On 28 July, 04:32, 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? 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...

I am not a pilot but maybe I can add something here.

Regarding GPS.

Obviously you can estimate airspeed with a gps and a weather forecast.
This estimate is not good enough to keep flying in an airliner
at 35,000 feet in a thunderstorm. There is only a small range
of airspeed over which the aircraft can safely operate. Maybe you
can look it up, I don't know any numbers. The problem is I believe
that the reducing air density, as altitude increases, *increases*
the stalling speed and the reducing temperature *lowers*
the speed of sound. The aircraft is limited
to an ever smaller range of safe operating speeds as
altitude increases. Look up terms like mach buffet, coffin corner.
The U2 spyplane I believe had only a 5 or 10 knot range when operating
at maximum altitude. It's more than that for airliners

So, the airspeed estimate from the GPS is not good enough
due to the strong swirling winds in a thunderstorm.

Unreliable Airspeed recovery

The airbus A330 has a specific "unreliable airspeed" flying
technique that should be used if the pilots think that the ASI
is not working. The A330 has 3 seperate ASI systems
and so it is expected that they will display different
readings if something is wrong with one or more of them.
You set an power level on the engines and
fly a particular attitude. (Or is it angle of attack?).

As you know aircraft are equipped with gyroscopic attitude
indicators. Many aircraft also have an angle of attack indicator
and I think the airbus A330 is one of them. The sensor is simply
a small weathercock operating on a lateral axis.

Google for Images with [angle of attack sensor] and you will
get loads of hits.