AIM Text
- Aircraft altimeters are subject to the following errors and weather factors:
- Instrument error.
- Position error from aircraft static pressure systems.
- Nonstandard atmospheric pressure.
- Nonstandard temperatures.
- The standard altimeter 29.92 inches Mercury (“Hg.) setting at the higher altitudes eliminates station barometer errors, some altimeter instrument errors, and errors caused by altimeter settings derived from different geographical sources.
Source: FAA Aeronautical Information Manual · current edition · paragraph 7-2-1.
The short answer
This AIM section is FAA advisory guidance on altimeter errors and high/low barometric pressure procedures. It covers instrument, position, nonstandard pressure and temperature errors, the 29.92"Hg standard setting, ground altimeter checks, and setting procedures below and above 18,000 feet MSL. It supports 14 CFR 91.121, 91.159, 91.177, and 91.144.
Research Notes
Common Questions
What do I set in my altimeter when barometric pressure exceeds 31.00 inches?
The AIM advises that when pressure exceeds 31.00"Hg a NOTAM is published, and all aircraft set 31.00"Hg for en route operations below 18,000 feet MSL, maintaining it until out of the area or until the final approach segment. Set 31.00"Hg before the lowest mandatory/crossing altitude or 1,500 feet AGL on departure or missed approach.
How big an altitude error does a wrong altimeter setting cause?
Per the AIM, an inch Hg error in the altimeter setting equals 1,000 feet of altitude—for example setting 29.90"Hg instead of 30.90"Hg. Flying from high to low pressure without resetting puts the aircraft closer to the surface than indicated: "GOING FROM A HIGH TO A LOW, LOOK OUT BELOW."
What ground altimeter check accuracy makes an altimeter questionable?
The AIM states that after setting the current reported altimeter setting and reading the altitude, if the difference from known field elevation is plus or minus 75 feet or greater, the altimeter's accuracy is questionable and should be referred to an appropriately rated repair station.