What's QNH?

Have a read of this:

http://en.wikipedia.org/wiki/QNH

😉

QNH is basically the setting your altimiter needs to be on to display accurately, and it is a world wide term:

http://en.wikipedia.org/wiki/QNH

Looks like your beat me to it! 😉

jarred_01 wrote:

Looks like your beat me to it! 😉

What would we do without Wikipedia 🙂)

Ah right. So it's like saying "altimeter two niner niner two" in the U.S. right?

By the way, is there a way to manually set the altimeter to get correct air pressure for that region in the Cessna 172? I know you can just press the b key, but I'd like to know how pilots would enter a QNH number in real life.

Thanks very much for your help, JTH 🙂

In 33 years of flying in the US, I have never heard barometric pressure referred to as QNH so I suspect it is not quite world-wide.

JTH: There are only two ways to accurately set barometric pressure in the altimeter. At any time, use the setting provided by ATC from ATIS, local controllers, etc. Second, on the ground at a place where the altitude above sea level is known, place your a/c at that location and then set the altimeter to the known elevation.

I think he means something different. On the actual altimeter (the instrument), there is a small knob (the cursor will change to a + or - sign). Turn this knob to change the altimeter setting to the correct QNH/baro setting.

99jolegg wrote:

jarred_01 wrote:

Looks like your beat me to it! 😉

What would we do without Wikipedia 🙂)

I wouldn't have a clue to be honest!

QNH

Altimeter indicates altitude above Mean sea level (AMSL). Actual atmospheric pressure particularly used for terrain clearance flying en-route as on aeronautical maps obstructions/terrain marked as altitudes. An (ASR) altimeter setting region is pressure current and forcast for that hour in the region.

QFE

Altimeter indicates height above a fixed point on the surface (usually an airfield). [/u]

Tartan, I think I get what you're saying. So when a controller says, "QNH 1516", he means set the altitude on your altimeter to 1516, right? But when might he use QFE? Would he say something like descend to 3,000 QFE? Then would you check your chart, see that the airport elevation is 300 ft for example, and descend to 3,300?

As for setting the altimeter, I think I know what you mean with the little knob now Bindolaf. But is Don Wood's method another way of setting the altimeter? I'm a little confused by those two posts.

Anyway, thanks as always to everyone for their help.

JTH: Bindolaf and I are saying the same thing in different ways. The knob he speaks of is how you set barometric pressure in the altimeter. It is also how you adjust the altitude readout on the altimeter when you are at a known altitude on the ground. I just assumed in my original answer that everyone reading would know the mechanical method for setting the altimeter.

I also compare a known barometric pressure to the altitude reading when I am at a location with a known altitude to make sure my altimeter is working correctly. As an example, when I am at an airport with weather observation, the barometric pressure I receive from ATC will be accurate. When I am at the runway threshold prior to takeoff, where the altitude is published for that location, I can then look at my altimeter to make sure the readout is within a few feet of the known altitude, If it is not and I verify I have set barometric pressure correctly, I know my altimeter is malfunctioning.

Type into google; "define qnh".
Or go to http://www.wikipedia.org/ and type in qnh

The latter is very detailed

Thanks for that Don, I understand what you're saying now.

By the way, could anyone clarify when you'd use QFH?

Say I was going to depart from an airport i would be on a QFE setting as provided by atc. Then once departed from the aerodrome flying northwards for arguements sake at a reasonable height say 4000ft i would fly on regional QHE. When reaching near the destination aerodrome i would change to that aerodrome QFE to decend, make my approach and land.

So the departing airport would give you a QFE altitude, say "300 QFE", which would mean 300 feet above the airport?

Then in the middle, the center will say maybe "4000 QNH", which is 4000 ft above sea level.

Then finally the arriving airport might say descend to "500 QFE", which is 500 feet above that new airport?

Is all that correct or am I missing something? I wonder why I haven't come across QFE on VATSIM yet 😕 Anyways, thank you for your continued help.

Not exactly...

When they said "QNH 1013" that doesn't mean that your altitude is 1013 above the airport. QHN 1013 is the standard pressure.

Shameless self promotion coming up:

ATC gives you a local barometric pressure (either in inches of mercury or hectopascals), because the altimeter (the instrument) cannot measure altitude without a point of reference. A radar altimeter can and does not need to be tuned.

The number you get (the QNH) is not an altitude. It is a pressure. It's the "decoder key", the starting number so the instrument can calculate your altitude correctly.

Read here if you want: https://forum.flyawaysimulation.com/forum/topic/8090/altimeter-2992/

This all makes perfect sense to me, however one thing I don't understand is how to actually apply the QNH number to get the correct altitude reading. What I mean is that for example today I got a QNH of something like 1013, whereas when I let MSFS autoprogam the altimeter by pressing the b key, it read something like 27.72.

So how does QNH 1013 = 27.72 inches of mercury?

Thanks once more 😉

Atmospheric pressure is measured in inches of mercury (used in the US), millibars (used in much of the rest of the world), or in pounds per square inch. They all measure the same pressure, just with different units of measure.

A standard day at sea level is 29.92 inches of mercury, 1013.2 millibars, or 14.7 pounds per square inch.

Pressure decreases by known, measured amounts as altitude increases. The altimeter measures altitude by comparing the pressure set in the altimeter by the actual ambient pressure. That's why pilots set current air pressure in their altimeter prior to takeoff, adjust it for current conditions as advised by ATC enroute, and prior to landing so that they have as accurate a readout of altitude as possible in all phases of flight.

Even though radar altimeters and GPS now provide a precise measure of altitude, they do not compensate for changes in air pressure. To be sure all aircraft are using the same measure to determine altitude, atmospheric air pressure is still the world standard.

So is there a formula for converting millibars to inches of mercury? Say QNH was 1246. What would I set my altimeter to then and why?

Thanks a lot.

JTH wrote:

So is there a formula for converting millibars to inches of mercury? Say QNH was 1246. What would I set my altimeter to then and why?

Thanks a lot.

Courtesy of Greekman : a little utility called "Convert".

I didn't read the whole thread but according to "Convert" 1246 millibar equals 36.79436 inches of mercury.

In the previous posts, the problem is in the interpretation of the altimeter setting. 1246 means 1024.6 millibars. Then convert this to inches with a calculator or converter. mb/33.856=in. mercury

http://www.pilotfriend.com/calculators/whizzwheel.htm

ATC should always give QNH altimeter settings.

Above 18,000 ft. (US) or other transition altitude in other countries, everyone uses QNE of 29.92 or 1013.2

QFE would be the result of a pilot setting altimeter to read zero at ground level. Not a normal setting for an altimeter and never given by ATC. 🙂

Thanks a lot CRJ, I've written that formula down so I can use it each time from now on.

By the way, in real life do pilots have a calculator with them in the cockpit and do they have to perform the Milibars to in. mercury conversion every flight?

I don't really get what you mean about QFE, but if ATC will never say it then I guess this is one thing I can live without knowing! But just out of curiosity, how/why would a pilot set the altimeter to read 0 at ground level?

Thanks once again 😀

In aircraft with an electronic flight instrument system (EFIS), you can select millibars or inches of mercury by knob or switch. Some traditional, round altimeters have both scales built in. If you don't have any of this, there are charts that can be used as a quick reference.

QFE is not used in the US. An example of when it may be used in general aviation in other countries: Small general aviation tower, good weather with plane(s) practicing landing in the traffic pattern. The QFE altimeter setting will cause each aircrafts altimeter to read height above airport. Since they are just circling, thats all they really need. The students or practicing pilots then know that when their altimeters says 500 ft., that's how high they are from the runway. With a normal alt. setting at a airport that has a field elevation of 1,800 ft.; the student must remember that when the alt. says 2,300 ft, he is only 500 ft. from the ground. I guess they figure it makes it easier for new or circling pilots.

Great, that explains it perfectly. Thanks a lot 🙂

During my PPL-A flight training in Germany, I was taught (and asked in the exam) about QFE. However, we never used it, and used QNH instead all the time. This is to prevent a possibly fatal confusion when you are doing your first enroute flights.

When you set your altimeter to the QFE, then it will show the actual altitude above the air field that gave you the QFE number. Your traffic pattern is maybe at "850 ft", and you would descend to "0ft" in order to land.

Now when you come in from another place, you will always be using the QNH (because you can get that from many places, the next ATIS or enroute advisory, or neighboring air fields). With the proper QNH and an airport elevation of 250 ft, your traffic pattern is at 1100 ft and in order to land, you descend to 250 ft.

Imagine a student who has set his altimeter to QNH, but thinks it's QFE. Before entering the traffic pattern, he will descend until his altimeter says "850 ft". But in reality, he is 250 ft lower, and he will mess up his final approach. Other traffic in the same traffic pattern will not see him, and when they are descending for landing, they will descend into the flight path of our student. In a Cessna, you don't really see what is underneath your own position, and you are likely to collide with it.

QFE, as previously correctly noted, is the pressure setting for a particular airfield such that at the threshold of the in use runway the alitmeter would read 0 (more or less...)

The more detailed definition is that QFE is the pressure at the airfield adjusted to standard temperature.

QNH is the QFE at an airfield, reduced to MSL (Mean Sea Level)

RPS (Regional Pressure Setting) is the lowest forcast QNH for the next hour in a particular ASR (Altimeter Setting Region)

On top of that, when operating above 3000', the SPS (Standard Pressure Setting) is used - a worldwide standard of 1013.2 mb

In the UK, civilian aircraft typically use QNH at airfields, while military aircraft use QFE. This has caused a few humourous moments when UK military aircraft overseas request the QFE from an ATC unused to it, and sometimes have to wait a few minutes while it's worked out!

CRJ Captain has the best explanation of this (as usual . . . I can never beat this guy to the punch 🙂 You hear QNH all the time in Europe, SW and SE Asia and it is usually given in millibars instead of inches of mercury (which pilots in the US are used to). QNE is used when you pass the transition altitude. In the US it is 18000'. However, in other parts of the world, it can be quite a bit lower. Germany is 7000', the UK is 3000'. I have only run into QFE a couple of times. The first time was flying a PAR approach (Talkdown is that the controller called it) to a military field in central England. I have been given GFE on two other occasions flying into former eastern bloc countries. In these instances I asked the contoller for a QNH setting instead and they provided it (I already have a radar altimeter and with QFE, they read the same). Look closely on the Jeppessen approach plates for Eastern Europe and Russia. Inside the IAF, you will see two altitudes for each segment of the procedure (one is in parenthesis). The lower of the two altitudes is usually the QNH, the higher QFE. If it is a QFE approach with no QNH altitudes (should be annotated in the notes somewhere), you can add the touch down zone elevation to the QFE altitude to get the QNH minimums.