300ft AMSL means 300ft above metric sea level.

Metric sea level is a constant.

Elevation is a constant.

The height of the mast above ground is constant.

The mast will always be 300ft above metric sea level.

The atmospheric pressure may change every day, the only effect this will have will be to change the QNH....i.e. changing the sub scale you have to dial into the altimeter to get the correct readings.

Altitude is relative to MSL. MSL is constant. Pressure changes. So you have to change the sub scale to 'restore' the correct relationship between altitude and MSL.

Excellent explanation from tech log.

To elaborate along the line of thought down which I believe Sanger was headed:

If the elevation of Sywell is shown as 424ft amsl, how do they know it will be 424ft when the pressure fluctuates each day?

Day 1.

The QNH is 1000mb. The aircraft is parked on the airfield. The altimeter sub-scale is set to 1000mb. The altimeter reads 424ft.

Day 2.

The QNH has changed overnight and it is now 1010mb. The same altimeter will now read 124ft. Has the elevation of Sywell changed? No, of course it hasn't. If you reset the altimeter sub-scale to 1010mb, the correct indication of 424ft will be shown.

Day 3.

Michael Fish is on duty and an intense low pressure system has moved across Northamptonshire. The altimeter from Day 2 now reads 724ft. What is the current QNH?

Pedant mode [ON]
AMSL = Above Mean Sea Level
[OFF]

Where the bloody hell have I gotten 'Metric' from.

Slap on the hand for that one!

Well, it was still a good explanation!

Coming back to the original question.

(1) Technically correct answer: subtract (airfield elevation / 27) from the QNH.

(2) Most sensible answer: ask the tower at your destination on RT what QFE is.

(3) Pragmatic answer, particularly if there's no tower: leave the altimeter on your last QNH. Mentally just add the airfield elevation to circuit height (or any other critical height) and fly to that.


Most countries don't use QFE at-all, it's become a bit of a British anachronism (although personally I quite like it), so (3) is actually the right answer in most of the world, most of the time and should not get you in trouble anywhere if you choose to do it that way.

Sorry i'm not sure how to work this?

Sorry to be a pain, Anyone help out with the above?

Hi Sanger, I'll have a go at explaining if it helps.

Consider the altimeter a barometer, that's all it is. It senses pressure wherever it is located. To get a readout from it, you need to tell it where the ground is. You do this by telling it the pressure on the ground, either at the airfield (QFE) or at sea level (QNH). It then works out the difference between the two and gives a readout of approx 30ft for every 1 mb.

You must not lie to the altimeter about where the ground is, or the readout it generates will be incorrect.

If on Day 2, the pressure on the ground at sea level has changed to 1010mb, but the subscale is still set to 1000mb. You are now lying to the altimeter. The altimeter still thinks sea level is at 1000mb. But it is actually 10mb further down in the atmosphere. 30ft x 10mb = 300ft therefore the sea level is 300ft further down than your altimeter thinks it is. It therefore under-reads by 300ft and shows 124ft.

On Day 3, the opposite has happened, the pressure has changed again and we are once again lying to the altimeter. We know that it is over reading this time by 300ft (724ft-424ft), therefore it must think the sea level is further away that it actually is. It is out by 300ft/30ft=10mb, and thinks sea level is at 1000mb, so the actual sea level pressure must be at 990mb, some 10mb further up in the atmosphere.

Hope this hasn't confused you further - you may find it useful to look at some diagrams on this if you can get hold of any, it may be easier to visualise.