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Learning hub · Meteorology · Lesson 1

Pressure, QNH and the altimeter

Your altimeter does not measure height. It measures air pressure and turns it into a number. Once you see that, QNH, QFE and flight levels make sense.

About 12 minutes, plus the quiz. Part of Meteorology and Instrumentation.

1. The idea

Air pressure falls as you climb. Near sea level it falls by roughly 1 hectopascal (hPa) for every 30 feet (a rule of thumb; the true figure is a little under 30). The altimeter is a sealed capsule that squeezes and stretches with that pressure, and its dial is marked in feet using a fixed, standard model of the atmosphere (the International Standard Atmosphere, ISA). It shows the height at which the standard atmosphere would have the pressure it is feeling.

The real atmosphere is rarely standard. Pressure at sea level changes from day to day and from place to place. So the altimeter needs a reference: a pressure setting you dial into its small window, which tells it where to start counting.

2. The three settings

  • QNH is the pressure at your airfield, adjusted down to mean sea level. Set it, and the altimeter shows altitude above mean sea level. On the ground it reads the airfield elevation. This is what you use for most flying in the circuit and on a navigation flight, because charts show terrain and obstacles above sea level.
  • QFE is the pressure at the airfield itself. Set it, and the altimeter reads zero on the runway and shows height above the airfield. Some schools and countries still use it in the circuit.
  • Standard (1013.25 hPa, often called “one-oh-one-three”) is the same setting for everyone. (In some textbooks QNE is strictly the altitude the altimeter reads on the ground with 1013.25 set, but the FAA handbook and SKYbrary use the word for the setting itself.) The altimeter then shows pressure altitude, and aircraft at high level read it as a flight level: 5,000 ft pressure altitude is FL050. Because everyone uses the same setting, aircraft are separated correctly even if the real pressure differs.

You fly on QNH below the transition altitude and on standard above it. When you descend through the transition level, you switch back to QNH. Between the two lies the transition layer. The transition altitude is different in each country (and sometimes each airport), so find it on the chart or in the AIP.

3. A worked example

An airfield at 340 ft elevation reports QNH 1003 hPa and 25 °C. The standard pressure is 1013.25, so the pressure here is about 10 hPa lower than standard. Ten hPa is about 300 ft:

Pressure altitude = 340 + (1013.25 − 1003) × 30 = 340 + 308 ≈ 650 ft

The airfield is 340 ft above the sea but the air feels like 650 ft of the standard atmosphere. Now the temperature. The standard temperature at 650 ft is about 14 °C and it is 25 °C, so it is 11 °C warmer than standard. Each degree of warmth adds roughly 120 ft to density altitude (a common rule of thumb):

Density altitude = 650 + 120 × 11 ≈ 2,000 ft

The engine and wings behave as if the airfield were 2,000 ft above sea level, not 340. The take-off run is longer and the climb is weaker. Try your own numbers in the calculator.

4. Try it yourself

Work each one out on paper first, then open the answer to compare. The numbers are invented.

  1. An airfield at 220 ft elevation reports QNH 1021 hPa. Using the lesson’s rule of thumb (1013 hPa standard, 30 ft per hPa), what is its approximate pressure altitude? What does it tell you?
    Show the answer

    Pressure altitude = elevation + (1013 − QNH) × 30 = 220 + (1013 − 1021) × 30 = 220 − 240 = about −20 ft.

    High pressure makes the airfield “feel” lower than it is. The air here is like the standard atmosphere at roughly sea level, even though the field is 220 ft above the sea.

  2. You leave an airfield with QNH 1018 set and fly towards an area where the QNH is 1006. You do not touch the altimeter and it shows 2,500 ft. Is your true altitude higher or lower, and by about how much?
    Show the answer

    The pressure fell by 1018 − 1006 = 12 hPa. At about 30 ft per hPa that is 12 × 30 = 360 ft.

    Flying towards lower pressure the altimeter over-reads, so you are lower than it shows: about 2,500 − 360 = 2,140 ft. High to low, look out below. Update the QNH from the nearest airfield or the area setting as you go.

  3. An airfield at 1,000 ft elevation has QNH 1003 hPa and a temperature of 29°C. The standard temperature at the pressure altitude is about 12°C. Find the pressure altitude and the density altitude with the lesson’s rules of thumb (use 1013 hPa as standard).
    Show the answer

    Pressure altitude = 1,000 + (1013 − 1003) × 30 = 1,000 + 300 = 1,300 ft.

    The day is 29 − 12 = 17°C warmer than standard. Each degree adds about 120 ft: 17 × 120 = 2,040 ft.

    Density altitude = 1,300 + 2,040 ≈ 3,300 ft. The engine and the wing behave as if the airfield were more than 2,000 ft higher than it really is, so expect a longer take-off run and a weaker climb.

5. Watch

A second explanation from another instructor often makes it click. This video covers QNH, QFE and standard settings.

Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.

6. When the altimeter misleads you

  • Flying towards lower pressure without resetting. The altimeter over-reads, so you are lower than it shows. Remember: high to low, look out below. Update QNH from the nearest airfield or the area setting as you go.
  • Flying into colder air. Cold air is denser, pressure falls faster with height, and the altimeter again over-reads. Be careful over mountains in winter.
  • Wrong setting. Forgetting to change from standard to QNH when you descend can put you hundreds of feet from where you think you are. A pressure difference of 10 hPa is about 300 ft.

Before any real flight, get the current QNH for your departure, route and destination, and add a safety margin above terrain and obstacles.

More to watch

A short second explanation of the altimeter settings and of when you change from QNH to standard and back, which goes with sections 2 and 3 of this lesson.

Open on YouTube if the player does not load. Not made by VectorEFB.

7. Check yourself

Five original practice questions. Answer them, press Check answers and read each explanation.

1. You land with the correct QNH set. What does the altimeter read?

QNH makes the altimeter show altitude above mean sea level, so on the ground it shows the airfield elevation. Zero on the runway is what QFE gives.

2. You fly towards an area of lower pressure and do not change the altimeter setting. Compared with what the altimeter shows, your true altitude is:

The pressure around you is lower than the setting assumes, so the altimeter shows more height than you really have. It over-reads and you are lower than it says: high to low, look out below.

3. An airfield at 500 ft has QNH 993 hPa. What is its approximate pressure altitude?

Pressure altitude = 500 + (1013 − 993) × 30 = 500 + 600 = 1,100 ft. Low pressure means the airfield “feels” higher.

4. Above the transition altitude the altimeter is set to:

Above the transition altitude everyone uses the standard setting, so levels are flight levels and aircraft are separated correctly.

5. A hot day raises the density altitude of your airfield. What do you expect?

Thin air gives the wing and engine less to work with. Take-off run and climb performance both get worse as density altitude rises.

Next: density altitude and take-off performance. Back to the learning hub.

For training and situational awareness only. Not certified for navigation. Always use official charts, the AIP, NOTAMs and a proper weather briefing for real flights.