Temperature correction to altimeter?

[Allan]

Can anyone explain this to me?

We had a major discussion today regarding the correction you shall apply to the altimeter when the temperature is below standard, i.e. 4%/10 degree below. Should this correction be taken from the altitude or height listed on the approachplate, e.g. OM crossing altitude 1400' but OM crossing height only 500'?
And what about an MSA of 14000', if ATC informs you that temperature at the airport is 10 degrees below standard, do you take 4% of the 14000 feet, or only the assumed height?

please clear this up for us :O

[FlyingForFun]

AIUI, you take your height above the airfield whose QNH or QFE you are using.

The QFE is the pressure at the airfield - when you're on the airfield, the pressure will equal the QFE, and the altimeter will read zero. As you gain height, the pressure will drop off slower than ISA due to the low temperature, so you need to add an amount to the indicated altitude proportional to the height you are above the airfield.

Likewise, QNH works in exactly the same way. It is the pressure at sea-level, corrected for the ambient temperature such that the altimeter will read airfield elevation when on the airfield. Therefore, it is the height above the airfield that determines how much correction you need to apply to the indicated altitude.

I think.

FFF
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[Crossunder]

You think, but I know! You're absolutely 100% correct!
If an aerodrome is situated 2.000ft AMSL and you're supposed to correct a step-down altitude of, say,
4.000ft, you'll only have to correct the 2.000ft AAL. Thus, a 10% correction would mean +200ft - not +400ft!
Try looking at your RAD ALT next time you fly into an AD at high PA (assuming the surrounding terrain is flat...) and you'll see for yourself :-)

[FlyingForFun]

You're absolutely 100% correct!That's very reassuring! But not quite true.

I said: "As you gain height, the pressure will drop off slower than ISA due to the low temperature." Of course what I meant was "the pressure will drop off faster than ISA."

FFF
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[Apollo 100]

Allan, the correction is applied to the height above the aerodrome elevation. I'll give you an example

Aerodrome elevation 700ft, QNH 1003, temp -20degC
Altitude given on plate at OM is 1900ft. Ques. what is your true height at the OM?

Firstly work out the Pressure Alt at the airfield (1000ft)
Get your whiz wheel and put 1000ft against -20deg in the "True Altitude" window On the inner scale read your indicated height ABOVE the aerodrome elevation (1200ft ... not 1900ft) and read off against the true height on the outer scale (I make it about 1060ft) add this to the elevation of 700ft to give you an accurate OM crossing altitude of 1760ft ie 140ft low.

Any ISA deviation colder than ISA will cause the altimeter to overread (=dangerous)

The same technique can be used to calculate true altitude anywhere but remember it is only the bit of air between the ground and your aircraft that you are correcting for, because by the time you reach the runway there will be NO error (assuming the met man got his QNH correct!)

Hope this helps

Apollo:O

[bookworm]

If you look at the altimeter as a measuring instrument, you set its reference point by adjusting the subscale to make it read the altitude at a particular station. That is then the QNH or altimeter setting for the station, and any properly calibrated altimeter will also read the station elevation with that QNH set.

As you move away from the station elevation, the temperature of the air between station elevation and your altitude starts to affect the accuracy of the altimeter. So what matters is the vertical distance between the station at which the QNH was determined and the altitude in question.

For almost all practical purposes, the QNH will be determined at the airport elevation at which you are about to land, so it's height aal that's important. I suppose there might be a case where you're using a remote altimeter setting for an airport at a different elevation, but the lateral barometric differences are likely to be as great as the vertical temperature errors.