dumb question, why aren't the flight instruments augmented by GPS instrumentation? It might not be as accurate but it'd be an easy backup system.
Last flight I was on I was watching our progress on google maps (yes Wifi/3G was off, the maps were cached) on my Android tablet - I got alt, heading, airspeed, all very accurate. And my garmin GPS I use while hunting can get coverage under dense push, in gullies, in heavily overcast weather.