10/22/2010

How high is your aircraft flying?

In the Lucerne transport museum the special exhibition to celebrate the centenary of Swiss aviation closes this weekend. If you get there you might see a reference to a QNH altitude: explanation.


When you are in an aircraft you expect the pilot to land on the runway, not some distance above it or below it. To achieve this, the pilot must know his exact altitude (above the mean sea level). The official name for this altitude is QNH, and I saw this abbreviation on an exhibit in the Lucerne Transport Museum, but without any obvious explanation. So here is the explanation.

An aircraft altimeter calculates the altitude according to the atmospheric pressure outside and comparing it to a base value. To do this correctly requires knowledge of the correct base value, in particular that for the region in which the aircraft will be landing, and for which the altitude is known. For Geneva airport the altitude varies between 1355 feet and 1410 feet above sea level: it has a little dip in the middle.

The name for this altitude reading is QNH. The literal meaning of this is "Query Nil Height", though in the UK pilots may remember it better as "Query Nautical Height", or even "Query Newlyn Harbour". This latter interpretation is because the UK National Tidal and Sea Level Facility, which references the mean sea level, is based there.

When an aircraft approaches Geneva airport it will, when it descends below a transition level (which may vary from airport to airport), request this QNH value, in milllibars, for the Geneva region. Remember, however, that this is NOT the pressure reading at the airport: I think it can be likened to what would be the pressure if one dug a hole down to the mean sea level!

However, in order actually to land in Geneva, the pilot needs to know his height relative to the ground. This goes by a different name: QFE (Query Field Elevation"), which may vary (slightly) from the QNH value. Thus, when the aircraft touches down, the QFE value should be exactly the runway altitude. If it is slightly higher, or lower, then the landing may not be as smooth as passengers would like. In fact, when an aircraft bounces on landing, the transponder will often broadcast that it has touched down and then gone back into the air. For the ARAG movement detection system, which is based upon analysing the transponder messages,, this has to be treated as one landing, yet be different from training flights (for light aircraft) where the pilot touches down then immediately goes back up.

And in the air, above the transition altitude? Well, up at higher altitudes it is important that the height reading, then referred to as the flight level, be identical for aircraft which are actually at identical heights. For this to be the case, all aircraft must use the same base setting for the (theoretical) pressure at sea level. This is defined as 1013 hectopascals (named after the French scientist Blaise Pascal), equivalent to 29.92 inches of mercury. This means that, although the pilot might announce that your aircraft is cruising at an altitude of 30,000 feet, this may be slightly inaccurate, but will ensure that it is 2000 feet below one which is crossing at 32,000 feet. Whew!

A little informative postscript: There is a computer programming language called Pascal, named in honour of Blaise Pascal by its creator, the famous Swiss computer scientist Niklaus Wirth. This developed from an earlier language called Algol (algorithmic language), which I learned about as a computer science student a few decades ago, and which was also defined at ETH Zurich. Another spinoff programming language developed from Algol was BCPL, which we used to write an early computer network at CERN.

 

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