Hi there awblain
The GPS in AF 477 during the descent would have been reading less than 100 its. They were actually descending faster than that VERTICALLY! The pitot static system icing up can render the Altimeter and the airspeeds invalid - don' t know what the situation was with air france. The big clue is that the ground speed was 350 knots too slow. It simply amazes me that many airline pilots don't realise the value of GPS Ground Speed to stabilise the flight path Way back in the 80s, we flew the 737-200 with total failure of airspeed and altimeters back to a save landing using G/S and of course, pitch and power but the real saviour was ground speed. Say your landing speed is to be 130 and the headwind is 30 knots you just fly the approach at 100 kts ground speed and you will land at precisely 130 kts airspeed. The Tristar used to do this routinely and funnily enough, the Airbus does the same in managed speed mode (if it is working , and this is the problem).
It flies the approach using groundspeed to fine tune the approach speed.
Cheers for now
John