737NG TR UNIT light

[framer]

Hi,
If the TR UNIT light illuminates in flight we know that either the TR1 has failed, or both TR2 and 3 have failed.
If it is. TR1 there will be no immediate effect as the DC Bus tie stays closed and we need to do an approach in a mode other than APP mode.
If it is both TR2 and 3, is there any loss of power to systems or does the TR 1 continue to power the DC 1 and 2 busses as well as the battery bus and standby DC bus?
I can't find any information on this in my manuals. The checklist does not differentiate.

[Jwscud]

TR1 will power DC Bus 1 and 2 through the cross bus tie relay.

The battery bus and DC Standby bus remain powered via the battery charger in TR mode from Transfer bus 2.

It's hidden in the battery Charger transformer/Rectifier on 6.20.9

[framer]

Thanks a lot JW,
With that being the case I imagine there will be no other indications than the original TR UNIT light for a failure of 2 and 3 which explains why the checklist is how it is. I should have been able to work it out from the diagrams but electrical is not my thing.
Cheers

[Jwscud]

It's not that clear in the FCOM as they don't give diagrams showing the failure case and it's one sentence in another section.

Has anyone tried arming APP in the Sim with a TR failure? What happens at GS capture?

[framer]

I have with a TR1 failure.
You get enough caution lights and red flags to make you think that something major has gone wrong. Definitely a good idea to do a different type of approach or if ILS was needed to get visual then do it in V/S.

[B737900er]

Yes, it goes dark! The systems that are powered on the failed side will go blank because the cross tie relay opens up to isolate the DC systems.

You can easily bring the system back online by quickly re-tuning both navaids by using the standby switch and then re-tuning them back to the ILS.

If you do it quickly there shouldn't be much deviation from the glide path. Hit VORLOC and us VS.

Jobs a good 'em.

[framer]

B737-900 are you talking about a failed TR1?

[B737900er]

Any of them! The cross tie relay is key in the scenario.

My answer was regarding the sim question.