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X-issue: 7.20
Date: Sun, 10 Jul 88 21:57:41 EDT
From: mnetor!utzoo!henry@uunet.UU.NET
Subject: Air France Airbus crash

> Does the Airbus model in question display altitude in feet or meters?
    [Question raised regarding whether the Air France Airbus was at 30
    feet or 30 meters...]

Unless I am greatly mistaken, in feet.  Altitudes and airspeeds are not
quite the same situation as fuel volumes.  The latter are a matter for
individual aircraft; the former are of vital interest to air traffic
control and other aircraft, and units are internationally standardized
(altitude in feet, airspeed in knots).  I wouldn't even expect readouts
in both units, because altitude and airspeed are safety-critical items
and even the slightest confusion about which number is which is unacceptable.

It's kind of unfortunate that aviation standardized on units that are now
obsolete, but this is one of those cases where the actual units are not
very important so long as they're standard.  For navigation one needs to
turn airspeed into map distance, and for the initial phase of takeoff and
the final phase of landing the absolute altitude is significant, but
otherwise comparisons are usually relative and the units of measurement
don't matter much.  For example, what matters about airspeed is not its
absolute value, but its value relative to safe limits, optimal values
for the particular phase of flight, and the value requested by traffic
control.  Even near-ground altitudes are relative to some degree:  50 feet
of altitude is a good takeoff-obstacle clearance for a Cessna, a dangerously
small one for a 747 (which can't do a hard turn without sticking a wing
down farther than that), and a routine operating altitude for military
aircraft in wartime.
				  Henry Spencer at U of Toronto Zoology
				uunet!mnetor!utzoo! henry

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