Abnormally Cold Air Effects on Altitude Readings

[aviationluver]

When flying into abnormally cold conditions, such as >10 deg C ISA deviation to the cold side, the true altitude will be lower than the indicated altitude.

Does anyone know why that actually is? My only guess is that it would have to do with the conditions in which the actual pitot-static and altimeters were made. Maybe the cold conditions cause a loss in calibration compared to the conditions that the instruments were manufactured in?

Or maybe with colder air, the air molecules are more unusually compacted?

Thanks.

[G0ULI]

Given that aircraft may find themselves being flown in temperatures ranging from -50°C to +50°C I don't believe that this can be a significant problem or it would have been commented on before.

Slight variations in altitude readings may be due to temperature but all aircraft at a given altitude and temperature would have the same error reading, so the relative heights and separations would remain safe.

Aircraft operating from airfields will generally be supplied with information about local barometric pressure to calibrate the altimeter for landing (QNH/QFE).

It is technically possible that errors can creep in with extreme temperature variations, but at the altitudes where this matters, the pilot should be looking out the window to judge height for landing, not following the altimeter reading into the ground.

[Capt Fathom]

Try This

Let me Goggle that for you!

[casablanca]

As for the why? That is too complicated for my first cup of coffee.
However it is quite noticeable in extreme conditions
At cruise altitude it is not a factor, unless terrain is very high, but yes cold temps affect the altimeter. Normally recommended to apply a correction when below zero....usually about 10% down to -10c, then the correction increases with temp drop.
So if temp -7 and doing an approach with mda of 330 agl, would add 33 feet to baro minimums.

It is also affected by high temps. Often we do the Ils approach at sea level airport at near 50 degree temps. At faf we are at 3000 agl,( actual)but when you look at altimeter it will read much lower like 2750.
In the case of cold weather you are lower than you think.....and a threat to strike terrain so hence the correction, but warm temps can also make your approaches high/ unstable when doing non precision approaches

[Mlambin]

Is it me or we are talking about the volume the air takes while it's cool down or warm up ?

This thread seems so complicated for a so simple thing.

[oggers]

Does anyone know why that actually is? My only guess is that it would have to do with the conditions in which the actual pitot-static and altimeters were made. Maybe the cold conditions cause a loss in calibration compared to the conditions that the instruments were manufactured in?

You are more or less correct. It is simply because the instrument is calibrated to ISA conditions. Therefore the instrument begins to over-read as you go up in colder than ISA conditions.

If you imagine two 'columns of air', one cold and another warm, the colder more dense column will be shorter. Any given pressure level (except at the base of the columns) in the colder column will occur lower down than the same pressure level in the warm column. Hence any time it is colder than ISA the altimeter will over-read as you go above the datum for the pressure setting.

[Johnny F@rt Pants]

I don't believe that this can be a significant problem or it would have been commented on before.

Probably because we've all been making (or should have) the standard corrections that are applied for these conditions. It most certainly is a significant problem if you don't apply the corrrctions.

[aterpster]

Think in terms of a vertical column air between you and the altimeter setting source. Cold air is more dense. Thus, the column compresses with cold temperatures. The longer the column (height above altimeter setting source) and the colder the air, the greater the column is compressed.

Old aviators adage, "Cold or low, look out below."

[Capt Pit Bull]

My word.

My only hope is that those who have said this is not a significant factor don't fly aeroplanes!

Seriously, with modern kit, most instrument errors are barely worse than of academic interest. However, cold weather ops, without the relevant correction, will put you into terrain.

The cause? Rate of change of pressure versus altitude depends on density. Every foot you climb you've got 1 less foot of the column of air above you, so the mass of that column of air decreases. Less mass above you = less weight pushing down per unit area = less pressure

If the air is denser than usual, that 1 foot of climb translates to a greater amount of mass than usual, so the pressure decrease is larger than usual. Thus, as far as the altimeter is concerned, we appear to have climbed further than usual.

So, lets run some numbers.

Lets say you are doing a ski charter into somewhere arse freezingly cold. I've sat on the ramp, close to sea level, at -45C. So ISA -60.

Lets say you are flying inbound to cross a ridge with an elevation of 4,000'

You are flying at 5,000'.

1,000 feet. Pretty safe, yes? A decent margin to deal with any trivial little altimeter errors.

4' per degree of ISA deviation per 1,000'.

4 x 60 x 5 = 1,200 feet.

You are 200' below the ridge rather than 1,000 above it.

Still think its trivial?

[aviationluver]

Thanks for your replies. I understand the errors, I am asking WHY the errors exist. Does the error exist because the altimeter and instrument manufacturers cannot handle large ISA DEV? Is it because cold air is too compressed for most instrumentation?

[Goldenrivett]

I am asking WHY the errors exist.

The atmosphere behaves differently to a sealed container.
The air is not contained by a solid boundary at the top - but is open (only earth's gravity keeps it in place.) The upper boundary can rise and fall with temperature variations whilst the surface pressure remains the same.

Hence any pressure sensing device (Altimeter) would indicate the same pressure (Altitude) at a lower height if the air below the Altimeter is colder than standard. (Air column has shrunk in length).

[Capt Pit Bull]

Thanks for your replies. I understand the errors, I am asking WHY the errors exist. Does the error exist because the altimeter and instrument manufacturers cannot handle large ISA DEV? Is it because cold air is too compressed for most instrumentation?

Read my post above! Rate of change of pressure with true altitude depends on density. ISA minus = denser air than usual = greater change in pressure for a given increase in true altitude.

[FullWings]

Does the error exist because the altimeter and instrument manufacturers cannot handle large ISA DEV?

No. It exists because we define (indicated) altitude in terms of air pressure. The altimeter, if properly calibrated and using the correct datum is indicating the correct altitude according to the definition.

The issue is that unless the atmosphere at that moment is identical to the ISA one, that indicated altitude will be different from the true altitude or elevation. The magnitude and direction of that difference will depend on the constitution of the part of the atmosphere that’s below you.

To put it another way, an indicated foot (or a metre) on a pressure altimeter does not automatically correspond 1:1 to an actual physical foot or metre, unless the atmospheric conditions are exactly the same as the model. This rarely (if ever) happens...

[Broomstick Flier]

Hi chaps,

I am not familiar with cold wx ops, as we do not have them in our network, but I already read a lot about it (just for the sake of learning it) and just wanted to clarify something pertaining ILS approaches:
The way I see it, when flying an ILS the G/S per se is not affected by the temperature, at is is an electronic beam, it will guide me on a 3º path to the runway.
Now comes the doubt, the check altitudes, on the other hand, will be affected by the temperature, and I will expect to cross them lower than mentioned on the chart, even if I am on the G/S? Can I assume that terrain clearance is guaranteed as I am following the G/S?

Thanks!

[Goldenrivett]

Can I assume that terrain clearance is guaranteed as I am following the G/S?

Affirm.

the check altitudes, on the other hand, will be affected by the temperature, and I will expect to cross them lower than mentioned on the chart, even if I am on the G/S?

When on the glide slope your Altimeter will show that you are apparently higher than the check Altitudes.

You have to raise your DA i.a.w. your published tables.

[FullWings]

Now comes the doubt, the check altitudes, on the other hand, will be affected by the temperature, and I will expect to cross them lower than mentioned on the chart, even if I am on the G/S? Can I assume that terrain clearance is guaranteed as I am following the G/S?

A quick rule of thumb: A pressure altimeter will under/over read by c.4% per 10deg C deviation from ISA. That 4% applies to the height of the aircraft over where the pressure setting for the sub scale was derived.

On an ILS at a fixed distance, warmer than ISA, the altimeter will read less than the check altitude/height and colder it will read more.

Edit: crossed with Goldenrivett: I agree.

[Piltdown Man]

You will always end up with a problem if you use a variable medium to measure a fixed one. Height is absolute. Air is not. Neither is metal or water. Probably light has its own problems. So because air pressure is used a measure a distance, the results it generates have to be corrected whenever the air used varies from its standard (calibration) values.

PM

[Broomstick Flier]

Makes sense now Golden and FullWings .. Thanks!

BF

[JammedStab]

A question for the cold weather experts here. If an aircraft is flying above a sea level airport at 5000', is the error the same as an airplane flying 5000 feet above an airport at 10'000 feet. If not, how much difference in error would there be between the two aircraft?

[eckhard]

That's a great question.

The error in indicated altitude arises when the air temperature at a particular altitude differs from the standard ISA model. This temperature difference affects the density and therefore the pressure of the air

The mechanism in the altimeter is constructed assuming a particular rate of change of pressure with altitude. Warmer-than-standard conditions will cause under-reading and colder-than-standard will cause over-reading, which is the dangerous case. This has been well explained in previous posts.

The important point raised by your question is that 'aerodrome QNH' is deliberately adjusted for any variation from standard temperature so that the altimeter will always read correctly when at the aerodrome surface. The error in indicated altitude will only become apparent when the aircraft is above the 'QNH reporting station'. This reporting station is the one providing the QNH, in other words, the aerodrome. The error will be small at low altitudes above the reporting station but can be significant at higher altitudes.

The magnitude of the error depends on the average temperature deviation from ISA of the column of air below the aircraft, or to be precise, the column of air between the aircraft and the reporting station. The error approximates to 4 feet per 1000 ft above the reporting station for every degree C deviation from ISA.

So, to get to your question, the error will be the same as both aircraft are 5000ft above their respective reporting stations. This assumes that the two columns of air have the same average deviation from ISA.

On the other hand, an interesting comparison could be made between two aircraft, the first flying at an indicated 10,000 above an airfield situated at sea-level and the second flying at an indicated 10,000ft above an airfield with an elevation of 5,000ft.

Assuming that both are using the QNH of their respective airfields, and that both are flying in an atmosphere with an identical colder-than-standard deviation from ISA, the aircraft will be at different true altitudes as the height above the QNH reporting station for the first aircraft is twice that for the second aircraft.

Assuming an average temperature deviation of -10C for the air beneath both aircraft, the first will have an error of 4x10x10=400ft whereas the second will have an error of 4x10x5=200ft.