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Altitude and Elevation

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Combrinck created the topic: Altitude and Elevation

Hi, thanks for your help.

This is my understanding - help me where I’ve gone wrong please:

If QNH is say 983 - and is set for 983 - for an aerodrome that is at sea level, the Altimeter should read 900 Ft while the aeroplane is still on the ground. In other words, if the plane takes off and reaches 100 Ft AGL the Altimeter should read “1000” Ft.
As the plane climbs to say 1100 Ft AGL, the Altimeter should read “2000” Ft. Thus, an altitude of 2000 Ft.
Now, say this aeroplane is flying Night VFR and needs to clear terrain or an obstacle that is at an peak elevation of 1500 Ft AMSL, the Altimeter will read the 2000 Ft (as noted above) and it will be assumed that there will be a ‘clearance’ of 500 Ft above the said terrain (1500 AMSL), whereas in reality it will not be clear of terrain. In reality it will be 400 Ft short of practical clearance and 900 Ft short of permitted clearance, because while the Altimeter will read “2000” Ft, the plane will only be at 1100 above actual sea level which will not be sufficient to clear the terrain at 1500 Ft above sea level.

The Altimeter - because of the very low QNH setting - will vastly overstate the plane’s clearance over the ground, and the plane should impact terrain.

Where have I gone wrong?
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John.Heddles replied the topic: Altitude and Elevation

I think that you might need to go back to the text books and review basic altimetry.

Your calculation has figured out the pressure height, which is quite irrelevant to your subsequent discussion

If the QNH is whatever, then that pressure is the pressure surface at MSL. If set on the subscale, the altimeter will read zero for an aircraft sitting on the ground at a sea level aerodrome.

Make the following changes in your initial text and it reads OK -

If QNH is say 983 - and the subscale is set for 1013 - for an aerodrome that is at sea level, the Altimeter should read 900 Ft while the aeroplane is still on the ground. In other words, if the plane takes off and reaches 100 Ft AGL the Altimeter should read “1000” Ft.
As the plane climbs to say 1100 Ft AGL, the Altimeter should read “2000” Ft. Thus,
a pressure height of 2000 Ft.

You then need to make a few minor changes to suit in the remaining text.

Engineering specialist in aircraft performance and weight control.
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Combrinck replied the topic: Altitude and Elevation

I see my error - thank you. So;

If indeed QNH on a particular day is 983, and the subscale is set on 983, then the Altimeter for the plane (on the ground at sea level) should read “0” Ft.
Thus, if the plane then ascends to 2000 Ft on the Altimeter it will indeed be 2000 Ft AMSL, and it should clear an obstacle at 1500 Ft AMSL.

Thanks.
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John.Heddles replied the topic: Altitude and Elevation

That sounds about right to me ...

Although it may sound a bit like nit-picking, we need to keep in mind that the altimeter is a barometer - not a tape measure. The "height" indication on the dial is only true if the atmosphere on the day is the same as the atmosphere assumed (ISA) for the design and calibration of the altimeter. If the atmosphere is a bit off-key (as it usually is) then there will be an error to a lesser or greater extent depending on the variation from standard conditions.

Engineering specialist in aircraft performance and weight control.
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Combrinck replied the topic: Altitude and Elevation

Yes, thank you.
So, in correctly inputting the correct QNH on the subscale for that aerodrome before take-off, the Altimeter mainscale on the ground will always read "0" Ft (noting small errors at times).
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John.Heddles replied the topic: Altitude and Elevation

So, in correctly inputting the correct QNH on the subscale for that aerodrome before take-off, the Altimeter mainscale on the ground will always read "0" Ft (noting small errors at times).

Perhaps if you were to reword your statement to read something like -

So, in correctly inputting the correct QNH on the subscale for that aerodrome before take-off, the Altimeter mainscale on the ground will always read the elevation of the aerodrome, which will be "0" Ft if the aerodrome is at sea level (noting small errors at times)

Engineering specialist in aircraft performance and weight control.
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Combrinck replied the topic: Altitude and Elevation

Yes, thank you. That is a more complete statement.

Appreciate the clarification!
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