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Altimeter 2992

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Why when you talk to air traffic control do they always say altimeter 2992 when you request altitude increase/decrease/change?

Pro Member Chief Captain
Jonathan (99jolegg) Chief Captain

Because that is the air pressure and affects your altitude. Experiment with it. You can change it up and down to the lowest pressure 25.00 or 35.00, the highest pressure. When you reach transition altitude (18000) you set your altimeter to 29.92. This means that when they say Flight Level xxx it is actually that altitude so every aircraft flying above 18000 is measuring altitude in the same way to avoid collisions. This isn't a very good explanation so sorry about that but maybe someone else can be clearer Wink

Pro Member Captain
Bindolaf Captain

The number is atmospheric pressure measured in inches of mercury. Atmospheric pressure is basically the force with which the mass of air above you "pushes you down".

Now, because of atmospheric conditions, pressure is not the same everywhere. So, let's say you take off from airport AAA. The "local QNH" or local altimeter setting may be - say 29,75. This is the *accurate local* pressure. Which means, that your altimeter, once set to 29,75 will show you your actual, real altitude.

Now, as you pass 18,000 ft (in the US) you need to switch your altimeter setting to 29,92. Why? Is that the real pressure over 18,000? No. But above that altitude it's not really necessary to know your real altitude. It IS important to know how far you are from other planes though.

Let me give a couple of examples:

Northwest 435 is flying at 18000 ft with an altimeter setting of 29,75
Delta 367 is flying at 19000 ft with an altimeter of 29,84
They converge. Are they really 1000 feet apart? No they are not. Try flying up to 18000 feet and switching from 29,75 to 29,84 and see how different the altitude is. If the difference is higher... there might even be a collision.

So you see, low on the ground, where you have to know your REAL altitude (so you don't run into a mountain), you use the LOCAL altimeter setting (QNH). High up in the skies we ALL use the SAME QNH, so we know where we all fly. Do we know our real altitude? No. Do we know how far apart we are from others really? Yes. And that's what is important.

By the way, 18,000 is the "Transition Altitude" in the U.S. and above that you don't say "altitude" anymore, you say "flight level". Thus, you fly at 17,000 feet, 18,000 feet and then FL190, FL200 etc..

Hope it helps

Pro Member Chief Captain
Jonathan (99jolegg) Chief Captain

Bindolaf wrote:

The number is atmospheric pressure measured in inches of mercury. Atmospheric pressure is basically the force with which the mass of air above you "pushes you down".

Now, because of atmospheric conditions, pressure is not the same everywhere. So, let's say you take off from airport AAA. The "local QNH" or local altimeter setting may be - say 29,75. This is the *accurate local* pressure. Which means, that your altimeter, once set to 29,75 will show you your actual, real altitude.

Now, as you pass 18,000 ft (in the US) you need to switch your altimeter setting to 29,92. Why? Is that the real pressure over 18,000? No. But above that altitude it's not really necessary to know your real altitude. It IS important to know how far you are from other planes though.

Let me give a couple of examples:

Northwest 435 is flying at 18000 ft with an altimeter setting of 29,75
Delta 367 is flying at 19000 ft with an altimeter of 29,84
They converge. Are they really 1000 feet apart? No they are not. Try flying up to 18000 feet and switching from 29,75 to 29,84 and see how different the altitude is. If the difference is higher... there might even be a collision.

So you see, low on the ground, where you have to know your REAL altitude (so you don't run into a mountain), you use the LOCAL altimeter setting (QNH). High up in the skies we ALL use the SAME QNH, so we know where we all fly. Do we know our real altitude? No. Do we know how far apart we are from others really? Yes. And that's what is important.

By the way, 18,000 is the "Transition Altitude" in the U.S. and above that you don't say "altitude" anymore, you say "flight level". Thus, you fly at 17,000 feet, 18,000 feet and then FL190, FL200 etc..

Hope it helps

The perfect explanation Yes

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