Cold temperature correction for true altitude

[AviationWeakestlink]

Please help me understand why we correct for cold temps in the air and not on the ground. I understand that altimeter is calibrated for close to isa temps and when its colder then isa small altimeter error will creep in. Generally 4% per 10c
rule of thumb formula gives 4 x isa deviation x altitude (agl) will give you needed correction. But how come we dont correct it on the ground ?
Assuming field elevation of 3000ft and -30c we should correct for approx. 432ft (4 x -36(isa deviation) x 3(altitude)) = 432
All the temps correction tables provide correction only for height above elevation ....but why ? Altimeter doesnt know if its on the ground or flying at 3000ft
What am I missing here ?
Thank you in advance for help !

[Piltdown Man]

You are correcting for air between the aircraft and the ground. On the ground, the altimeter setting you are passed will result in your altimeter reading airfield elevation. The problem comes when the air is appreciably colder than ISA. On a day when the air is say ISA-15 and wish to you raise your aircraft 200 feet above the ground, your altimeter will not read 200 feet. It will be showing that you are higher than 200 feet. It will over read, in fact it will read about 220 feet (a typical DA for a CAT I ILS). So not correcting approach minimums on colder than ISA days will result in you busting your DA/MDAs. Possibly a bigger nasty comes when flying in a mountainous region when thousands of feet of air has to be corrected, like when over the Rockies or the Alps. Then it would be nice to know your real altitude and not your indicated one. Also, remember you should also correct acceleration altitudes when departing.

Does it make sense now?

PM

[oggers]

All the temps correction tables provide correction only for height above elevation ....but why ? Altimeter doesnt know if its on the ground or flying at 3000ft
What am I missing here ?

The altimeter setting provided has already been adjusted so that there will be zero error (nominally) at airfield elevation. Therefore you only correct for height above airfield as opposed to your true altitude above sea level.

[eckhard]

rule of thumb formula gives 4 x isa deviation x altitude (agl) will give you needed correction. But how come we dont correct it on the ground ?

The answer is right there in the rule of thumb. You are on the ground, therefore the correction equals 4 x ISA Dev x 0 which equals zero.


Let's do a little thought experiment:

Your airport ABC is on top of a 3000ft cliff. At the bottom of the cliff is another airport, XYZ which is at sea level.

The OAT at ABC is -30C. Assume that ISA Dev is constant, therefore the OAT at XYZ (at sea level) is -24C.

Assume the QNH at XYZ is 1013hPa.

What will be the QFE at ABC?

In a standard atmosphere, the pressure reduces by approx 1hPa for every 27ft elevation.

Therefore, there should be 3000/27 = 111hPa difference between the armpospheric pressure at XYZ and ABC.

So, the QFE at ABC should be 902hPa.

Imagine your surprise when you set 902 hPa on the sub-scale of an altimeter fitted to an aircraft in the hangar at ABC and find that it reads 468ft instead of zero. Why is this?

Because of the cold temperature, the atmosphere 'shrinks' and the distance between successive hPa surfaces reduces from 27ft to a little over 23ft. There are now 128hPa difference between sea level and 3000ft elevation, so the QFE at ABC is actually 885hPa.

Now let's imagine that a pilot takes off from XYZ, planning to climb to the top of the cliff and land at ABC. He likes to use QNH for take-off and landing. He knows that ABC is only a few miles from XYZ, so assumes that the QNH must be the same, as there are very few isobars on the weather chart. He sets his altimeter to 1013. It reads zero as he takes off from XYZ.

The weather is CAVOK so as he levels at 3000ft and flies towards the cliff. Something is wrong. He is below the top of the cliff! He climbs until he is just level with the runway and lands, somewhat shaken by the experience. Imagine his surprise when he sees that his altimeter is now reading 3468ft!

He asks ATC at ABC for the 'aerodrome QNH' and they reply, "996hPa".
When he sets 996 on his sub-scale, the altimeter duly reads 3000ft.

Why is the QNH so different at two places that are so close together, when there are no isobars on the weather chart?

The answer is that 1013hPa is not the QNH, but the QFF. That is the value of surface pressure, reduced to an equivalent seal level reading, using actual temperature deviations from ISA. The isobars on the chart are lines of constant QFF.

To calculate the QNH to transmit to a landing aircraft, the met man at ABC reads the actual QFE (885hPa) and reduces this to an equivalent sea level value by using ISA temperature deviations. So, he adds 111hPa to 885 and calculates that the QNH at ABC is 996hPa.

Why does he use ISA deviations? Because the mechanism in the altimeter also assumes the ISA model, so the data is consistent.

[AviationWeakestlink]

First of all great answers from you and I cant express enough how much I appreciate yout time and effort in providing me helpful answers.

I understand now that QNH is not really QNH solely based on pressure but it also includes and is adjusted by temperature to provide correct elevation. This was the problem...I always thought that QNH is only function of pressure.

Thank you once again !