Re: External inspection

Date:         13 Feb 99 02:26:08 
From:         JF Mezei <>
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Gerard Foley wrote:
> GPS speeds in automobiles are extraordinarily accurate.  At aircraft
> speeds they are reasonable, but the mile markers go by too fast for me to
> click my stopwatch ):-)) to check.  You're right, of course, that the
> ground speed isn't much use on takeoff, unless you know there's no wind!

The average error on a GPS system actually goes down at higher speeds. The
small deviation due to SA and natural phenomena is more significant at low

During takeoff, if your airspeed indicator has failed, the pilots should
notice this and abort, shouldn't they ?

But in flight, if they fail, they should be able to use a GPS. While the
altitude may be close to a 1000m (3000feet) off from barometric altitude, (I
have seen it at 800metres off), you can use it to monitor if your are fairly
stable or dropping altitude (or climbing). And you may not be able to directly
calculate indicated airspeed from the true ground speed, but you should be
able to get a good idea based on weather reports and perhaps radio contact
with nearby planes on the same track.