747-400 Stab Trim Indicator
 

Trying to understand the inner workings of this device, but with only the wiring schematics as a guide...

My training notes say that an OFF flag will appear and only the black part of indicator tape will show if power is lost. If the tail signal is lost, then only the black part of the tape will show (although my notes don't specify the nature of the signal loss... single broken wire?).

I understand that the OFF flag is actually stuck on the end of the tape.

The schematics only show power going to the selsyn(?) transmitter/indicator wiring. I would have thought that a power loss would result in a frozen tape.... or perhaps if there is a spring in the indicator mechanism, it would drag the tape to the OFF section. However, if there is a spring, then why doesn't a failure produce an OFF flag also?

I know they don't pay some of us enough to think about such things, but.... just curious.

Cheers.
NSEU

I've had a couple of these fail over the years they usually just freeze where they are . I've never seen an off indication at all .

Thanks, Jack.

I'll try pulling the CB and see if that brings up the flag. I have a photo of the flag taken many years ago, but I can't recall the details... possibly on an unpowered aircraft.

Cheers