The one that fell off was in a tommy. Which I have 1100 hours SEP. And wasn't really an issue as I was IFR well "I follow roads" and the A9 being the road in question. And the was zero backup apart from looking outthe window and speaking to ATC.
And in the one the FO killed 3400 hours but it is as much use as a chocolate fireguard because it gets calibrated with both generators off, windshield heating heat off etc. Which realistically would mean your into a double generator failure or god for bid a double engine failure. But there is no backup as such to it. It backs up two flux valves, gps, ATC. Its main use is for jamming charts behind it when your doing a LPC/OPC to cover up the windscreen for who ever is flying.