QNH — Altimeter Sub-scale Setting — Sea Level Pressure

What QNH Actually Sets

An aircraft's altimeter is fundamentally a barometer — it infers altitude from air pressure, and air pressure changes constantly with weather. QNH is the specific altimeter sub-scale setting that, once dialled in, makes the altimeter read the aircraft's height above mean sea level (AMSL) rather than an arbitrary or misleading value. It's derived by taking the actual atmospheric pressure measured at an airport and mathematically reducing it to what that pressure would read at sea level — hence "QNH," one of a set of legacy Q-codes from early radiotelegraphy that aviation still uses as compact shorthand.

QNH vs QFE vs QNE

Three related settings are easy to confuse. QNH gives altitude above sea level — the setting used for most enroute and approach phases below the transition altitude, and the one that lets a pilot compare their altimeter reading directly to charted terrain and obstacle heights (which are also published relative to sea level). QFE is a local-airfield setting that instead makes the altimeter read height above the airfield itself (zero at touchdown) — used operationally in some regions and by some general aviation and military operators, though less common in international airline operations. QNE, more commonly called "standard pressure" (1013.25 hPa / 29.92 inHg), is not tied to any local pressure reading at all — it's a fixed reference used above the transition altitude so that all aircraft in that airspace are working from the same baseline, which is what makes vertical separation between aircraft reliable regardless of the actual weather below.

The Transition Altitude Switch

Below the transition altitude (a fixed value that varies by country and airspace — commonly around 18,000 ft in the US, often much lower, e.g. 3,000–6,000 ft, in parts of Europe), pilots fly on local QNH so their indicated altitude matches charted terrain and minimum safe altitudes. Climbing through the transition altitude, pilots switch their altimeter to standard pressure (QNE) and thereafter describe their vertical position as a Flight Level rather than an altitude — FL350 rather than "35,000 feet" — precisely because everyone at that level is using the same fixed reference rather than their own local QNH.

Why Getting QNH Wrong Is Dangerous

Because QNH ties an aircraft's indicated altitude to real terrain height, using a stale or wrong QNH value is a genuine controlled-flight-into-terrain risk, particularly during approach and in mountainous terrain — a large pressure error can translate into an altimeter reading that is off by hundreds of feet relative to the ground. ATC and ATIS broadcasts update QNH regularly for exactly this reason, and pilots cross-check it at multiple points during an approach.

Where Pilots Get QNH From

QNH is broadcast continuously via ATIS (Automatic Terminal Information Service) recordings at towered airports, issued directly by ATC on initial contact and during approach, and published in METAR weather reports — giving pilots several independent, frequently-updated sources to cross-check against.