Altimeter & Altimetry Settings

The pressure altimeter is the most operationally important of the pitot-static instruments, and the way it is set is a recurring source of both routine airspace compliance and serious accidents. Its mechanism is a stack of sealed aneroid wafers that expand as the surrounding static pressure falls and contract as it rises. Because atmospheric pressure decreases predictably with height — roughly one inch of mercury per 1,000 feet in the lower atmosphere — that expansion can be geared to a pointer or digital tape reading in feet. The instrument and its settings are described in the FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25), Chapter 8, Flight Instruments.

The Kollsman window is the small adjustable barometric-scale window on the face of the altimeter. Turning its knob mechanically shifts the reference pressure the instrument measures from, letting the pilot compensate for the day's actual sea-level pressure. This is where the altimetry settings come in, and they are best understood through the ICAO Q-code shorthand used worldwide. QNH is the setting that makes the altimeter read height above mean sea level, so that on the ground it displays field elevation; it is the everyday setting below the transition altitude. QNE is not a pressure value you dial from a report but the practice of setting the standard datum of 29.92 inches of mercury (1013.25 hectopascals), which makes the altimeter display pressure altitude and is used at and above the transition altitude so all aircraft share a common reference for flight levels. QFE sets the altimeter to read zero on the airfield, so it shows height above that specific field — a setting still used at some European and military fields but rare in the United States.

The transition altitude and transition level manage the switch between these references. Climbing through the transition altitude, a pilot resets from the local QNH to the standard 29.92 and begins flying flight levels; descending through the transition level, the pilot resets back to the local QNH. In the United States the transition altitude is fixed at 18,000 feet MSL, above which flight levels begin at FL180. Elsewhere the transition altitude is far lower and varies by country and even by airport — often 3,000 to 6,000 feet — which is a common trap for pilots crossing between systems and a reason briefing the transition altitude is standard before international flights.

The altimeter is only as accurate as its setting and the temperature, which is the point of the memory aid "high to low or hot to cold, look out below." If a pilot flies from an area of high pressure into an area of lower pressure without updating the Kollsman window, the altimeter continues to read the old, higher reference and the aircraft is actually lower than indicated. The same true-altitude loss happens flying from warmer air into colder air, because the altimeter assumes standard-temperature air columns; in very cold conditions true altitude can be significantly below indicated altitude, which is why cold-temperature altitude corrections are published for certain approaches. Both cases put the aircraft closer to terrain than the instrument admits, making altimeter setting and temperature awareness a direct defense against controlled flight into terrain.

For flight schools, altimetry is a topic students often think they have mastered until they fly somewhere new. A domestic student who has only ever set QNH from ATIS can be surprised by a QFE field, a low European transition altitude, or a cold-weather approach where indicated altitude overstates true height. Instructors build the habit of reading back and setting the altimeter with every clearance and of re-briefing the transition altitude on any cross-border trip, because a single missed reset is enough to bust an altitude or, worse, an obstacle clearance. The concept ties directly into pressure altitude and density altitude on the performance side, so it is worth teaching as one connected system rather than three isolated facts.

Operationally, altimeter-setting errors sit behind a meaningful share of controlled-flight-into-terrain and altitude-deviation events, which is why current altimeter settings from METAR and ATIS are treated as safety-critical information and why altimeters and static systems carry the recurring inspection required under 14 CFR 91.411 for IFR flight. Schools and clubs operating near high terrain, or in climates with large temperature swings, reinforce cold-temperature corrections and the "look out below" rule as part of their standard weather and route briefings.

Aviatize's Training Management module lets instructors track altimetry competence as discrete, graded curriculum items — setting QNH versus standard, applying the transition altitude, and reasoning through the "high to low, hot to cold" errors — so gaps surface in the record rather than in the airplane. Ground Training & Checking keeps the flight-instruments and altimetry syllabus consistent across instructors and locations.

For operators flying internationally or into high-terrain fields, Aviatize's Safety Management module can capture altitude-deviation and altimeter-setting occurrences as reportable events, turning near-misses into trend data the safety manager can act on rather than isolated stories that never reach the risk register.