Aircraft Altimeter

There are two types of pressure settings that are used in flying and you’ll hear them whenever you enter new airspace – QNH and QFE.

But what do these mean? And what’s the difference?

Understanding the abbreviations will help!

– QNH = Query NAUTICAL Height – so think SEA level.

– QFE = Query FIELD Elevation – so think above the FIELD or Aerodrome.

These are two of many ‘Aeronautical Q codes‘ that were devised during the days of morse code to standardise radiotelephony practices and to keep transmissions shorter.

If you set your altimeter to the given QFE when you are at ground level, it will show ZERO (Aerodrome height) If you set it to QNH it will show your altitude above sea level, sometimes hundreds of feet.

So why use two types?

Good question! Pressure changes within regions, and it’s important to know the pressure setting you are flying with so your altimeter reads the same elevation as others around you. ATC will keep all pilots constantly updated when pressures change.

QNH = The pressure measured at station then reduced down to mean sea level pressure. When set on your altimeter it will read your ALTITUDE. Sat on the tarmac at your airfield the altimeter will display the airfields elevation above mean sea level.

This is the most commonly used pressure setting in the commercial world. Its probably the most useful setting to have, as nearly all aviation references to elevation are in relation to mean sea level. The mountain peaks on a map, airfield elevation, target elevation, minimum safe altitudes enroute etc. Incidently, QNH is given as a regional pressure setting and should be updated with new ones if you leave its area of reference into a new QNH pressure region. The QNH is the LOWEST FORECAST pressure at mean sea level for a given day to ensure that safe terrain seperation is maintained regardless of the days variation in pressure.

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QFE = Is mean sea level pressure corrected for temperature, adjusted for a specific site or datum like an airfield, being the most obvious example. When this is set on your altimeter, it will read your HEIGHT not altitude. It will read zero at airfield elevation and after take off will read your HEIGHT above that specific airfield. If you fly to another airfield of different elevation and/or different QFE pressure, you will have to ensure you reset that particular airfields QFE if you want your altimeter to read zero on touchdown.

QFE is very good for new pilots who are remaining in the circuit around an airfield and keeps things simple for that task.

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QFE Example: Airfield A with elevation 250ft above mean sea level. Airfield B elevation 300ft AMSL. A to B = 10miles. Assuming a uniform atmospheric pressure in the region.

Take off from A, altimeter reads 0ft on runway and after take-off reads HEIGHT above airfield A. Go and land at B and your altimeter will read 50ft on the runway. This is because B’s HEIGHT is 50ft higher then A.

In this example, if we set the regional QNH, then the altimeter will read ALTITUDE and therefore the airfields altitude AMSL. Airfield A, altimeter will read 250ft. Airfield B will read 300ft. This is why QNH is the primary pressure setting used in aviation at lower levels. It is far simpler working in a setting that gives ALTITUDE, so you can reference your vertical position from everything on a map or chart. (All airfield plates (charts) have their altitudes AMSL on the plate.)

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This is all good and well knowing that QNH is the best pressure setting to use in a region for vertical situational awareness. But it is not always possible to get the regional pressure setting QNH from accurate means and a reliable network of meteo stations. Remote airfields and isolated combat zones are just 2 examples where it’d be difficult to get an accurate QNH when you dont have access to good forecasts and numerous pressure sensing stations.

If pressure info isn’t available then you can get QFE easily by selecting an altimeter setting that reads zero on the airfield. The number in the altimeter pressure window is your QFE.

To get QNH, you just need to know your elevation AMSL and set that in your altimeter. Airfield elevation = 250ft. Set altimeter to read 250ft. Pressure in the altimeter pressure window shows your QNH. (You have to remember that this wont be the lowest forecast QNH pressure for the day and just be cautious at low level. But thats why a radio altimeter is handy!)

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There are other Q codes used for aviation pressure settings

QNE = the Internation Standard Atmosphere (ISA). It is the average mean sea level pressure around the globe. It is planet earths mean atmospheric pressure at sea level basically. This pressure setting is refered to as STANDARD in aviation. STANDARD is set from QNH when climbing up through the “Transition Level”. Your altimeter will then read your FLIGHT LEVEL. A reading of 25,000ft is FL250. 5,000ft = FL050. 13,500ft = FL135.

Part of the above information has been taken from this blog as it summarises the information better than I ever could.

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