AIM ¶ 7-2-1 — Altimeter Errors
AIM 7-2-1 explains altimeter errors pilots must know: instrument, position, nonstandard pressure & temperature. Why 29.92 "Hg is used at high altitudes.
AIM 7-2-1 introduces the errors and weather factors that affect aircraft altimeters. As a student pilot, you should understand that your altimeter doesn't always show true altitude — it can be off for several reasons:
- Instrument error — mechanical imperfections inside the altimeter itself.
- Position error — caused by the aircraft's static pressure system not sensing perfectly undisturbed air.
- Nonstandard atmospheric pressure — pressure that differs from the standard 29.92 "Hg.
- Nonstandard temperatures — air warmer or colder than the standard atmosphere causes the altimeter to read high or low ("from hot to cold, look out below").
At higher altitudes, all aircraft set their altimeters to the standard setting of 29.92 "Hg. This common reference eliminates station barometer errors, some altimeter instrument errors, and discrepancies that come from using altimeter settings issued by different geographical sources. The result is consistent vertical separation between aircraft in the flight levels, even if the indicated altitude isn't true altitude. This is why understanding altimeter errors matters operationally: terrain clearance and traffic separation depend on it.