Learning to fly, or thinking of learning? Post your questions, comments and experiences here

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By Uzayr
#1687290
Hello,

Anyone who can please assist with a good explanation of why at higher temperature aircraft appears higher than it is, and vice versa in cold temperatures.

Thanks
By MachFlyer
#1687292
Warmer air is less dense so an altimeter will under read, if flying towards colder air it will over read, same with pressure, high to low watch out below.

Be aware I’m only a student but have very recently taken these exams so I “think” this is correct but happy to be corrected of course :)
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By Rob P
#1687305
I vaguely recall that it's something to do with the atoms / molecules getting more 'excitable' with the rise in temperature and thus taking up more space each. So fewer molecules in a given area.

But honestly that's so dimly remembered that I could well be talking complete nonsense. With luck someone with more scientific knowledge will be along shortly

Rob P
By WGN
#1687332
The altimeter works by having a fixed amount of air in an expandable capsule which is called an aneroid. This capsule expands and contracts when the atmospheric pressure changes. The capsule is connected to a gearing mechanism (like a watch cog) which alters the dials on the face of the altimeter that work on a scale measured in feet. As the aircraft descends into higher atmospheric pressure the capsule compresses causing the pointer on the scale to read a lower altitude.

The pilot manually selects the atmospheric datum on the ground to match the current atmospheric pressure (be it QNH, QFE or while in the air Flight level). This altimeter setting is updated during flight through updates gained via radiotelegraphy for any changes in atmospheric pressure throughout the day or changes across different regions.

Decreases in temperature reduce the speed of molecules leading to lower pressure (imagine balls in a box bouncing around, the slower they bounce the less pressure they exert). So in a situation where a plane flying at 1000ft with an altimeter setting of 1013hPa flys into colder air (where the static pressure is lower) the capsule in the altimeter will expand causing the altimeter to read a higher altitude. As the pilot hasn’t changed the altimeter setting datum and is aiming to fly at 1000ft they will lower the altitude of the plane to maintain an indicated height of 1000ft as shown on the altimeter. In actual fact, the plane will be much lower.

Point me out if I’ve got that wrong. I’m still reading through the Pooleys APMs at the moment.
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By lobstaboy
#1687351
Yes that's correct, and it needs to be pointed out that:
The altimeter with a subscale set to 1013 and indicating 1000' is telling you that you are flying 1000' above wherever (vertically) the pressure is 1013hPa - ground level for QFE, sea level for QNH etc.
BUT that is assuming you are flying in the International Standard Atmosphere (ISA). Altimeters are constructed assuming that the pressure and temperature drop with height follows the ISA. If you fly into air that is hotter than the ISA for the actual altitude you are flying at then, because the hotter air is less dense, the altimeter will show you as higher than you actually are because the less dense air will be creating less pressure on the capsule in the altimeter (or, put another way, the change of pressure with height will not follow the ISA assumption that the altimeter is based on, so the altimeter is fooled).
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By GrahamB
#1687356
Sorry, but when flying into hotter air the altimeter will under-read, not over-read.

Think of the atmosphere as a set of surfaces, each corresponding to a given absolute pressure. Those surfaces will rise and fall like contours according to temperature - in cold air they will be compressed closer together, in warmer air they will stretch out further apart.

As you travel along at a given pressure setting on your altimeter, maintaining a constant indicated altitude, you are flying along one of those contours. If you pass into warmer air you will follow that contour upwards; i.e. your true altitude will increase while your indicated altitude remains the same - your altimeter will therefore under-read. Similarly, if you pass into colder air, the contours become compressed and you will descend while indicating a constant altitude - your altimeter will over-read. This is addition to any change in local QNH which in itself would move the datum up and down if not corrected.

As in pressure settings varying as you travel enroute 'High to Low - Beware Below'
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By lobstaboy
#1687360
GrahamB 's explanation is the same, only he describes what would actually happen assuming you are flying by reference to pressure altimeter. The OP question in that sense is ambiguous- it depends on what "appears higher" is referring to.
Anyway, the idea of visualising the pressure surfaces as GrahamB suggests, is the best way to think through any altimetry scenario.
By Uzayr
#1687541
Thank you for the explanation, but what is the scientific explanation for this?

Thanks.

[quote="GrahamB"]Sorry, but when flying into hotter air the altimeter will under-read, not over-read.

Think of the atmosphere as a set of surfaces, each corresponding to a given absolute pressure. Those surfaces will rise and fall like contours according to temperature - in cold air they will be compressed closer together, in warmer air they will stretch out further apart.

As you travel along at a given pressure setting on your altimeter, maintaining a constant [u]indicated[/u] altitude, you are flying along one of those contours. If you pass into warmer air you will follow that contour upwards; i.e. your true altitude will increase while your indicated altitude remains the same - your altimeter will therefore under-read. Similarly, if you pass into colder air, the contours become compressed and you will descend while indicating a constant altitude - your altimeter will over-read. This is addition to any change in local QNH which in itself would move the datum up and down if not corrected.

As in pressure settings varying as you travel enroute 'High to Low - Beware Below'[/quote]
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By GrahamB
#1687566
A column of cold air is more dense, i.e. heavier, than a warmer one so the air at any point is compressed more by the air above, thus increasing the pressure gradient.