Originally Posted by Flying Stone
Your GS is 500kts. Are you safe? Could be TAS of either 650 or 350 with a 150kt wind. Neither of which will keep you flying for long at cruising FL.
Hang on. Nobody is suggesting you suddenly find yourself plopped into a scenario with now idea how you got there or what the current conditions were and now had use the GPS.
You will obviously be happily flying along, knowing you are doing the right speed, then the pressure instruments go awry. The GPS GS is a great indicator in that case to keep you flying until you descend or otherwise sort out the problem.
The other scenario is after takeoff. You are starting from a relatively accurate known point: get it cleaned up and maintain 250 GPS GS, allow for the low level wind if you like. Safe as houses.
GPS GS is a fantastic aid if you have no IAS. You could even use your phone GPS speed.
Display of AOA is the best possible back up
In my aeroplane, the FPA is not reliable during a UAS event. I assume that AOA (unless it was just a raw readout from the vane) would also be dodgy.