How can I calculate QNH from QFE?

Can someone please explain in simple terms how to work out your QNH from QFE?

For example, you take off at 0' but leave the circuit, what do you set the altimeter to? How do you know the QNH?

• If you have it set to QFE, turn the knob to add the field elevation to the current altitude. Or set it to the field elevation on the ground. (Or get the QNH from a nearby ATIS.) :) – falstro Dec 30 '16 at 21:21
• I found this googling helpful, especially this rule of thumb: "Divide airport altitude (in feet) by 30 feet. Add resultant number of millibars onto the QFE that you are given. Then you have QNH.". But please (!) wait for more profound answers, this was just what I found at Google. – PerlDuck Dec 30 '16 at 21:27
• Appreciate your help guys, i've googled and confused myself silly. – user13555 Dec 30 '16 at 21:39
• @user6035379 I think it's pretty much anywhere outside north america. Though QFE is very rarely used (I think the UK uses it more than others, it's also fairly common with gliders) – falstro Dec 31 '16 at 11:55
• @user6035379 Very common in the UK when operating within the airfield zone. It makes flying circuit height etc very easy. 1000 on the altimeter, job done. We switch to QNH once we transition to en-route or otherwise leave the zone. – Simon Dec 31 '16 at 13:06

3 Answers

In metric:

1 milibar is approximately equal to 30 feet. This calculation is therefore also approximate but is good for airfield elevations to several hundred feet since we round to the nearest millibar.

Divide the airfield altitude in feet by 30 to get the number of millibars above MSL. Add this to the QFE to get QNH or subtract it from QNH to get QFE.

For example, the airfield elevation is 200 feet. Dividing by 30 gives us 6.66r. The QFE is 1023. Add 6.66 to get 1029.66 and round up to 1030 millibars, which is the QNH.

In imperial:

1 inch mercury is approximately equal to 900 feet. This calculation is therefore also approximate but is good for airfield elevations to several hundred feet since we round to the nearest hundredth inches.

Divide the airfield altitude in feet by 900 to get the number of inches above MSL. Add this to the QFE to get QNH or subtract it from QNH to get QFE.

For example, the airfield elevation is 300 feet. Diving by 900 gives us 0.33r. The QFE is 30.12. Add 0.33 to get 30.45 which is the QNH.

If the airfield elevation is below sea level, subtract rather than add and vice versa.

I just read about QNH and QFE so I thought I’d throw this out there to start the discussion. We don’t use QFE in the US but I think my logic is correct.

From the FAA Instrument Procedures Handbook

Barometric Pressure for Local Altimeter Setting (QNH)

A local altimeter setting equivalent to the barometric pressure measured at an airport altimeter datum and corrected to sea level pressure. At the airport altimeter datum, an altimeter set to QNH indicates airport elevation above mean sea level (MSL). Altimeters are set to QNH while operating at and below the transition altitude and below the transition level.

For flights in the vicinity of airports, express the vertical position of aircraft in terms of QNH or QFE at or below the transition altitude and in terms of QNE at or above the transition level. While passing through the transition layer, express vertical position in terms of FLs when ascending and in terms of altitudes when descending.

When an aircraft that receives a clearance as number one to land completes its approach using QFE, express the vertical position of the aircraft in terms of height above the airport elevation during that portion of its flight for which you may use QFE.

note that transition level is when you set the altimeter to a standard value. It varies by country. In the US it is 18,000' and 29.92 inches. Not relevant to this discussion.

In the US we don’t use QFE, but as a comment indicates, you would probably get that from the ATIS. If no ATIS is available, to convert QNH to QFE you would move the altimeter so that you decrease your altitude by the field elevation. Here’s an example:

My field elevation is 212'. If I set the altimeter to QNH it will show 212' as the altitude. To get QFE I need to change the altitude to 0'. In other words whatever altitude is showing on the altimeter, move the knob to make it show 212' less.

Note that when you turn the altimeter knob the altitude goes in the same direction as the pressure setting in the Kollsman window. In the US we use inches of mercury but the movement is the same: As an example, right now the altimeter setting at KSBP is 30.08. The setting at KSBA (62 nm away) is 30.01. If you fly to KSBA you would notice a change in altitude of -70 feet when you get the new altimeter setting from approach control.

• "start the discussion" and "I think" are not good phrases to use in a Q&A site. Otherwise, good answer. You might consider an edit? – Simon Dec 31 '16 at 11:33

From google of 'how to calculate QNH from QFE: Divide airport altitude (in feet) by 30 feet. Add resultant number of millibars onto the QFE that you are given. Then you have QNH.