GPS and ground speed

On Mon, 27 Jul 2009 20:32:52 -0700 (PDT), D Ramapriya
wrote:

Since

(a) Ground Speed can be determined by GPS,
(b) the relationship between the aircraft's attitude and the angle of
attack should theoretically be unvarying and
(c) the attitude indicator is a gyroscopic instrument,

is it possible to assume with any correctness that during cruise, an
aircraft can only be at *a* particular ground speed at *a* particular
altitude at *a* particular attitude?

dont think so. the attitude and speed that the aircraft will return to
unaided is dictated by the decalage and that will vary with the
elevator trim position. each different combination of thrust and trim
position will result in a different speed.


If the answer is Yes, is it
possible to develop some formula where the pilot can at least arrive
at a rough airspeed figure using the GPS should his pitot tubes get
blocked or iced for whatever reason? I acknowledge that this will only
be a rough estimate since headwind and tailwind can't be measured.

I'm thinking about the recent Air France crash and wondering if at all
it could've been prevented using such a calculation to roughly
estimate the actual airspeed instead of having to rely on entirely
inaccurate IAS...


if you were forced to use a gps unit for speed you could correct the
apparent value using the metrological forecast winds for the altitude.
it is a simple speed triangle calculation that could be done on a
circular sliderule like the jeppeson cr5.

in all of aviation the experience has been that a properly maintained
minimal component count in the systems has resulted in better
serviceability than a myrriad number of interconnected poorly
maintained systems. you quest should be to perfect the existing.

you just have to accept that at the leading edge of a technology you
sometimes find leading edge problems that no one has an answer for.
for those moments you just hope that the guy up front has a name like
Bartels or Sullenberger or the thousands of other pilots who are just
as capable but havent been called on to become famous.

Stealth Pilot