747 & 767 Elec Bus Isolation During APProach

[QAVION]

Need some real pilot input on this one, folks... My manuals are letting me down.
On another site, a Boeing 777 pilot was relating how his Check Captain selected "APP" on his MCP in cruise (on a real flight) and the electrical busses isolated for Autoland (then and there, well above the expected 1500' plus time delays). Our Boeing 747-400 Maintenance Manual tells us that Bus Isolation occurs when either the A/P is triple armed or engaged, but haven't actually seen this happen in real life.... and our AOM specifies engagement.

Anyway, I believe all three CMD buttons on our 747-400's & 767's light up when the "APP" button is pushed at any altitude (above the A/L range), indicating that all three A/P's are armed (or rather one is in command and the other two are armed). Does this mean that Bus Isolation is active?

Since there are so many conflicting manual references here, I just wanted to know what the real story was on those 747-400's and 767's (with the option of arming all 3 A/P's above 1500'). On the 747-400, this would be easy to prove as there is an Elec Synoptic. On the 767, would there be any indication in the cockpit that Bus Isolation has taken place? (E.g. a change in loads on the ELEC/HYD Maintenance page on the Lower EICAS (active on some aircraft above 10,000'))?

Perhaps something for long hauler 400 and 767 pilots to look into when the flight gets a bit boring? :D With so many conflicting manuals, I'd prefer that sim experience be discounted at this point (as they may have been programmed according to a specific manual).

Thanks.

Regards.
Q.

[bsevenfour]

QAVION,

On the 747-400 when approach is engaged all 3 autopilot lights illuminate. However only the auotpilot originally selected is engaged while the other two are armed for use.

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 close and allow generator 4 through the synchronous bus to power the associated AC bus and as a result the autopilot. As all 3 auotlpilots still have independent sources of power then an autoland can still be accomplished.

Three unusual aspects to this isolation :

- The BTB open lights do not illuminate when this isolation occurs.

- You do not get an ELEC BUS ISLN message.

- Finally if you do select the electrical synoptic after isolation has occured you merely get a message on it saying " Electrical Synoptic Inhibited For Autoland " ( this is true for the -400s we fly not sure if it is universally so).

For the isolation to occur higher than 1,500ft would not make much sense. The idea of the isolation as previously described is that each autopilot has an indepndent source of power while engaged. As above 1,500ft only one autopilot is engaged while the other two are merely armed there is no real need to provide independent power supplies for two autopilots that are not actively involved in controlling the aircraft.

Hope this helps.

[QAVION]

"For the isolation to occur higher than 1,500ft would not make much sense. "

Agreed, bsevenfour. So why do the 777 busses isolate above 1500' and why does the Boeing 744 Maintenance Manual state that the busses will isolate when all three are "armed, or engaged"? (Ch 22-11-00 page 5 "C. AFDS Autoland Bus Isolation", Item(6b))

Are you simply quoting from your manuals (like I am) or have you looked at the Elec synoptic after pushing "APP" above 1500' on a non-sim 744 (with the type of autopilot which illuminates all the CMD switchlights above 1500'). What I really need is for someone to tell me that they pushed the APP button recently ...and for some strange reason, they had to look at the Elec Synoptic after doing so... not something, I'd imagine, a 744 pilot would normally do).

No offense (honest)... just trying to clarify the issue once and for all.
(I've already had a 744 pilot tell me that he knew how something worked, but after I asked him to recheck, he realized he didn't).

Thanks.
Rgds.
Q.

[QAVION]

P.S. Re the 767 Bus Isolation process... this image was kindly provided by one of the gentlemen over at the AVSIM site.

http://ftp.avsim.com/dcforum/User_files/3d3f70f854f3b3a9.jpg

Rgds.
Q.

[CCA]

I'm not on the 400 any more but I did once put the elec page up above 1500' and watched it go from the normal display to inhibited for autoland at 1500' not sure about 777 but hasn't it only got one A/P button and all all autopilots are active but the ARINC sorts it all out, sorry about the poor discription it was a vague disscussion in a bar regarding the 777.

[QAVION]

Thanks CCA.

Just out of interest, was the A/P the type which lights up all 3 CMD switchlights above 1500'?

A further report just came in regarding the 767. One gentleman tells me that one of his VHF comms momentarily cuts out when the APP button is pushed (above 1500'), also indicating (in a somewhat disturbing manner) that Bus Isolation is taking place on this aircraft.

Anyone here fly 757's? :D (Just wondering if the 747-400 is the only modern Boeing without this feature?)

Cheers.
Q.

[CCA]

Yep all the lights came on at APP arm

[QAVION]

"There was a couple of 744 pilots and engineers involved with the creation of PS1

Er... thanks, Michael.... (actually... I was one of them :D ). However, as they say... we had some of the answers all of the time and all of the answers some of the time, but never all of the answers all of the time ;)

BTW, PS1's designer, Hardy Heinlin, hinted, a few months(?) ago, on the Aerowinx forum that the display of the isolated busses on touchdown doesn't actually occur. However, he went to so much bother programming the unseen effects on the electrical system of bus isolation, he felt he had to showcase bus isolation (graphically). Unfortunately, since PS1.3's release, it has been noted that Bus Isolation isn't cancelling (as it should) when certain system failures are generated.

"Cheers" ;)
Q.


CCA.... Thanks for the follow-up :)

[Cornish Jack]

G'day Q.
Bsevenfour has the guts of the thing. Bodstrup is correct in what happens on PS1 but it does NOT happen on the real aircraft. PS1 is useful in that it gives you an electrical synoptic view that can't be seen in real life - i.e. the ACTUAL electrical configuration during LAND3 ops and the change which occurs on A/P de-selection.
The three A/P lights on the MCP illuminate as soon as the APP switch is pushed but only one A/P is engaged until 1500' when all three start 'working in committee' (assuming 'Land3' annunciated).
PS - still haven't found the ADP info..... not surprising with half a room full of manuals and notes :eek:

[None]

As I understand the 767, when the APP button is selected the electrical system is separated (the autopilot systems are connected to separate power sources...see QAVION's third post above). This results in the number one comm being momentarily interrupted (the standby DC bus is changing its power source).

Separation is different than isolation because transitioning to single generator operation will result in the electrical system reverting to normal operation.

Isolation occurs below 200 feet RA (AGL). If transition to single generator operation occurs here the center autopilot remains powered by the standby invertor and battery system.

[QAVION]

"still haven't found the ADP info..... not surprising with half a room full of manuals and notes"


G'day, CJ.
I think I have the ADP thing almost sorted. ADP ops is reliant only on TE flap movement... not LE flap movement. However, some 747-400's have time delays on both the "flaps out of up" logic and the "flaps in transit" logic. This means that when the flaps are moved from, say, 25 to flap one (on the ground), the ADP's still run for 30 or more seconds after the flaps have reached 1 unit, and if UP is selected within this time, it gives pilots and unsuspecting engineers the impression that the ADPs are looking at LE position. Apparently this delay is a modification which helsp reduce wear and tear on the hydraulic system components.

Re the Bus Isolation, it looks like I am going to have to accept that the Boeing 747-400 Maintenance Manual is wrong (or, like the ADP logic, there are variations from airline to airline). One thing I did learn from this exercise is that, even in our own 767 fleet, there are a ludicrous number of variations (e.g. there are 3 different bus combinations when it comes to powering MCP's).

Anyway, thanks to all :)

Cheers.
Q.