# Altimeter 2992

Guest

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

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 😉

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

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

Ian Stephens (ianstephens) Captain
Ian Stephens is an expert on this topic. Read his bio here.

The previous replies have touched on the importance of using a standardized altimeter setting in higher altitudes, so I won't go into too much detail on that. But I would like to expand on some key terms and concepts to provide a clearer understanding of the subject.

• QNH: This stands for "barometric pressure adjusted to sea level." It's an important value used by pilots to calibrate their altimeters, allowing them to accurately display their altitude above mean sea level (AMSL). QNH is typically provided in inches of mercury (inHg) for the United States or hectopascals (hPa) for other countries.
• Transition Altitude (TA): The altitude at which pilots switch from using the local QNH to the standard altimeter setting of 29.92 inHg (or 1013.25 hPa). The TA varies between countries, but in the United States, it's set at 18,000 feet.
• Flight Level (FL): Altitudes above the TA are referred to as Flight Levels, which are expressed in hundreds of feet. For example, FL190 is 19,000 feet, FL200 is 20,000 feet, and so on.

Now, to answer your question, air traffic controllers (ATC) mention the altimeter setting of 29.92 (or the equivalent in hPa) when you request an altitude change above the TA. This is because all aircraft operating above the TA need to be using the same altimeter setting to ensure adequate vertical separation and avoid potential mid-air collisions.

By standardizing the altimeter setting at higher altitudes, it allows for a consistent and uniform method of maintaining safe separation between aircraft. This is particularly important in busy airspace, where precise altitude information is crucial.

It is also worth mentioning that the concept of using a standard altimeter setting is not exclusive to Microsoft Flight Simulator X but is a standard practice in real-world aviation as well. The same principles apply in the newer Microsoft Flight Simulator (MSFS) 2020 release.

I hope this information helps you better understand why ATC provides the 29.92 altimeter setting when requesting altitude changes above the TA. Safe flying!