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

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By Keef
#594718
All excellent learning stuff! JB is definitely heading for a guru badge.


Being a bear of little brain (and with older than average cells in what's left), I work on "High to low, watch out below" - whether temperature or pressure. That works for me and requires no detailed thought while the brain is fully occupied trying to navigate.
#1822261
This puzzled me too. Think about a super simplified example. There are two columns of air, side by side. Each column has two layers that are distinct, one on top of the other.
At their bases, both have the same weight of air, (measured by a weighing scale called a barometer). The column on the right is shorter than the one on the left. How?
In this example, the air on the right is colder in both layers. Colder air is denser/thicker than warm air so it takes less vertical space to add to the same weight. Bricks vs pillows.
If you put two more barometers in the middle of the top layer of each column, they will also measure the same weight as each other. But the one on the right, in the colder, denser air will be closer to the base of the columns or ground, as it takes less room (vertical distance) to pack in the same weight.
An altimeter is just a barometer weighing the air above it and converting that weight into a distance vertically from the last weight it measured using a standard ratio. If the weight is changing faster than standard, the distance is also changing faster than standard and the barometer is reading behind the actual descent.
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#1822798
I think the altimeter is under reading.
High to low beware below. Correct. It’s over reading, ie: you are lower than you think
In this case it is “low to high” under reading, ie: you are higher than you think.

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By VRB_20kt
FLYER Club Member  FLYER Club Member
#1822807
You're flying along in warm air. The altimeter is showing 3,000' and you really are flying at 3,000. You fly into a column of cooler air and you keep an eye on the altimeter. You keep it rock steady at 3,000' but now in reality you have descended (see Jonathan B's diagram of pressure lines earlier on). So the altimeter is showing 3,000' but in reality you're flying at (say) 2,800'. The altimeter is showing 200' more than the true aircraft altitude and thus it is over-reading.
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#1822811
No, in reality you have climbed until the actual pressure matches the 3000ft pressure

You land a seaplane. At sea level the pressure is 1013, altimeter reads zero on the water.
If the pressure at sea level increases you have to climb to reach the 1013 pressure point where your altimeter will read zero.
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By GrahamB
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#1822815
Crash one wrote:No, in reality you have climbed until the actual pressure matches the 3000ft pressure

You land a seaplane. At sea level the pressure is 1013, altimeter reads zero on the water.
If the pressure at sea level increases you have to climb to reach the 1013 pressure point where your altimeter will read zero.

This is about changes of temperature, not changes of pressure.

The 'pressure surface' mental model is a good one; think of the way the surface of the ground undulates with peaks, troughs, cols, ridges and valleys. The points in space representing a given atmospheric pressure, which is calibrated to depict a specific altitude in your altimeter, would look very similar, with the height at any point relating to the temperature of the air in the column beneath it. As you fly around this surface at a constant indicated altitude, your true altitude will vary accordingly, reducing where it's colder and increasing where it's higher.
#1822895
Quite true your true altitude will reduce in the cold (and therefore higher pressure) area and your altimeter will still be reading the original higher level.
Therefore if you fly a constant altitude on a radar altimeter over flat terrain the barometric altitude reading is under what it should be in cold areas.
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By GrahamB
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#1822909
Crash one wrote:Quite true your true altitude will reduce in the cold (and therefore higher pressure) area and your altimeter will still be reading the original higher level.
Therefore if you fly a constant altitude on a radar altimeter over flat terrain the barometric altitude reading is under what it should be in cold areas.

A reduction in temperature does not necessitate an increase in pressure. The two are not directly connected. In a high pressure system descending air warms adiabatically.

I think you need to rethink your last sentence.
#1823041
You’re right.
My apologies, my mistake.
Now I’m trying to figure out how to describe why for my brain to get round it.
ISA has an even/regular reduction in pressure of 1mb per 30ft of altitude.
A column or bubble of “non ISA” warmer, low pressure air is effectively lower than it should be, giving a wrong, higher altitude, reading compared to what it should be at that height.
Assuming the pressure reduces because of higher temperature, and altimeters only read pressure.
Hard work this thinking business innit.
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By GrahamB
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#1823047
My advice to you in trying to get your head around the basics is to dissociate temperature and pressure. You can have air which is at a higher temperature, but lower pressure, than ISA, or the other way round, or at both higher, or lower than ISA.

Think about them one at a time; on a cold day with high pressure the two may cancel one another out as far as true altitude is concerned.

For the most part, true altitude is only of interest to the VFR pilot when planning a route across high terrain in conditions significantly differing (lower) than ISA. For IFR pilots it’s essential to consider it where aircraft performance, availability of oxygen, icing layers, minimum terrain clearance and other factors may reduce the choice of flight levels available over such terrain to none.
#1823163
If the pressure in the sealed chamber in the altimeter rises due to warm air increasing the pressure in it, the altimeter will read that as a lower pressure outside and display a higher altitude.
The sealed chamber/linkage system can only read the difference in pressures from inside to outside.
Knowing how the altimeter works is easier for me than just going for cliches like “high to low beware below” etc. I’d rather know why because increases of pressure or temperature do opposite things in free air and the same thing in a sealed chamber, therefore don’t compute.
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By VRB_20kt
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#1823171
This is about the temperature of the air outside the aircraft rather than that in the altimeter. Essentially the colder air is, the denser it is. Consequently an x metre high column of cold air exerts the same pressure as an x+n metre column of warmer air. (or an x metre column of warm air exerts less pressure than an x metre column of cold air). The big changes that we notice routinely are the ones linked to height and to surface pressure. We rarely notice or bother about the changes arising from temperature.

But in an ideal world of constant surface pressure if you fly from a hot area to a cold area and maintain a constant altitude reading, the aeroplane will have descended. And if you fly with constant altitude reading into a warmer space, the aircraft will climb.
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By GrahamB
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#1823195
Crash one wrote:If the pressure in the sealed chamber in the altimeter rises due to warm air increasing the pressure in it, the altimeter will read that as a lower pressure outside and display a higher altitude.
The sealed chamber/linkage system can only read the difference in pressures from inside to outside.
Knowing how the altimeter works is easier for me than just going for cliches like “high to low beware below” etc. I’d rather know why because increases of pressure or temperature do opposite things in free air and the same thing in a sealed chamber, therefore don’t compute.

All but the cheapest, nastiest altimeters have temperature compensation built into the mechanism, so I’m afraid you are misleading yourself again.