Q.E.D.

A commenter recently asked about the altimeter setting and Q-codes. I have probably blogged on this before, but Blogspot doesn't show it in my archives so it's just as easy to explain again. It is to do with the way the altimeter works.

The instrument in the panel that indicates my altitude is a barometric altimeter. It operates purely by air pressure, mechanically comparing the pressure in its sealed reference capsules to the air pressure sensed immediately outside the airplane. For this purpose there are little holes on the outside of my fuselage called static ports, and tubes conduct the pressure from the static ports up to the altimeters.

As the airplane climbs, the outside air pressure drops, but the pressure inside the reference capsules stays the same and this difference moves the hands around the face of the clocklike altimeter to indicate my altitude. Electronic altimeters work the same way, except that the pressure difference is converted to a digital signal. All the altimeter knows is the difference in pressure. If the pressure goes up, the needles show a lower altitude. If the pressure goes down, the needles show a higher altitude. This also happens without the airplane ever leaving the ground, just because the air pressure changes as weather systems move through.

In order to always start at a known place, there's a knob on the altimeter that moves what the altimeter considers to be the zero altitude up or down. So you could be on the ground with the altimeter reading zero, and turn the knob so that the altimeter read a thousand feet, and then crank it back the other way so the altimeter read two hundred feet below sea level. While you turn the knob, and the altimeter hands are moving, a small dial with numbers is also moving, visible through a cutout in the face of the altimeter. On an North American altimeter the numbers usually range from 28.10 to 31.00. On a European altimeter they range from about 950 to 1050. (International altimeters have two windows and two dials driven by the same knob). Turn the knob so a bigger number appears in the window and the hands go up to indicate a higher altitude. Select a lower number in the Kollsman window and the hands point to a lower altitude. Whee! The pilot will have to set it correctly before takeoff.

But what is correctly? There are actually two systems as to what the altimeter should read when you're parked. One is that it should read zero when you're on the ground, no matter where that ground is. Under that system, a circuit pattern flown at a thousand feet above ground level would always read 1000' on the altimeter, regardless of the elevation of the airport. If the altimeter reads two hundred feet, you're two hundred feet above ground. The number that you would set in the Kollsman window to make the altimeter read zero on the ground at a particular airport is called the QFE for that airport. The QFE system is not very popular. I believe the British military uses it, as does Russia, so probably much of the former Eastern bloc. I highly doubt that a Canadian or American controller would be able to give you a QFE if you asked for one, and many probably would not understand the question. Even if you could get the QFE, there are many airports in the US where you couldn't set it on an ordinary altimeter, because the numbers in the Kollsman window don't go that high.

The far more popular way to set an altimeter is using the QNH, but North American pilots don't learn that term, either. We just call it "the altimeter setting." The altimeter setting is the number you set in the Kollsman window so that the altimeter shows airport elevation while you are on the ground. If the airport is 1200' above sea level, then the altimeter shows 1200' when you're parked, and 2200' when you're in a thousand foot circuit pattern.

Neither the QFE nor the QNH for a particular airport is fixed. It varies from hour to hour with the weather patterns. That's why the controllers and the ATIS broadcast the altimeter setting.

There's a third way to set your altimeter, which everyone does one they climb into the flight levels. That is to set 29.92 and leave it, so that you don't have to keep changing it as you go through different pressure systems en route. If there's a Q-code describing this setting I don't know it. We call it "twenty-nine nine two."

Update: The terse and anonymous first commenter below has indicated that twenty-nine nine two is called QNE. And in verifying that, I'm reminded that with 29.92 set on my altimeter, it is showing the pressure altitude but most people don't call it "setting the pressure altitude" unless they are doing it momentarily for the purpose of determining the pressure altitude for a calculation.