[PaulSS]

I think (but my brain is frazzled by an all-night flight) the problem lies with the ISA deviation applying to ISA conditions. Since, in this example, the QNH is not ISA you have to 'reduce' the conditions to ISA, which is what GH's 'column of air' calculation does. Having done that you can then consider the 2 degrees/1000' as a valid means of working out the temperature side of the problem. In other words, you have to make the atmosphere 'Standard' in order to use 'Standard Atmosphere' calculations.

The only very minor mistake I can see is that 0.4% should read "-15*0.004= -0.6%", not "0.04".......even if the written answer is the same

Maybe I just need more sleep

[GolfHotel]

I may be completely wrong here but the question asks “What is the true altitude”.
Surely altitude is distance above sea level, relative to QNH / FL 1013. The airfield elevation is irrelevant, isn’t it?

But it’s only the height of the air that is affected by the change in temperature. So you have to work out the true height and add back the height of the airfield to get the altitude.

If the temperature/pressure of the air above terrain changes with the elevation of the terrain then your altimeter would be jumping up and down as you passed over land.
The temperature changes whether there is granite in it or not.

Air is a fluid and self levels to a certain extent so no way will that happen.

The topic is about an exam/revision question and the methods used. I don’t think anyone believes it will give an accurate answer or even be useful in the real world for finding height.

Possibly the largest source of error in the answer is the assumption that the atmosphere is similar to the ISA. There is also the question the if there is wind then as it blows over the mountains the pressure will drop making the altimeter over read even more.

What it will do is give the pilot who appreciates the issues a healthy understanding that his altimeter set at a flight level may well be overreading by a considerable amount.

And as it will NOT jump about as described there is a substantial risk of hitting mountain tops.

[GolfHotel]

I think (but my brain is frazzled by an all-night flight) the problem lies with the ISA deviation applying to ISA conditions. Since, in this example, the QNH is not ISA you have to 'reduce' the conditions to ISA, which is what GH's 'column of air' calculation does. Having done that you can then consider the 2 degrees/1000' as a valid means of working out the temperature side of the problem. In other words, you have to make the atmosphere 'Standard' in order to use 'Standard Atmosphere' calculations.

The only very minor mistake I can see is that 0.4% should read "-15*0.004= -0.6%", not "0.04".......even if the written answer is the same

Maybe I just need more sleep

You’re spot on that it should be 0.004. My dyslexia cutting in again.
But the 6% is right.

[Crash one]

But it’s only the height of the air that is affected by the change in temperature. So you have to work out the true height and add back the height of the airfield to get the altitude.

If the temperature/pressure of the air above terrain changes with the elevation of the terrain then your altimeter would be jumping up and down as you passed over land.
The temperature changes whether there is granite in it or not.

Air is a fluid and self levels to a certain extent so no way will that happen.

The topic is about an exam/revision question and the methods used. I don’t think anyone believes it will give an accurate answer or even be useful in the real world for finding height.

Possibly the largest source of error in the answer is the assumption that the atmosphere is similar to the ISA. There is also the question the if there is wind then as it blows over the mountains the pressure will drop making the altimeter over read even more.

What it will do is give the pilot who appreciates the issues a healthy understanding that his altimeter set at a flight level may well be overreading by a considerable amount.

And as it will NOT jump about as described there is a substantial risk of hitting mountain tops.

Golf hotel.
We are obviously at cross purposes here.
Please re-read my post. I said “the temperature changes whether there is granite in it or not”.
Meaning therefore there is no need to consider the 1600 feet of airfield elevation.
The only things that need consideration are FL (ISA 1013) and QNH. And temp of -30.

[GolfHotel]

Golf hotel.
We are obviously at cross purposes here.
Please re-read my post. I said “the temperature changes whether there is granite in it or not”.
Meaning therefore there is no need to consider the 1600 feet of airfield elevation.
The only things that need consideration are FL (ISA 1013) and QNH. And temp of -30.

Hi Crash
I don't think we are so much at cross purposes its more like I'm trying to politely point out that my understanding is different to yours. No offense intended but I think your wrong.
The question is from a MET course for the IR I think. The way of working it out I have explained above is that taught in the course and will get the marks.
As I've said above the ground will not expand or contract at the same rate as air. The QNH will have been calculate based on the ISA from the ground level pressure (QFE) of the airfield. We do not know what the actual pressure is at MSL.
One of the things hammered home is it is ONLY the thickness of the air that is adjusted for temperature. It may be the course is incorrect. Sorry if I can't explain it to you but I am convinced it is right.
Regards GH

[Crash one]

Golf hotel.
I can understand what you are getting at but. If the column of air over the land were to shrink or expand due to temperature, it would cause a partial vacuum or pressure rise, this would pull or push air in/out from outside until the balance was restored.
I’m sorry. This may be the IR course method but I just don’t agree with it no offence, let’s not fall out over it.
In the scenario described, there were the variables quoted. FL, QNH, temperature.
The fact that the airfield elevation was quoted, in my opinion, is a red herring.
Just suppose the airfield was perched on top of a volcanic plug. How can that possibly affect the pressure/temperature above the airfield other than by having a bonfire/radiated heat, locally. Even then, the temperature was given -30 ?
If your method gets the correct answer, then fine. But I just don’t see how airfield elevation has anything to do with it. As I said earlier, if terrain elevation were to alter the pressure/temperature then the altimeter would move up and down as you flew level over variable terrain, which doesn’t happen!
I don’t have an IR, so what the hell would I know
Sorry, I just can’t buy it