Bertie the Bunyip wrote:
Flydive wrote in :

Bertie the Bunyip wrote:
Mxsmanic wrote in
:

Andy Hawkins writes:

Just out of interest, how do you measure speed on the ground?
Obviously the airspeed indicator isn't much use in these
situations...
If you have fancy avionics, they'll tell you your actual ground
speed,
as read from an IRU or GPS.
Says mr know nothing..


Bertie

But he is right


Not really. We use them as a rough guide but they're not all that
reliable on the ground. Only as a guide. Older widebodies had ground
speedos just like a car. Just a little clock on the capt's panel down
the bottom, usually. All newer ones use IRS, but
IRS info can be out by up to 10 knots by the time you land. A
negligable error in flight, but huge on the ground. Also ****s up your
wind indications on appraoch. Not a problem for a pilot, BTW, but it
does screw up the automatics if they get confused about the crosswind on
some airplanes.
It's not uncommon for them to show movement while you're parked after a
long flight particualrly if radio update isn't all the best towards the
end. Like if you're flying into an area without a lot of DMEs. newer GPS
stuff is better, but it's not really neccesary anyway except in 74's
wher it's quite difficult to tell your GS.



Bertie

True in part, the latest FMS use the GPS as primary source, so the GS
indication is quite accurate on the ground.