Pressure Altitude.

[TurningFinals]

What is it?

How do you calculate it?

What are it's uses?

I'm an assistant at a UK ATC unit and a few visiting aircraft have asked for it recently, it's just out of interest really.

Thanks!

[bingofuel]

Are they referring to QNE, elevation of the airfield with 1013.2 hPa set?

[TurningFinals]

Bingofuel yes, I think that's what it was.

[TurningFinals]

And by the way diginagain, I do apologize for asking a question in the Question section of the forum.


:ugh:

[HEATHROW DIRECTOR]

TurningFinals.... Interesting one. I never heard that request and I can't imagine why they'd need QNE in the UK. Isn't QNE usually used at very high airfields? (Remember I'm 10 years out of date!).

[Exaviator]

TurningFinals, I think what you are looking for is QFE, the barometric pressure which, when set on the aircrafts altimeter, will show zero feet with the aircraft on the ground, Local Station Pressure/"Field Elevation" as against QNH which shows airfield elevation.

If you have an altimeter in the tower wind it to zero elevation and read off the barometric reading or check out this website for a calculator QNH/QFE :ok:

[Flexi]

The pressure altitude is the actual altitude corrected by the "pressure error". It is used for performance calculations and can be "calculated" by setting the altimeter to the standard setting of 1013,25hPa/29,92 inches.

This gives you quite a good explanation:

Pressure & Density Altitude Effects, Pressure Lapse Rate, Take-Off & Landing Performance, Airplanes - EAI (http://www.experimentalaircraft.info/flight-planning/aircraft-performance-3.php)
Pressure Altitude

Pressure altitude is altitude or elevation corrected for non standard pressure. In short: set the kollsman window on the altimeter to 1013 hPa / 29,92 inHg and you will have pressure altitude. Easy when you have an aircraft or altimeter nearby, but what when you only have the regional QNH and local elevation/altitude?

With a small but effective formula it is easy:
Pressure Altitude = Altitude + (1013 - QNH) × 27.[/FONT]

Its obvious that when QNH is higher than 1013 hPa (QNE) that result will be negative and deducted from altitude, or, put another way: add 27 ft for every hPa below 1013 hPa (common in low pressure areas) or when QNH is lower than 1013 hPa -> Pressure Altitude will be higher (27 ft/hPa) and vice versa.

Hope that helps!

[BOAC]

TF - I am at a loss to understand why anyone would ask you for it! Some companies use it to calculate take-off and landing performance but it is so easily extracted from QNH and field elevation as Flexi says without bothering ATC.

[747 forever]

I spent alot of time studying it and finally understand it, however when I change the baromice settings on the PMDG 747 (fsx) to a lower setting the altitude tape shows a lower altitude. Why is that? I thought the lower the pressure the higher the altitude

[fireflybob]

"Pressure Altitude" is really a pressure rather than an altitude.

It's the altitude in an International Standard Atmosphere you'd have to fly at, in order to experience the pressure.

A simple way of deducing Pressure Altitude is to set 1013.25 millibars (ahem Hectopascals) and then read indicated "altitude".

I spent alot of time studying it and finally understand it, however when I change the baromice settings on the PMDG 747 (fsx) to a lower setting the altitude tape shows a lower altitude. Why is that? I thought the lower the pressure the higher the altitude

Because when you wind off pressure (on the altimeter), you wind off altitude and vice versa.

[sycamore]

You will also need it to correct for landing heights at airfields with very low atmospheric pressure,and the aircraft altimeter subscale cannot be set below 950 mb/hp.

[keith williams]

I spent alot of time studying it and finally understand it, however when I change the baromice settings on the PMDG 747 (fsx) to a lower setting the altitude tape shows a lower altitude. Why is that? I thought the lower the pressure the higher the altitude


The altimeter indicates your altitude relative to the point in the atmosphere at which the pressure is equal to that set on the subscale.

Pressure increases as altitude decreases.

So if you increase the setting on the subscale your are setting the altimeter to indicate relative to a lower point in the atmosphere. This causes your indicated altitude (relative to this lower point) to increase.

And if you decrease the subscale setting you are setting the altimeter to indicate relative to a higher point in the atmosphere. This causes your indicated altitude (relative to this higher point) to decrease.

If you keep on reducing the subscale setting until the indicated altitude is zero, your subscale setting will be equal to the atmospheric pressure around the aircraft.

[TurningFinals]

Thanks everyone.

It had most of us in the tower scratching our heads. It is American visitors we have at the moment that are asking for it, and no one can understand why.

So for example if the regional QNH is 999mb, and the field elevation is 41ft, the pressure altitude is -

41 - 14 x 27?

[BOAC]

Plus .....................see #7

[fireflybob]

So for example if the regional QNH is 999mb, and the field elevation is 41ft, the pressure altitude is -

41 - 14 x 27?

Firstly, in this case you need to be using the aerodrome QNH, not the Regional.

Let's assume the Aerodrome QNH is 999 mb and you are sitting on the ground at said aerodrome with 999 mb set - the altimeter (should) read 41ft.

We now wind on 1013 mb, an increase of 14 mb which (using 27 ft per mb) equates to 378 ft. As you have wound on 14 millibars, you have wound on 378 ft which added to the 41 ft we started with equals 419 ft.

So in this case the pressure altitude is 419 ft. (ie you would have to be at 419 ft in an International Standard Atmosphere to experience this pressure).

Quite a few performance calculations require Pressure Altitude (depending on the graphs/tables one is using) - this combined with temperature leads you to effectively entering such calculations with Density Altitude.