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

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By JonathanB
FLYER Club Member  FLYER Club Member
#594086
Just in case it's not clear (and following Jowloo's hint!) I thought a quick diagram might help!
Image

The lines across the air masses indicate a pressure level, indicated altitude, same temperature. Hopefully you can see that the levels get pushed closer together due to the increased density of the cold air causing the air to sink and compress.


Hmmm, perhaps I should start some training courses!!! (Not that I want to compete with Irv, or have the experience to do so!).
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By mo0g
#594108
Charles Hunt wrote: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


This is exactly what I was saying in my logic explanation. Thats why high to low pressure does what our logic suggests.

I cannot work out why high to low temperature isnt the opposite of the above. Higher temperature = lower pressure isn't it? Colder air is more dense and therefore higher pressure.

I can easily remember this for the exam if I imagine the a/c sitting on top of the 3 different columns of air, and the colder column will be more dense and therefore lower.. but the logic still escapes me.
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By mo0g
#594113
JonathanB wrote:Just in case it's not clear (and following Jowloo's hint!) I thought a quick diagram might help!

The lines across the air masses indicate a pressure level, indicated altitude, same temperature. Hopefully you can see that the levels get pushed closer together due to the increased density of the cold air causing the air to sink and compress.


Hmmm, perhaps I should start some training courses!!! (Not that I want to compete with Irv, or have the experience to do so!).


I can understand this, I just cannot correlate it to the high to low pressure scenario. As far as the altimeter is concerned, isnt this the opposite to that diagram? You are going from more dense to less dense air, ie the reverse direction to that diagram? I suspect that trying to explain the reading just from the altimeter perspective is where we are getting confused, maybe it will all become clearer when we have done the met course..
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By JonathanB
FLYER Club Member  FLYER Club Member
#594118
[quote='mo0g']I cannot work out why high to low temperature isnt the opposite of the above. Higher temperature = lower pressure isn't it? Colder air is more dense and therefore higher pressure.[/quote]

Not lower pressure, lower density. In the warm column the air is less dense so the same mass of air is spread out more across the height of the column compared to the cold column. The same mass of air is in both columns meaning the surface pressure is the same, but because the cold column has more dense air it sinks down more making the column shorter and the pressure levels through the column pushed together and lower than in the warm column - as per my diagram.

[quote='mo0g']I can easily remember this for the exam if I imagine the a/c sitting on top of the 3 different columns of air, and the colder column will be more dense and therefore lower.. but the logic still escapes me.[/quote]

Am I helping or hindering?!
:?
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By JonathanB
FLYER Club Member  FLYER Club Member
#594123
[quote='mo0g']I can understand this, I just cannot correlate it to the high to low pressure scenario. As far as the altimeter is concerned, isnt this the opposite to that diagram? You are going from more dense to less dense air, ie the reverse direction to that diagram? I suspect that trying to explain the reading just from the altimeter perspective is where we are getting confused, maybe it will all become clearer when we have done the met course..[/quote]

It is hard to get your head round! Took me a while until it clicked.

Remember that with lower pressure as you climb the altimeter shows a higher altitude. Pressure is related to the weight of the mass of the air above you. Denser air is heavier and so sinks, leaving less air above.


In the cold air if you were at the same [b]true[/b] altitude the pressure would be lower than in the warmer air and your altimeter will read higher than you really are. However usually you are flying at a constant altitude which means in you are in reality flying at a constant pressure. The level of this constant pressure is dragged downwards in the colder air due to the more dense air sinking more, so you will be lower than the indicated altitude - imagine the aircraft flying along the lines on my diagram. I have tried to space the lines equidistant in each column to show how it works.
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By mo0g
#594124
JonathanB wrote:Am I helping or hindering?!
:?


You have helped, thanks. It was the distinction between pressure and density that I was missing!
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By Morley
#594125
Jonathan is god! His very first explanation was the best and most logical.

The altimeter is simply a barometer and if you fly to the altimeter and move into colder air you will have to descend (in real terms) to maintain the same displayed altitude cos the colder air is simply closer together. When it's cold even molecules huddle closer together!

It's why things look closer from the air in winter :wink:
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By JonathanB
FLYER Club Member  FLYER Club Member
#594126
The invoice will be in the post! :lol:
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By JonathanB
FLYER Club Member  FLYER Club Member
#594127
Thanks Steve, your cheque will be in the post! :lol:
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By AndyR
FLYER Club Member  FLYER Club Member
#594135
He is truly a sky god :wink: Gen Nav in particular!

Irv is going to have someone to take over from him in years to come methinks :lol:
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By Charles Hunt
#594152
Many thanks to Jonathan for the explanation. His diagram worked for me. You have to consider what is happening in the air masses.

I was drawing diagrams of the altimeter and trying to work out what would happen within, and this didn't help at all.

Charles
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By Dave Phillips
#594155
A cute little formula is:

H = 96T/P

Where H is the vertical change per hPa
T is the temperature at that level (Kelvin)
P is the actual pressure in hPa.

So, all other things being equal, if you reduce the temperature, H will reduce.
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By keybuk
#594170
mo0g wrote:I can understand this, I just cannot correlate it to the high to low pressure scenario.


Remember that the pressure you're quoted is at Sea Level or Aerodrome Level. ie. it is not the pressure you are flying through, but the pressure at a nominal surface.

Assuming all other things remain constant (datum, temperature, density, and thus most importantly, the rate at which pressure decrease with height) you will be doing this:

Image

Note that the spacing between the pressure levels remains the same in both the area of high surface pressure and low surface pressure, whereas in Jonathan's diagram it is the cold air that reduces the spacing.

Given the same temperature, you'll have the same density, so the difference between an area of high surface pressure and an area of low surface pressure, is the amount of air above you. There's less air in the low, so the pressure levels are lower (but still spaced the same as before).

Given a lower temperature, as in Jonathan's diagram, the density increases as well. This brings the pressure levels lower AND closer together.

So if flying from an area of high surface pressure to an area of low surface pressure, without changing your datum, you will be lower than it claims since there is less air above you.

And if flying from an area of high air temperature to an area of low air temperature, you will also be lower than it claims since there is also less air above you.
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By JonathanB
FLYER Club Member  FLYER Club Member
#594172
Note that keybuk's diagram refers to high pressure and low pressure as the spacing of the pressure surfaces/lines is the same.

My diagram shows warm and cold air masses and the pressure lines get squeezed - the surface pressure can be exactly the same.
By AWoL
#594597
I had difficulty with the problem of altimeter readings in different temperatures.
Here’s how I see it.

Take a barometer and walk up a hill on a nice day, say about ISA temp of 15 deg C.
When you’ve reached an elevation of 30’ you’ll notice that the pressure registered on the barometer will have dropped by 1 millibar.

On a cold day, you will only have to climb to an elevation of 28’, to register a drop in pressure of 1 millibar. The reason why this is the case is in the diagram provided by Jonathan B and the reasoning for the next example is to be found in the same place.

On a hot day, we have to climb to an elevation of 32’ to register a drop of 1millibar.

Now, on a cold day, we take the barometer onto an aeroplane without an altimeter, and I ask the pilot to climb until the barometer registers a drop of 100 millibars. I tell him to level off.
How high are we above the take-off point asks the pilot. I tell the pilot, A rise of 28’ caused a drop of 1millibar, so by my reckoning a drop of 100 millibars gives us a height of 2800’.

Same set up on a hot day. We level off when I get a drop of 100millibars on my barometer. What’s the height? 32’ gave 1millibar so for 100 millibars, I make it 3200’

We do the same on a middling day, say around 15 deg C. Up we go and I tell him to level out when the barometer is 100 millibars less than at ground level. How high?
The nice day reading was 30’ for 1 millibar, therefore by my reckoning, a hundred millibar drop gives a figure of 3000’ and that’s what I tell him

Summarising, on all occasions we got a drop of 100millibars, yet the heights attained depended on temperature. On the cold day we got to 2800’, on the middling day we got to 3000’ and on the hot day we got to 3200’

The next point to take in is that altimeters are a bit of a cheat. They are simply barometers in disguise. They do not measure height above a fixed point. Yes they show height above a point, on the dial, but the only thing that affects them is a change in pressure, exactly the same as the barometer. It’s the scale on this barometer that calls itself an altimeter that is the problem. It would add a ridiculous amount of complexity to put three scales on the dial of this minibarometer, one for cold, one for middling, and one for hot. So we put on the one for middling. That means that if there is a drop in pressure of 100 millibars, this barometer will register 3000’ on the dial, regardless of temperature.

But as we saw before on a [b]cold day[/b], when we went up to 100 millibars difference, that gave a height of only 2800’ However we know that this thing that calls itself an altimeter will register 3000’ for a100millibar drop in pressure. So it [b]over-reads.[/b]

Now take the [b]hot [/b]day. I’m in the back with my barometer. I tell the pilot to level off when pressure has dropped by 100millibars. I know, and you know, that the reading on his altimeter will be 3000’, but you and I know that we’re really at 3200’. The altimeter, with its fixed scale, [b]under-reads[/b].

The scale is inflexible and can’t be altered to suit the conditions, and so needs a bit of interpretation.
[b]Altimeter over reads in the Cold[/b]…. You’re lower than indicated. Think Terrain.
[b]Altimeter under reads in the Heat[/b]…..You’re higher than indicated. Think CAS
[b]Cold, lower [/b](than you think),
[b]Hot, higher [/b](than you think).