B-737NG F/D dissappear at G/S intercept
On approach you lose both engine driven generators and follow the abnormal procedure. You fail to get any generators back but manage to get the APU going. Once it becomes available, you power both AC transfer busses (B-737NG) with it and re-engage A/P "A" in command.
You are ready for the ILS approach and you get vectors to intercept the localizer. You intercept it and once established inbound, you have the G/S armed with the APP mode engaged. At the precise moment the G/S is captured, both F/D command bars dissappear and you are left with raw data information to complete the approach.
We know that the cross tie relay opens at the G/S intercept point isolating DC bus1 from DC bus 2. We also know that when the APU is the only power source, the galley busses and if needed, the main busses are automatically shed.
My question is, why do we lose both F/D at the G/S intercept point?