Learn › Flight levels & transition altitude

Flight levels vs altitude

Why a climbing airliner suddenly stops talking about feet — transition altitude, transition level and QNH, explained.

Listen to an approach frequency for a minute and you will hear two ways of giving a height: "descend altitude four thousand feet" and "climb flight level two four zero". They are not two units — they are two different references, and the switch between them is one of the quiet rituals of every flight.

An altimeter is a barometer

An aircraft's altimeter measures air pressure, which falls as you climb, and converts it to feet. But the pressure at sea level changes with the weather: on a low-pressure day the same altimeter reading means a lower real height. So every altimeter has a knob to tell it what pressure to treat as "zero", and what you set there decides what the numbers mean.

• QNH — the local pressure adjusted to mean sea level. With QNH set, the altimeter reads altitude above sea level, which is what charts, terrain and obstacles are measured in. Low down, near the ground, everyone uses QNH.
• QFE — the pressure at the airfield itself, so the altimeter reads height above the runway (zero on touchdown). It is used in a few places and by some military operators; airline flying mostly uses QNH.
• Standard pressure — 1013.25 hPa (29.92 inHg), the same everywhere. With it set, the altimeter reads pressure altitude, given in hundreds of feet as a flight level: FL350 is 35,000 ft on the standard setting.

Why flight levels?

At cruise there is no terrain to worry about; what matters is the distance between aircraft. If every aircraft sets the same standard pressure, two of them at FL350 and FL360 are 1,000 ft apart in practice — whatever the weather — even though neither is exactly at 35,000 or 36,000 ft above the sea. Using local QNH up there would mean resetting altimeters every few minutes as the pressure changes from region to region, and any aircraft that missed a change would be at the wrong height relative to its neighbours.

Transition altitude, level and layer

The changeover happens at fixed heights published for each airspace:

• The transition altitude (TA) is the highest altitude flown on QNH. A climbing aircraft sets 1013 as it passes through it and from then on talks in flight levels.
• The transition level (TL) is the lowest flight level available above the TA. A descending aircraft sets QNH as it passes through it and from then on talks in altitudes.
• Between them is the transition layer, which nobody is cleared to fly level in. Because the TL is a flight level, its real height moves with the pressure: on a low-pressure day the TL is set higher to keep the layer thick enough.

Transition altitudes differ widely. The United States and Canada use 18,000 ft everywhere; across Europe they are much lower, commonly between 3,000 and 7,000 ft depending on the country and airport. Some airports with high terrain or busy arrival flows use high values — Riyadh, one of the airports in this simulator, has a transition altitude of 13,000 ft and a transition level of FL150, so its arrivals are worked in feet much higher than most.

The phraseology tells you which side of the line an instruction is on: below the TL a controller gives "altitude four thousand feet, QNH one zero zero eight"; above it, "flight level one two zero". Readbacks include the QNH, because a wrong pressure setting is a wrong altitude.

The semicircular rule

Cruising levels are also arranged by direction, so that opposite-direction traffic is never at the same level. Under the ICAO table, aircraft on a magnetic track between 000° and 179° fly odd levels — FL310, FL330, FL350 — and those between 180° and 359° fly even levels — FL320, FL340, FL360. Visual flights use the same split plus 500 ft (FL55 eastbound, FL65 westbound). A few states rotate the split to suit their main traffic flows, so it is always checked in the national rules.

RVSM: 1,000 ft at the top

Altimeters lose precision as the air thins, so for decades aircraft above FL290 were separated by 2,000 ft. Reduced Vertical Separation Minima (RVSM), introduced across the world around the turn of the century, cut that to 1,000 ft between FL290 and FL410 for aircraft with approved, more accurate altimetry and autopilots — adding six extra cruising levels where airliners spend most of their time. An RVSM-approved aircraft files the letter W in item 10 of its flight plan.

Cold, low and lower than you think

An altimeter set to QNH assumes a standard atmosphere. In very cold air, or flying from high to low pressure with the old setting still dialled in, the aircraft is lower than the altimeter says — hence the old memory aid "high to low, look out below". Published minimum altitudes are therefore corrected for low temperatures, one more reason the QNH is always part of a descent clearance.

In the simulator

The ATC Radar Simulator uses each airport's real transition level: below it, levels are shown and spoken in feet ("descend altitude four thousand feet"), above it as flight levels ("climb flight level one two zero") — exactly how a real approach controller works the stack. The airport pages list each field's transition level.