Re: Boeing 737 crash

From:         lhe@sics.se (Lars-Henrik Eriksson)
Organization: Swedish Institute of Computer Science, Kista
Date:         01 Feb 95 02:16:21 
References:   1 2 3 4 5 6
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In article <airliners.1995.68@ohare.Chicago.COM> irksome@netcom.com (Ian Urquhart) writes:

   Please excuse my ignorance but what is QNH and QFE?  I hold a US 
   commercial certificate and instrument rating and have not come across 
   these terms.  There is only one altimeter setting and that's it. 
   Simple.  Do Canadian pilots deal with this QNH and QFE business? 
   Just interested.

QNH is what is called "altimeter setting" in the US. "QNH" is actually
the term that is used, at least in Europe, for this information. To be
precise, QNH is defined as the altimeter setting that will make the
altimeter show the airport elevation on the ground at the airport.

QFE is the actual air pressure at the airport, so that an altimeter
with a QFE setting will (except for temperature effects) show the
altitude over the airport.

Formally, QFE is the altimeter setting that will make the altimeter
show zero when on the ground at the airport. Or, to be precise, to
make the altimeter show its own height over the "QFE datum". The QFE
datum is typically the official airport elevation, but at least when
precision approaches are being made, it should be the threshold
elevation of the landing runway.

Note that the difference between QFE and QNH is fixed for any
particular QFE datum, so the QNH is computed by simply measuring the
air pressure at the airport (QFE) and adding the fixed offset.

There is also a third pressure code that is used in meteorological
contexts, namely QFF. QFF is, informally, the pressure that would have
been measured if the measuring station was located at sea level. QFF
is different from QNH, because it is not computed by adding a fixed
offset to the QFE, but an offset varying with air temperature.

The air pressure which is given by isobars on meteorological charts is
QFF.

Sometimes the code QNE is also used. This is simply the standard
setting - 1013.23 hPa (don't know what it is in inches of mercury).

--
Lars-Henrik Eriksson                            Internet: lhe@sics.se
Swedish Institute of Computer Science           Phone (intn'l): +46 8 752 15 09
Box 1263                                        Telefon (nat'l): 08 - 752 15 09
S-164 28  KISTA, SWEDEN                         Fax: +46 8 751 72 30