747-400 EIU logic

[blue up]

This question relates to a Sim but ought to be the same as the live aircraft.

Whereabouts is the decision made for an EIU to be selected ? On our Sim it naturally will sit with the centre and left DUs fed from the left EIU until the left fails, when it then passes all of those 4 DUs to the centre EIU. Is this non-standard? I assumed that the centre displays would be powered normally from the centre EIU. We have an odd fault in that the left EIU sometimes trips off during engine starts and can be reset by the breaker. The CMC display test shows which DUs are powered by which system and when the fault occurs it suggests the left EIU issue is not with the box itself, but more likely in whatever system selects it. Changing EIU units over between left and centre racks doesn't move the fault. I hope this makes sense.

It is staring to give me a headache!

Thanks

B

[sph33r]

Hello, without having any knowledge of the 747, Ill apply what I would do in a similar situation... the swap over not moving the fault means its not that particular LRU, the screens operating under control of the other EIU means your screens are operating so the big Q...what else is in between these two points, that's where I'd look... and hope it's an LRU, wiring's a bitch.

[Aeroshizzle]

not a mechanic, but looking through my manuals

for normal ops, the EIU selector remains on auto, if you don't have auto then your corresponding Left or Right
only 1 of the 3 EIU supplies data to all the EICAS displays
Left EIU, supplies, Left PFD, ND, and the 2 EICAS displays, Centre is on standby, Right supplies Right PFD, ND

Once the Left EIU fails, it will auto switch to the Centre, and if that fails, switches to the Right.
Auto switching is inhibited during a standby power situation

hope that clears the transfer logic

[blue up]

Centre always planned to be on stby. Hmmmm. I can see a certain amount of logic in there, but...

More playing tomorrow, I think, in between changing power supply boxes and fixing a helicopter computer. Busy week.

Thanks, guys.