Elec Bus Isolation 747-400

[QAVION]

Hi, FELlOw PPRuNers.

Can anyone tell me if Elec Bus Isolation happens on the 747-400 when all three A/P's are _armed_ for approach... or does this only happen when all three A/P's are _engaged_ at 1500' (when LAND3 is annunciated)?

I thought it was the latter, but the Boeing Maintenance Manual says otherwise. If you have a copy handy, the reference is 22-11-00, possibly page 5 (depending on your airline).

Presumably, this is co-incident with the Electrical Synoptic page being blanked, so this should be an easy question for 400 pilots ;-)

Thanks.
Regards.
Q.

[aviator]

Looking in my Flight Handbook it states

Autoland
During an autoland, with three autopilots selected,AC and DC buses 1,2, and 3 are automatically isolated. AC and DC bus 4 continue to power their respective tie bus. This provides an independent power source for each autopilot. There is no indication of this isolation. The electric synoptic is inhibited at this time.

During a 3 engine autoland approach, electrical connections are automatically made to ensure AC and DC busses 1 2,3, and 4 are powered and an independent source for each autopilot is maintained.

With a LAND 3 indication, the level of redundancy of the autopilot system is such that the occurence of any single fault would not prevent the autopilot system from making an automatic landing (fail operational).

Since LAND 3 comes on at 1500 feet RA it seems to me that the isolation takes place at this time.

No direct quote - just reading between the lines...

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I have a need for speed!

[QAVION]

"Since LAND 3 comes on at 1500 feet RA it seems to me that the isolation takes place at this time."

The "seems" part is the bit I was trying to avoid, Aviator ;-)
I was hoping for some eye witness accounts (preferably not in a simulator). This way I could write to Boeing and tell them that their manual needs correcting... or at least clarifying.

By the way, on our aircraft, with one autopilot engaged before Loc intercept and the APP button pushed, all three CMD buttons light up. I believe there are variations to this. Hopefully, my eyewitness would be flying the same type.

Rgds.
Q.

[spannersatcx]

It TAKES place at 1500'

[QAVION]

"It TAKES place at 1500'"

So you're saying that the Boeing Maintenance Manual is wrong about the "arm" statement, Mr Spanners? Perhaps you can tell my old mate, John Karas at CX Maintenance Watch(Avionics) to look into it also ;-) (Tell him the "Riddler" sent you).

Cheers.
Q.

P.S. I'd also like to know when he's going to sign my 747-400 experience books!

[BoxProf]

I can tell you that if you have the electrical synoptic up at the start of the approach, it will blank as you pass through 1500'. To me, that indicates that 1500' is when isolation actually occurs. By the way, I understand that there is actually some testing that starts at 1500', and you generally won't see LAND3 until around 1350'.

[QAVION]

"I can tell you that if you have the electrical synoptic up at the start of the approach, it will blank as you pass through 1500'."

Thanks, BoxProf et al....

Now if only I can find the time to write to Boeing 8-)

Cheers.
Q.

[BoxProf]

Qavion - "If I can only find the time to write to Boeing"

Make sure you include the year born, software version, company specs, serial number....oh, and I understand they will tailor your answer depending on whether you are a pilot or tech. I only say this because we made the mistake of calling them one day from class on a question....

Hey, at least they didn't give the answer in French (g). - BoxProf

[Cornish Jack]

Just a little additional info....
The three A/P bars light up at APP selection but there is still only ONE A/P operating as evidenced by the PFD FMA of CMD. That changes to LAND 3 at some point AFTER 1500' RA - the interval being the requirement for the self-check confidence test.
The elec bus isolation occurs so that, in the event of any single engaged bus failing, the #4 bus will take over the failed bus via the Sync bus and the failed gennie's BTB.

Apologies, if that's "teaching granny to suck eggs" etc.

[QAVION]

"Just a little additional info.... The three A/P bars light up at APP selection but there is still only ONE A/P operating as evidenced by the PFD FMA of CMD."

I think I'll have to look up the legal definition of "armed" here. I understood that the illumination of the 3 CMD switchlights indicated that all three A/P's were "armed" (and that the arm solenoids in the A/P servos were powered, with the servo position synchronised to the flight control position). How this is related to the FD/CMD/LAND 3 annunciation during the period between when the APP button was pushed and 1500', I don't know. (Hey, is anyone from Boeing/Collins listening in on this conversation??? Help!) :-)

"Apologies, if that's "teaching granny to suck eggs" etc."

Never too old for reminders, CJ... In fact, the older I get the more I need ;-)

Cheers.
Q.

[Cornish Jack]

Sorry Q - just back off hols and the brain is still not fully engaged!! (is it ever?)

You are quite right, of course, the other two A/Ps are ARMED at APP selection, but only the original is actually controlling the aircraft, "as evidenced etc"