STAB trim cutout - still able to trim??

[John H Watson]

Or are you saying there'll be no bus isolation command at all because there's no LAND 3 capability? But then it shouldn't say LAND 2. It should say NO AUTOLAND.

(1) During the approach mode, three independent ac and dc power sources are required to
accomplish a fail-operational Category III autoland operation.

I can't find anywhere that it says that LAND 2 requires bus isolation. All the books say the same or similar thing:

From the AMM

(6) These are the conditions for the bus isolation to occur:
....
(c) All the autopilot channels are not NO LAND 3 and not NO AUTOLAND (the main EICAS
display does not show NO LAND 3, or NO AUTOLAND).

LAND 2 simply gives redundancy of sensors, not isolation of power sources. You won't get NO AUTOLAND.

P.S.: I think there has been proven in a BA sim that a single engine failure, e.g. #2, will keep the LAND 3 mode active. Bus 4 will open its BTB and feed bus 2. The redundancy is reduced, but it's still LAND 3, not LAND 2.

I have no disagreement with this. An engine failure can still provide a valid solution for LAND 3. As far as we know, if you have an engine failure, you won't get ">NO LAND 3" on the EICAS because all sensors/computers remain powered prior to APP being pushed. Bus isolation can occur and the sensors will still remain powered. On the other hand, having a dead AC Bus 1, I'm sure one or more of the sensors will have already failed prior to APP being pushed.

[Hardy Heinlin]

LAND 2 simply gives redundancy of sensors, not isolation of power sources.

I'm impressed. I didn't know this. Thanks to the inhibited EICAS ELEC synoptics during autoland nobody has ever noticed it :-)


Edit: I'm still so impressed. Please confirm that I'm not dreaming:
When, say, a single radio altimeter fails during autoland, and everything else is normal, none of the AC/DC busses will be isolated anymore, although autoland continues with LAND 2?

Edit 2: It must be true. Another hour has passed, and I'm now already used to it (I just modified it). I also learned that multichannel TOGA will cancel the bus isolation (but the A/Ps still keep controlling the rudders).

[Britjet]

Hardy - I was just going to see if the Stab Trim cutout stopped alternate trim. That is all..
Peter

[Hardy Heinlin]

But you did notice that the stab trim cutout comments in this thread are from last year and that they are not the subject of the current discussion anymore? :-)


|-|ardy

[John H Watson]

Thanks to the inhibited EICAS ELEC synoptics during autoland nobody has ever noticed it :-)

I have to admit, I've always had trouble getting my head around this stuff at the best of times. I'm starting to wonder, though, if the Elec Synoptic is actually inhibited during Land 2 (as the busses won't be showing anything unusual apart from the failures)

The 767 is the same. The 767 notes say "Bus isolation request is initiated for triple channel approach only". The 767 notes are good for removing ambiguities when it comes to A/P stuff.

Things didn't start making sense when you said that bus isolation would occur with the ELEC BUS AC 1 message. This would mean as soon as you pushed the APP button (this can be done in cruise), that you would lose DC Bus 1, too. DC Bus 1 has around 70 things attached to it.

(I just modified it)

WOW... I expected that hundreds of lines of code would have to be modified/deleted.

[Hardy Heinlin]

Not quite hundreds. For the bus isolation command there were just 50 lines :-)

Very nice. I can now even autoland with 3 engines out -- as long as the airspeed is high enough.

Do you think I'm on the safer side if I inhibit the ELEC synoptics only when "LAND 3" is on the PFD FMA? (That automatically considers conditions like multichannel TOGA with "CMD", and degradation to LAND 2 below 200 with "LAND 3" remaining displayed, and such degradation with "LAND 3" disappearing.)


Cheers,

|-|ardy

[John H Watson]

Ah.. just checked the Boeing Maintenance Manual:

When autoland status is annunciated on the Primary Flight Display (PFD), the entire
electrical synoptic display is replaced by a ELECTRICAL SYNOPTIC INHIBITED FOR
AUTOLAND message

So not necessarily LAND 3

[Hardy Heinlin]

Then I assume multichannel TOGA also belongs to "autoland" in this context (ELEC synoptics inhibit).

[John H Watson]

Possibly.

TOGA is multichannel, but on that Engineering page I mentioned, GA cancels Bus Isolation, so I guess you won't see much even if it is unblanked.

Do we know what happens on the real deck? 

[Britjet]

Just to confirm that operating both stab trim cutout switches causes the alternate trim to be inoperative.
(BA sim).
Peter

[John H Watson]

Thanks, Peter. Engineering concurs

[Hardy Heinlin]

Modification of SRM power logic and autoland bus isolation is now available in PSX 10.43:

http://aerowinx.com/board/index.php?topic=4191.0


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