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'). 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 the AOM specifies engagement.Anyway, I believe all three CMD buttons on our 767's light up when the "APP" button is pushed at any altitude, 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 the 767 (with the option of arming all 3 A/P's above 1500'). Are there any indications in the cockpit, other than the "Land 3" annunciation, that Bus Isolation has taken place? (E.g. a change in loads on the ELEC/HYD Maintenance page on the Lower EICAS)? Perhaps something for a long haul 767 to look into when the flight gets a bit boring?Cheers.Ian.

Hope this helps, From My 757/67 pilot's reference manual. I took it with my digital camera, as I don't have a scanner. Jim

Thanks, Jim.It really does look like the busses isolate upon pushing the the APP button doesn't it.I'm more than happy with this, but it would be interesting to know if there are any noticeable changes to the values on the Maintenance ELEC page when the APP button is pushed.Cheers.Ian.

G'Day Ian,Jim is correct in saying the power transfer takes place as soon as you press the APP button, regardless of phase of flight.On the flight-deck you can hear the relays clicking when you press the button, and the left VHF is unpowered for a fraction of a second (you notice this if making a radio call during the transfer).

Thanks, HP.It's interesting to note that a lot of 767 manuals tell you that, during Bus Isolation, that the Centre Bus is fed by the Standby Inverter/Battery, giving the impression that the battery is being drained. I had originally thought that the Left Bus was actually providing power to the battery and that the battery wasn't actually being drained, but after trying to figure out the answer to Laurent's quiz from last week, I did a bit more reading on the subject. I now believe that the Right Bus provides power to Center Bus.... via the the Ground Service Bus, the battery charger (which is running in Transformer-Rectifier mode) and the Standby Inverter (via the Battery relay). (Easy, huh? :-)).Because of this, the A/P's are not really powered by separate power sources during Autoland... However, if the Right Bus fails, then the aircraft may start draining the battery.Here's a "work in progress" on the 767 electrical system in relation to Laurent's question of whether the aircraft will perform an Autoland if the Right Bus is totally dead....http://www.ozemail.com.au/~iriddell/767/RightACBusFail.gifThe grey colouring represents powered busses.I've shown the APU running, providing power to the Ground Service Bus and other busses. There are still some things I'm not sure of. Once these are sorted out, I will be able to figure out what parts of the Autoland system are powered/not powered.Cheers.Ian.