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.
Plain English
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.
Context Anchor
Seen in instrument procedure guidance, especially when discussing the lowest usable flight level and high altimeter-setting conditions.
Derivation
" Hg means inches of mercury, the traditional unit for atmospheric pressure used in U.S. aviation. It comes from old barometers where air pressure was measured by how high it pushed a column of mercury (chemical symbol Hg, from the Latin hydrargyrum). 31.00 inches is the standard mechanical limit built into U.S. altimeters.
Why Pilots Care
Maintains adequate terrain and obstacle clearance when high pressure causes actual altitude to be higher than pressure altitude would suggest.
Analogy
Think of it like reading a tire pressure gauge, except the number is air pressure outside the airplane and the old measuring reference is a column of mercury instead of pounds per square inch.
Grounding Statement
When the air outside is squeezing down harder than the altimeter knob can keep up with, the instrument quietly under-reads your altitude unless you follow the high-pressure procedure.
Intuition Check
31.00 " Hg is not a flight level or an altitude. It is a pressure value used to help the altimeter show altitude correctly.
Example Sentence 1
When the reported altimeter setting exceeded 31.00 " Hg across the region, the crew applied the high-pressure altimeter procedure to determine the lowest usable flight level.
Example Sentence 2
Controllers issue the adjusted flight level once surface pressure reaches 31.00 " Hg.