View Full Version : Qnh?

20th Jun 2002, 19:37
Hi all,

Just started studies for my JAR-CPL, and one of the first subjects we tackled was altimetry. Basically it's all clear - and most of it was PPL-stuff anyway, except for the temp. corrections - but there is one little question that puzzles me.

There we go: when QNH is given, I read anywhere that QNH is basically a calculated pressure level (which might have to do with the temperature corrections). I always thought that QNH was the actual pressure level at sea level, but indeed, when you start correcting for non-standard temperature, your QFE might be wrong.
So, is QNH basically the real, actual pressure level, without temp. corrections taken into account, or is it indeed a calculated level, so with temp. corrections your airfield level is indeed the level you see on your altimeter? What's practice?

Hope for an answer, thanks.

20th Jun 2002, 20:42
So, is QNH basically the real, actual pressure level, without temp. corrections taken into account,

Yes, but

your airfield level is indeed the level you see on your altimeter

also yes!

If you want the altimeter to read the airfield elevation when at airfield level, you don't need a temperature correction. Just take an altimeter at the airfield, set the airfield elevation and read off the subscale. That's the QNH. It's not (on a non-ISA day) the sea level pressure, but you're not interested in the sea level pressure. As long as your altimeter and the altimeter that was used to get the QNH have the same nominal pressure vs height relationship, your altimeter will read airfield elevation at airfield level with the QNH set.

If you were to set the sea level pressure on your subscale, your altimeter would assume that the pressure vs height relationship between sea level and airfield level was ISA. But if it's not (and it's usually not), the altimeter will be wrong at airfield level.

20th Jun 2002, 20:44

If memory serves me right the definition of QNH is :

the QFE reduced(or increased in certain areas)to mean sea level pressure using ISA lapse rates and parameters.

QFF is the QFE reduced to mean sea level pressure using ambient lapse rates.

Hope this helps


20th Jun 2002, 22:17
Dunno if this adds anything to the other excellent answers, but if you get an altimeter, as bookworm says, place it at ground level at your airfield and twiddle the knob till it shows the airfield elevation, the subscale reads the QNH.

Dig a hole from a/f level down to sea-level, and drop the altimeter down it.

Because of lapse rate differences etc., it is very unlikely that the alti will now read zero. Confused? ;)

21st Jun 2002, 05:45
I'll have a go, join the crowd, that sort of thing! :)

Say you have an airfield at 1000' above sea level, and someone has drilled one of Hug Monster's holes down to sea level. If you lower an altimeter down the hole to sea level, and wind the sub-scale so that is reads zero, the pressure the altimeter will display is QFF. QFF is actual sea level pressure - this is the pressure shown on mean sea level pressure charts.

Now if you raise that altimeter up the hole to the airfield, and say the day is a hot one, it will not read 1000'. As the temperature of the air in the hole is higher than standard, it is less dense. As it is less dense, the rate of pressure drop with altitude is lower than the altimeter expects - so the altimeter will show an altitude lower than 1000'. Lets say it is a 23C day (i.e. ISA + 10) at the airfield. The altimeter will show 960' , once you have raised it up the hole.

Now, as a pilot, you know the actual height of the airfield from your charts/map, and you want the altimeter to show you how far you are above that actual elevation - so this pressure setting (with its 40' error today) isn't much use to you.

If you take the altimeter (sitting at the airfield) and wind it to show 1000' - the height of the airfield - then the pressure shown will now be the QNH. Much more useful, as now when you descend to the field, the atimeter will wind down to the airfield height as you touch down.

If you then take your altimeter (sitting at the airfield), and wind it so that it shows a zero altitude, the pressure shown will be QFE.

How it works - the tower/met observer measures QFE directly (or makes a minor correction from his instrument to the airfield datum). Then correcting for a standard atmosphere below the airfield, they calculate QNH - which they pass to pilots. Then they correct for temperature to find QFF - which they pass to the met people.

23rd Jun 2002, 19:08
Thanks a lot for the info. It was as I thought it would be, but having a few more replies makes it clearer.

Greetz, Eagle