Air Law question

[JP1]

What do you think the answer is to this question?

In unusually cold air, an altimeter is likely to indicate:

A that the aircraft is higher than it really is
B that the aircraft is lower than it really is
C normally, as they are always temperature compensated
D normally, as the difference caused by temperature variation is insignificant

as asked from AirQuiz.

[bean_ian]

I reckons its A.

[JP1]

And the reasoning is..??

[Islander2]

The pressure altimeter is calibrated to the standard atmosphere. If the ambient temperature is below the standard atmosphere temperature for a particular altitude, the altimeter will over-read. Hence temperature below standard is the dangerous case.

ICAO Doc 8168:

"The calculated minimum safe altitudes/heights must be adjusted when the ambient temperature on the surface is much lower than that predicted by the standard atmosphere. In such conditions, an approximate correction is 4 per cent height increase for every 10degC below standard temperature as measured at the altimeter setting source. This is safe for all altimeter setting source altitudes for temperatures above -15degC."

[Whirlygig]

Warm to cold, don't be bold!

Cheers

Whirls

[Wessex Boy]

High to Low, check Below!
I think B as cold air will be denser and therefore read as higher pressure=lower height

[Islander2]

I think B as cold air will be denser and therefore read as higher pressure=lower heightThat's WRONG, and dangerously so. I would strongly advise some reading around the subject before flying IMC in very cold weather. For starters, try ICAO Doc 8168, as I referenced in post #4.

In measuring pressure, the altimeter is actually reading the weight of the column of air above it. If the temperature is above standard at the altimeter setting source (let's take that as MSL, for example), the column of air from sea level upwards will be expanded compared with ISA ... so at a given altitude there will be more air above the altimeter than in ISA conditions, the altimeter reads a higher pressure and, ergo, under-reads the altitude. For a temperature below ISA at the altimeter setting source, which is the dangerous case, the column of air is shrunk compared with the ISA column, there is less air above the altimeter, the altimeter reads a lower pressure ... ergo it over-reads.

[IO540]

Another way to think about it is this:

When the atmosphere is cold, it is (like most things in life ;) ) thinner, so for a given altimeter reading you end up flying lower down.

This incidentally is one reason why GPS altitude can differ significantly from the altimeter, at reasonable heights e.g. 10,000ft, even if you have exactly the right QNH of a station directly below. The GPS is probably within 100ft but the altimeter might be 500ft out. It doesn't matter in reality because everybody uses the baro figure so inter-aircraft separation is maintained.

[Radix]

............

[Airbus38]

Taken from there, it implicates that the error will be greater when you're flying up higher; is that the correct assumption?

Yes, I guess so.

Difference is 4ft per thousand for every 1*C difference from ISA - that is to say that if at 1000 ft in ISA-10 , error is 40ft, whereas at 10,000ft in ISA-10, error is 400ft.

[Wessex Boy]

Just checked my AFE Air Law (Studying it for my re-take after a 17 year gap from flying) and I think I mis-understood the semantics of the question.
As the temperature decreases, your actual height will decrease, so the Altimeter will over-read as in A Above.
'High to Low, Down you Go'
My reasoning of colder air being denser so the same pressure level is lower in cold air than in warm air was correct.

[bookworm]

Taken from there, it implicates that the error will be greater when you're flying up higher; is that the correct assumption?

What matters is the height difference between you and the station at which the QNH was measured. So at ISA -20 degC, 1000 ft above a 7000 ft airport, using that airport's altimeter setting will cause the altimeter to overread by just 70 ft, not 500+ ft.

[Radix]

............

[eyeinthesky]

QUOTE
Taken from there, it implicates that the error will be greater when you're flying up higher; is that the correct assumption?
UNQUOTE

Yes, and that is one of the reasons why there used to be 2000ft vertical separation above FL290. This was to ensure that inaccuracies in pressure altimeters, including those introduced by colder than ISA temperatures, were not critical. As you may know, we now use only 1000ft vertical separation above FL290, and part of the certification process for an aircraft to operate in that airspace includes Air Data Computers which compensate for the temperature and ensure that the aircraft really is flying at the correct level.

[IO540]

Surely, if everybody is using barometric altimetry, the separation will be maintained because the errors will be the same.

AIUI, the need for all the extra stuff to do with RVSM (which I am reliably informed can add five or even six figures to the cost of a turboprop) is because normal altimeters have such cr*p repeatibility in such thin air that separation would be compromised.

An air data computer is only a few k.

[maxdrypower]

I have to say , when I did my ppl I used acombo of airquiz and the ppl confuser. I found that every question (that I could remember) on the exam was in the confuser. However the same could not be said of airquiz. So depends on your point of view , if oyu wish to just pass the exams use the confuser , if oyu wish to have an in depth knowledge of your subject beyond the exams then use both , in my humble opinioon the latter is preferable

[JP1]

Thanks for the replies.

In terms of studying for Air law, I think the airquiz site is excellent for learning the subject material. It identifies what you don't know; you can then simply learn the answer.

The argument of not understanding the subject material does not hold. If you identify that you don't know what 2 red spheres hanging from a mask signifies, then identifying that it means gliding is in progress is all that is necessary, there are no underpinning hanging ball theories that must be mastered to apply the knowledge to a practical situation. I agree for other subjects that airquiz might not be the way.

I was looking for an answer based on physics since at first sight I thought the altimeter should under read since cold air is more dense. But I think I have now managed to apply the physics to the problem which seems to fit with the correct answer.

Anyway I think the best that came out of this was the post by whirlygig, that will make be remember the important fact.

[Islander2]

JP1:
I was looking for an explanation based on Physics .Ooops, awfully sorry! If you'd have made that clear, I wouldn't have bothered with the theology-based explanation in post #7 :confused: