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Moderator: AndyR

#593785
I'm currently revising for my air law exam by taking the Air Quiz online exams to highlight gaps in my knowledge. However, there is one question on temperature effects on altimeters which either Im not understanding right, or the air quiz answer is wrong.

The question is

Which of the following statements concerning altimeters is true?
A In air colder than ISA, an altimeter will under-read the true altitude
B In air colder than ISA, an altimeter will over-read the true altitude
C In air hotter than ISA, an altimeter will over-read the true altitude
D Normal temperature variations do not affect altimeter accuracy

My logic is, colder air = denser = higher pressure. The effect of this is that the altimeter thinks it is at a lower altitude (where it would normally be that higher pressure). The altimeter is therefore under-reading the true altitude. You are flying higher than what is being indicated.

The air quiz answer is telling me it is B, the altimeter is over-reading the true altitude.

Am I right or is Air Quiz right?

If I am wrong, can someone please correct my logic so I have it square in my head!

Thanks

Mark
User avatar
By AFKAE
#593786
Air Quiz is right.

You can use "high to low look out below" for pressure and temperature.
User avatar
By mo0g
#593795
AFKAE wrote:Air Quiz is right.

You can use "high to low look out below" for pressure and temperature.


I can remember that no problem, but I'd still like to understand the logic in terms of the altimeter here. :?

high to low pressure makes sense, but lower temps = higher pressure, so even that statement is contradictory?!

As I said, I can just take it as it is for the purposes of the exam, but I'd prefer to understand why/how too.
User avatar
By mo0g
#593797
I guess I need to go off and find out how barometers work, I suspect that will sort me out!
User avatar
By JonathanB
FLYER Club Member  FLYER Club Member
#593808
A column of cold air will be shorter than the same column of warm air.

The pressure levels in the cold air will therefore be pushed closer together and lower than the same pressure levels in the warm air.

Therefore the altimeter (which is essentially displaying pressure) will read a higher value than you're actually at in the cold air as the pressure level is lower than that in ISA.

I can understand your confusion with the density as I had similar problems with this, I think the key is thinking that the weight of the air above is what gives you pressure, and with cold air that column of air is shorter, pushing the pressure levels lower.
User avatar
By Pianorak
#593820
All of the above.

An altimeter is [b]an instrument which measures static pressure and in ISA conditions[/b] converts that pressure into an altitude value.
Remember that ISA (a purely theoretical model) is +15°C at MSL. Assuming you fly in a warm air mass of 19°C and from there to a colder air mass of let‘s say -1°C but right now your are flying in an air mass of 9°C and your altimeter indicates 3,000 ft.
[b]ISA[/b] air pressure, density and temperature values [b]all[/b] decrease as height is gained. (leaving aside inversions etc.).
In the above example you will be flying higher in the warm air, ie at 3120 ft and lower in the cold air, ie at 2880 ft.
User avatar
By Charles Hunt
#593872
Fascinating how we all see things differently.

At the standard atmosphere, I see all the molecules in the air banging against the bladder/bellows (or whatever it is) and this being converted into an altimeter reading.

OK, now as we cool the air, you get more molecules inside the bladder/bellows, each one pushing against the bladder and so the altimeter reading goes up; ie over-reading.

( The trouble with this logic (?) is that at a lower temperature each molecule will have less energy, so will not push as hard on the bladder, thereby reducing pressure - presumably this is the lesser of the two effects)

Now try another route.

Answer D is incorrect.
If answer A is correct, then C must also be correct: colder-under, hotter-over.

If only one answer is correct, and we have discounted D. B must be correct, never mind what it says.

Charles
User avatar
By Charles Hunt
#593987
This is theory - it's the practice I need help with!

At the moment I can no more land a 'plane than fly.

Charles
User avatar
By KNT754G
#594000
[quote='Charles Hunt']Fascinating how we all see things differently.

At the standard atmosphere, I see all the molecules in the air banging against the bladder/bellows (or whatever it is) and this being converted into an altimeter reading.

OK, now as we cool the air, you get more molecules inside the bladder/bellows, each one pushing against the bladder and so the altimeter reading goes up; ie over-reading.

Charles[/quote]

Charles, you were on the right lines but your logic fouled up.

Cooler air = more molecules banging against the bellows/capsule correct.
Hence LOWER READING because the bellows/bladder/capsule thinks the pressure is HIGHER, hence LOWER altitude.

Cold air - altimeter under reading.

Think of it another way, very cold air = likelihood of catching assorted nasty winter diseases hence unsafe.

Very sold air also = altimeter under reading and aircraft lower than actual and likelihood of catching cumulogranitus hence unsafe...
User avatar
By Charles Hunt
#594018
Thanks for explanation - initialy I was thinking of molecules inside the bellows, but in an altimeter the changeable pressure is outside the bellows - back to the text books!

On a practical note, - winter weather likely to catch a cold - is the sort of thing that stands some chance of sticking in my brain!

Thanks again

Charles
User avatar
By Charles Hunt
#594035
Aaaaaarghhhhh!!!!!

Having gone back to the text book, I fear I may have confused myself, and anyone else following this thread.

Let me go back to square one.

Inside the altimeter, there is an expandable capsule containing a fixed amount of air.

At ground level there is high pressure on the capsule, giving a low altitude reading.

As you climb higher, the pressure on the capsule reduces giving a higher altitude reading.

At lower temperatures we now have more dense air inside the altimeter pushing on the bellows, which would give a lower altitude which is wrong!!!

I'm going to lie down for a bit and maybe try again later.......

Charles
By Jowloo
#594052
Methinks JB has recently done his ATPL :wink:

I always get myself into a muddle with altimeters and it gets worse at ATPL level because they deliberately over-complicate the questions to try to confuse you. With me, they usual succeed. I end of foaming at the mouth surrounded by men in white coats.

Hint 1: Draw yourself a little diagram. That helps to understand it in your mind's eye rather than in words.
User avatar
By JonathanB
FLYER Club Member  FLYER Club Member
#594062
KNT... not sure you have it quite right.

When you fly into [b[bolder air[/b], you will be at a lower [i]true[/i] altitude if you maintain the same [i]indicated[/i] altitude. This means in air temperatures lower than ISA, you will be lower than the indicated altitude and therefore the altimeter is [b]overreading[/b]. The display shows you as higher than you actually are.

Yes cold air is denser, but this means that it sinks more dragging the pressure levels down and pushing them closer together. A column of the same mass of cold air is not as tall as the equivalent mass of warm air. Static pressure is based on the weight of air above you so as the denser cooler air sinks, at the same [i]true[/i] altitude there is less air above you in a cooler air mass. You only see an indicated altitude though and will therefore descend as you fly a constant indicated altitude into cooler air.

[b]In air colder than ISA, an altimeter will over-read the true altitude[/b]
User avatar
By JonathanB
FLYER Club Member  FLYER Club Member
#594064
KNT, just re-read you definitely have it back to front.

In terms of terrain clearance...

An under-reading altimeter is safe - if it indicates lower than you actually are then you're flying higher than it indicates.

An over-reading altimeter is unsafe - it indicates higher than you actually are so you have less clearance from terrain than you think.