[mairdie]

There was a discussion about the Russian plane that crashed. Here's an article putting the blame on, possibly, a frozen speedometer.

Pilots blamed for frozen speedometer that may have caused Russian passenger jet to crash killing all 71 on board - and they knew there were problems BEFORE it took off

[Billyv]

This would point to a frozen pitot tube resulting in bad airspeed/altitude/VVI indications on one or both pilot position instruments. Don’t know the configuration of this aircraft, but the US mil trains to recognize and deal with this possibility. If the aircraft could be seen from the ground, the pilots could see also, not too big a problem to deal with if VFR and trained at all.