QNH, QFE and standard pressure explained

How a pressure altimeter works

Inside the case is a sealed, flexible capsule. As the aircraft climbs, the pressure around it falls, the capsule expands, and a linkage turns the needles. The instrument is really a barometer with a dial marked in feet, calibrated to the International Standard Atmosphere (ISA): 1013.25 hectopascals (hPa) and 15°C at sea level, with pressure and temperature falling at set rates as you go up.

Near the ground, pressure drops by roughly one hectopascal for every 27 ft of climb. The figure grows a little with height, and many pilots round it to 30 ft for mental arithmetic. The trainer uses 27 ft throughout.

The subscale, the small window you adjust with the knob, tells the altimeter which pressure counts as zero. Wind it up by one hectopascal and the needles move up about 27 ft without the aircraft moving at all. That is the whole idea behind the three settings: you choose the datum, and the altimeter measures from it.

QFE, QNH and 1013: three datums

QFE: height above the aerodrome

QFE is the pressure at aerodrome level. With it set the altimeter reads zero on the runway and, in the air, your height above the aerodrome. A 1000 ft circuit reads 1000 ft straight off the dial, with nothing to add up when you are busiest. In the UK it is used in the circuit and the traffic pattern, where the school or aerodrome chooses to.

QNH: altitude above mean sea level

QNH is the aerodrome pressure reduced to sea level. With it set the altimeter reads the aerodrome elevation on the runway and, in the air, your altitude above mean sea level (amsl). Everything on a VFR chart is measured this way: spot heights, maximum elevation figures and the airspace bases given as altitudes. That makes QNH the setting for leaving the circuit and flying en route below the transition altitude.

1013: flight levels

Standard pressure is 1013.25 hPa, set as 1013, or 29.92 inches of mercury (inHg). With it set the altimeter reads pressure altitude, and dropping the last two zeros gives a flight level: 4500 ft on 1013 is FL45. It says nothing reliable about terrain, but every aircraft above the transition altitude uses the same datum, so their vertical separation holds whatever the weather.

The words on the radio follow the setting: “height” with QFE, “altitude” with QNH, “flight level” with 1013. Mixing them up in a level report is an easy way to confuse a controller.

Try it: Open the altimeter settings trainer and pick “On the ground”, then “Downwind in the circuit”, to see the three readings side by side

Transition altitude, transition level and the transition layer

The transition altitude is the altitude at or below which you fly on QNH and report altitudes. The transition level is the lowest flight level available above it. The gap between the two is the transition layer, and nobody cruises in it.

In the UK the transition altitude is 3000 ft across much of the country, with 5000 ft or 6000 ft in or beneath some controlled airspace; the charts and the UK Aeronautical Information Publication (AIP) give the figure for each area. An old proposal to raise it to 18,000 ft did not go ahead. The current airspace modernisation strategy from the Civil Aviation Authority (CAA) aims instead for one transition altitude across UK airspace, giving 6000 ft as an example, but nothing has changed yet. Check the current AIP rather than assume it has changed where you fly.

The transition level moves with the weather. A flight level sits lower on a low-pressure day, so a falling QNH pushes the transition level up to keep the layer thick enough. With a 3000 ft transition altitude and a layer of at least 1000 ft, the trainer’s table gives FL40 at 1013 hPa, FL45 at 1000 hPa and FL50 at 980 hPa. Real transition levels come from the AIP, the controller or the ATIS (the recorded aerodrome information broadcast), so treat the trainer’s numbers as a picture of the principle.

1. Leaving the circuit, set QNH if you flew it on QFE, and check the altimeter now reads your altitude.
2. Climbing through the transition altitude, or when cleared to a flight level, set 1013 and report levels as flight levels.
3. Descending through the transition level, or when cleared to an altitude, set the QNH and report altitudes.
4. Joining the circuit, set QFE if that is the aerodrome’s practice, and check it reads your height.

Each change of setting moves the needles by about 27 ft for every hectopascal the knob turns, with the aircraft exactly where it was. What actually changes is the level the altimeter counts from.

Try it: Pick “Through the transition altitude” in the altimeter settings trainer, then move the QNH slider in free play and watch the transition level change

High to low, look out below: the pressure trap

The altimeter only knows what is in the subscale window. If the pressure where you are is lower than the setting you left in, it over-reads: the aircraft is lower than the dial says. If the pressure is higher, it under-reads and you have more room than you think.

The error is simply the difference in hectopascals times about 27 ft. The trainer’s “1013 left set, low pressure” lesson shows the one that catches people. The QNH is 983, thirty hectopascals below standard, and 1013 is still set from the cruise. The pilot levels at an indicated 2000 ft; the aircraft is really at 1190 ft, 810 ft lower, which is enough to put a hill in the way.

Hot to cold, don’t be bold: the temperature error

No subscale fixes this one. The altimeter assumes the standard temperature profile. In cold air the pressure falls off faster with height, so between the aerodrome and the aircraft there are fewer real feet than the instrument counts. The aircraft is lower than indicated.

A working figure is about 4% of your height above the aerodrome for every 10°C below ISA. The trainer’s “A cold day” lesson sets −25°C, 40° below standard, and shows the gap between where the aircraft is and where the altimeter puts it. It matters most on instrument approaches, where minima close to the ground may need a cold-temperature correction; the ILS approach guide covers the approach side. On a day warmer than standard the error goes the other way.

Where each setting is used in the UK

• QFE: in the circuit and near the aerodrome, where the school or aerodrome uses it. Plenty of UK aerodromes and schools fly the circuit on QNH instead, so follow local practice.
• QNH: departing, en route below the transition altitude, and anywhere near airspace whose base or top is an altitude. The CAA’s infringement guidance treats it as the setting for staying clear of airspace.
• 1013: at and above the transition altitude, when cruising at a flight level.

How it differs in the United States

American pilots set the local altimeter setting in inches of mercury, which is QNH by another name, taken from a station along the route. The transition altitude is fixed at 18,000 ft MSL, and at and above it everyone sets 29.92 inHg and flies flight levels. QFE is not part of normal US practice, so the altimeter reads field elevation on the ground. One inch of mercury is roughly 1000 ft, and the same pressure and cold-temperature warnings apply.