744 AC Bus Isolation on Approach

[stratofactor]

Below 1500ft. elec bus isolation occurs when all 3 autopilots become engaged. During this isolation bus tie breakers 1,2 and 3 open to ensure each autopilot has an independent power source. In the meantime generator 4 powers the Synchronous bus as a back up power source. Should generator 1,2 or 3 subsequently fail it's associated BTB will lose and allow generator 4 through the synchronous bus to power the associated AC bus and as a result the autopilot.


How does the system work if dispatched with generator 4 inoperative? Which generator will power the sync bus? Will LAND 3 be available?


Thanks

[BuzzBox]

It's been a while but if my memory serves me correctly, with Gen 4 inoperative the system automatically reconfigures and one of the other generators remains connected to the synchronous bus during bus isolation. I can't remember which one. Independent power sources are still available for all three autopilots, so LAND 3 is still available.

[JammedStab]

Land 3(fail operational) is available with 3 electrical power sources. No distinction is made for which three generators are required. As well, the MEL for one generator inoperative makes no mention of landing restrictions. Therefore there must be another generator that powers the Sync Bus if #4 is unavailable. Which one I don't know. And seeing as the electrical synoptic will not display when Land 3 is active, it won't help by looking at it in the sim if you want to try.

That being said, generator #4 normally doesn't power any autopilots which is why it is the only one powering the sync bus for Land 3. Therefore if one of the other three gererators is lost(meaning one of the generators powering an autopilot when busses are isolated is lost), then I suppose the #4 BTB remains closed as usual along with a second BTB, being the one for the failed generator's bus. Never thought about that before.

[Spanner Turner]

4 engined aircraft.......what a wonderful thing.
Queen of the skies!

[MarkerInbound]

The FCOM says if any DC or AC source fails while isolated the system automatically reconfigures to maintain independent power sources for the three autopilots. Doesn't say which one takes over. As pointed out above there are no MEL restrictions on approaches with one gen inop. The QRH CAT II/III briefing page says you need three generators so system logic will take it from there.

[EEngr]

there are no MEL restrictions on approaches with one gen inop.

For generators 1, 2, or 3 inop, number 4 will pick up that load bus. For #4 inop, the #4 load bus will be powered by #3 (if I recall correctly). All you lose with one generator inop is the ability to continue a normal autoland in the event of another generator (or its associated engine source) loss.

[JammedStab]

Originally Posted by EEngr

All you lose with one generator inop is the ability to continue a normal autoland in the event of another generator (or its associated engine source) loss.

Not sure what you mean by "normal autoland".

Our QRH CAT II/III briefing page for shows CAT II/III fail-passive with two electrical power sources(ie. two generators lost). It is not monitored by the ASA. I would have assumed that this would be LAND 2 meaning autoland capable. Are you saying that the ASA will say NO AUTOLAND?

I think you will have autoland but not LAND 3 fail-operational and the electrical system will re-arrange itself accordingly.

[EEngr]

Not sure what you mean by "normal autoland".

Bear with me, I'm just the electrical guy.

Engaging autoland causes the FCCs to send an isolation request signal to the bus power control units. If the BCUs can reconfigure the electrical system to provide three isolated channels, they will do so and provide a confirmation signal once configured. If one or more sources are lost, the power system will reconfigure to provide power to all buses. If this results in a loss of three channel isolation, the confirmation signal will be removed.

The response of the FCC/autoland system and resulting cockpit warnings are a bit beyond my expertise. But I believe that the behavior and warning messages depend on the altitude at which the 'out of configuration' condition occurs. Above the decision altitude, the failure is handled the same way a failure to enter isolated mode would be (a BCU FAIL message if I recall correctly). Below, the failure is annunciated but is a fail active condition, where the autoland system remains engaged.

[NSEU]

For #4 inop, the #4 load bus will be powered by #3 (if I recall correctly).

My engineering manuals concur with this. Gen 3 will be providing power to the sync bus and Bus 4 during A/L.

If another generator fails above "alert height" (200' RA), bus isolation will be cancelled and LAND 2 will be annunciated. With Bus Isolation removed, all busses will be powered (i.e. all computers/sensors will still be powered). You will only lose your power source "triple redundancy".