Originally Posted by
peterh337
You can check the altimeter against a GPS.
GPS altitude is normally very
accurate.
I know one uses barometric altimetry for good reasons but all GPWS systems use GPS altitude and its accuracy as a standalone source, not subject to an incorrect QNH setting etc, is inescapable.
In this one instance, I'd turn it around.
My interest (engine failure in IMC) would be putting the aeroplane on something flat and large in one piece. I'll talk to somebody and declare an emergency, at which point I simply don't care about airspace busts or deconflicting with other traffic - that's all somebody else's problem.
So, I'd simply use GPS as my primary altimeter, particularly making use of my (up to date!) terrain and obstactles database. I'd also use it as my primary speed indicator - since it gives me groundspeed and distance, and either from there or my VSI I have RoD, it should be pretty easy to answer the vital question "will I get there, pointed in the right direction, with air still under the tyres?" without faffing around with mental arithmetic that I just don't have time for at this point.
Most other times, the altimeter is primary - but not this time. Ditto airspeed - so long as I stay adequately away from Stall/Vne, I'll start worrying about the airspeed around 1000ft when I'm sorting my approach out.
G