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Barometric Altimeter Errors

AIM ¶ 7-2-2 — Barometric Altimeter Errors

AIM 7-2-2: How high and low barometric pressure cause altimeter errors. Learn true vs indicated altitude risks for pilots flying near terrain.

In Plain English

AIM 7-2-2 explains how extreme barometric pressures cause your altimeter to lie to you — a critical concept for terrain and obstacle clearance.

  • High pressure (above 31.00 "Hg): Common in cold, dry air masses. Many altimeters can't be set above 31.00 "Hg, so when the actual setting is higher, your true altitude is higher than indicated. (Less dangerous — you have extra clearance.)
  • Low pressure (below 28.00 "Hg): Considered abnormal. The FAA does not recommend flight when the altimeter can't be set below 28.00 "Hg. Here, your true altitude is lower than indicated — you are closer to the ground than your altimeter shows.
  • Cold temperatures make the low-pressure error worse, potentially placing the aircraft significantly below indicated altitude.

The AIM emphasizes extreme caution near terrain or obstructions in low pressure and/or low temperature conditions. This is a recommended practice and safety advisory — not a regulatory limit — but ignoring it can lead to controlled flight into terrain (CFIT). Remember the memory aid: "High to low (or hot to cold), look out below."

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AIM Source Text
FAA AIM ¶ 7-2-2
High Barometric Pressure: Cold, dry air masses may produce barometric pressures in excess of 31.00 “Hg. Many aircraft altimeters cannot be adjusted above 31.00 “Hg. When an aircraft's altimeter cannot be set to pressure settings above 31.00 “Hg, the aircraft's true altitude will be higher than the indicated altitude on the barometric altimeter. Low Barometric Pressure: An abnormal low-pressure condition exists when the barometric pressure is less than 28.00 “Hg. Flight operations are not recommended when an aircraft's altimeter is unable to be set below 28.00 “Hg. In this situation, the aircraft's true altitude is lower than the indicated altitude. This situation may be exacerbated when operating in extremely cold temperatures, which may result in the aircraft's true altitude being significantly lower than the indicated altitude.
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Oral Exam Questions a DPE Might Ask
Q1What happens to your true altitude when the barometric pressure exceeds 31.00 "Hg and your altimeter can't be set that high?
Per AIM 7-2-2, if the altimeter cannot be set above 31.00 "Hg, the aircraft's true altitude will be higher than the indicated altitude on the barometric altimeter.
Q2At what barometric pressure does the AIM consider an abnormal low-pressure condition to exist, and what is the operational recommendation?
Per AIM 7-2-2, an abnormal low-pressure condition exists when barometric pressure is less than 28.00 "Hg. Flight operations are not recommended when the altimeter cannot be set below 28.00 "Hg, because the aircraft's true altitude will be lower than indicated.
Q3Why does the AIM advise extreme caution when flying near terrain in low pressure and low temperatures?
Per AIM 7-2-2, low pressure causes true altitude to be lower than indicated, and extremely cold temperatures can exacerbate this error, resulting in true altitude being significantly lower than indicated — a serious risk near obstructions or terrain.
Related Paragraphs in Chapter 7
7-2-1
Altimeter Errors
7-2-3
Altimeter Errors and Settings
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