will be unaffected by temperature, only by pressure changes
- CTrue altitude will be lower than indicated altitude, posing a terrain clearance hazard
- DTrue altitude will be higher than indicated altitude
Show answer & explanationAnswer & explanation
Correct answer: C. True altitude will be lower than indicated altitude, posing a terrain clearance hazard
In colder-than-standard air, the atmosphere is denser and pressure levels are compressed closer together, so the aircraft's true altitude is actually lower than the altimeter indicates, even though the altimeter setting is correct for pressure. This is summarized by 'high to low, look out below' for temperature as well as pressure.
Why the other options are wrong
- A. An altimeter setting corrects for pressure but not for non-standard temperature.
- B. Temperature does affect true altitude by changing air density and the spacing of pressure levels.
- D. This describes flying into warmer-than-standard air, the opposite effect.
Altimeter Temperature Error
When flying from warm air into significantly colder-than-standard air, true altitude is lower than indicated altitude because cold, dense air compresses the vertical spacing between pressure levels.
- Cold air: true altitude lower than indicated (hazard near terrain)
- Warm air: true altitude higher than indicated
- Remember: 'From hot to cold or high to low, look out below'
Memory trick: 'High to low or hot to cold, look out below!'