Plain English Meaning

This is the highest pressure setting most altimeters can physically dial in. When the air pressure outside is higher than this number, pilots can't set their altimeter to match it, so special rules kick in to keep aircraft safely separated from terrain.

Full Definition

An altimeter setting of 31.00 inches of mercury, which is the upper limit that most U.S. aircraft altimeters can be mechanically set to. When the local altimeter setting along the route of flight exceeds 31.00 " Hg, special procedures apply because the altimeter cannot be adjusted to the actual reported pressure, and aircraft will indicate lower than their true altitude.

Context Anchor

Seen in instrument procedure guidance, especially when discussing the lowest usable flight level and altimeter-setting .