I'm trying to get my head around the concept of isolating the TR units during an ILS approach. It applies to both the classic and the NG B737.
From the B737-800 Operations Manual:
The cross tie relay automatically opens, isolating DC Bus 1 from DC bus 2, under the following conditions:
*At glide slope capture during a flight director or autopilot ILS approach. This isolates the DC busses during approach to prevent a single failure from affecting both navigation receivers and flight control computers.
*Bus transfer switch positioned to OFF.
So before conducting an ILS approach we check that there are no TR failures. If for example we find that TR1 had failed, we would normally choose not to select APP mode if we were wanting to fly an ILS approach because we know that if we did, at Glide Slope capture, the DC Bus 1 would subsequently be isolated to only the TR1 unit and result in DC Bus 1 being unpowered causing all sorts of issues.
So what I don't understand is why isolate? What is wrong with another TR unit powering DC Bus 1 when TR 1 fails? Same thing if the failure happens during the ILS approach. TR 1 fails. TR 2 or TR 3 takes up the slack and the approach continues. What's the big deal?
If the cross tie relay stays closed, how can a "single failure" as referred to above create a problem. It seems to me that it's when the cross tie relay is closed that a single TR failure (TR 1 or TR 2+3 in the case of the B738) causes a problems.
I'm obviously missing something!
By the way if TR 1 had failed, and we wanted to fly an ILS approach, we'd simply use LOC and either V/S or VNAV anyway.
Also I can not find a list of items powered by the DC BUS 1 and DC BUS 2 for either the B734 or the B738. Does anyone know where I can find a list. It's very frustrating that such information is not included!
Thank you.