FAR 91.144 — High Barometric Pressure Restrictions

FAR 91.144 addresses a rare but important situation: when barometric pressure exceeds 31.00 inches of mercury along your route of flight. Most altimeters can't be set above 31.00 inHg, which means in unusually high-pressure conditions your altimeter will read lower than your true altitude — a serious terrain and traffic separation hazard.

Key points:

• If any information indicates that barometric pressure on your route currently exceeds, or will exceed, 31 inches of mercury, you may not operate an aircraft or begin a flight contrary to requirements published in a NOTAM issued under this section.
• The FAA issues a special Notice to Airmen (NOTAM) spelling out the operational procedures (such as altimeter setting and altitude correction guidance) that pilots must follow during these conditions.
• The Administrator may waive the restriction to allow emergency supply, transport, or medical services to isolated communities, provided the flight can be conducted with an acceptable level of safety.

Operationally, this matters because flying with an altimeter pegged at 31.00 while actual pressure is higher causes the indicated altitude to be lower than reality — risking CFIT and loss of separation in IFR conditions.