Why center autopilot in 757/767

[mono]

NSEU,

One the 767 it does indeed control the aileron lockout.

On the 757 it controls the elevator assymetry actuator. This is also airspeed dependant rather like the aileron lockout.

Both terminologies are correct it just depends what a/c they are fitted to!!

PS. I am an engineer (and the term is from the AMM) and reverted to type in that I gave it the name I first heard as the 757 was the first of the 2 on my licence.

[NSEU]

Thanks, mono... I'd ask what its function is, but I don't want to hijack this thread

[SuperRanger]

Gvendur,

sorry for jumping in late. i think Jetstream had answered your question. in case, you missed it.

it is all about "Fail Operational" and "Fail Passive" during a LVP.

google reveal this.

SR

[chemical alli]

so a curly question then is how do you get a triple a/p engaged on a 75/76 with only two different power sources for autoland,that should keep you guessing for a while.as each autopilot must have a its own dedicated pwr source for auto land.

[Jetdoc]

Power for the Autopilot/Flight Director System is provided by the engine or APU driven generators via the left and right AC buses. The left and right transformer rectifier units (TRUs) provide DC power to the left and right DC buses respectively. During normal operation, airplane cruise or takeoff, the center AC and DC buses are powered by the left buses. During autoland operation, the center AC and DC buses are powered by the static inverter and hot battery bus. Center bus switching is provided by the FCCs internal isolation request and system relay logic. The switching provides each FCC with an independent power source during autoland operation.

[chemical alli]

go jetdoc your the man.anymore 767 questions and i am coming to you

[NSEU]

During autoland operation, the center AC and DC buses are powered by the static inverter and hot battery bus.

Here's a tricky question for the experts...

If you pushed the APP switch in cruise, initiating bus isolation.... would the battery go flat before you got to the runway?

[Spanner Turner]

If you pushed the APP switch in cruise, initiating bus isolation.... would the battery go flat before you got to the runway?

Well, Bus isolation only happens when valid approach data is available to each FCC. (don't think there would be valid glideslope and/or localiser signals available in cruise)

In any event, when isolation happens (valid triple autoland) and the centre DC bus is powered from the battery bus - the main battery charger goes into TR mode (transformer/rectifier) to supply the centre bus which prevents any real draining of the battery !

[Jetdoc]

Pitch Channel – Autoland
Multichannel engage is initiated when one channel is engaged in CMD and one or more A/P channels are manually placed in CMD with APP selected. The switches remain in CMD only if APP is selected. Pitch multi-channel engage occurs at 1500 feet radio altitude if G/S and LOC are captured and two or more channels are armed.
Since multichannel engage occurs at 1500ft or below, with the above conditions, there will be no bus isolation request and no drained battery.

[NSEU]

Since multichannel engage occurs at 1500ft or below, with the above conditions, there will be no bus isolation request and no drained battery.

This certainly makes for a good debate. You might like to discuss this with your fellow pilots/engineers/tech experts.

Irrespective, as Spanner Turner says, during bus isolation, the Battery Charger is providing power to the Hot Battery Bus, Static Inverter, Center Bus, etc. The Battery Charger switches to TR mode when the battery relay closes (as part of bus isolation). So where does the Battery Charger get its power from.... The Battery Charger is provided with power from the Right AC Bus. So, in reality, two FCC's are being powered by the Right Bus.
I believe the only time the battery is really used is if there is bus failure below 200'. Thus satisfying the independent source rules.

Incidentally, I've been told by pilots from two different airlines, that it's not a good idea to push the APP switch when you are using the VHF comms, as there will be a temporary loss of signal. This would also suggest that bus switching occurs whenever the APP switch is pushed, rather than at 1500'.

BTW, do the FCC's need valid ILS signals for all three CMD switches to light up... or just a selected frequency/course? (Haven't had the chance to try this in the sim).

Rgds.
NSEU

[Jetdoc]

• During a category III autoland, the main ac buses are prevented from being supplied by a common source. The autoland condition is started by the flight control computers. The computers provide a ground to the isolation request relay . The isolation relay energizes, allowing a 28-volt dc signal to be sent to the bus power control unit (BPCU).
• Multichannel engage occurs when all A/Ps have detected LOC and G/S capture and 1500 feet from a valid radio altimeter (RA). The first channel in command must receive acceptable CMDENG criteria from the other two FCCs. All A/Ps engage pitch, roll, and yaw channels. The ASA displays LAND 3 (Fail Operational) or LAND 2 (Fail Passive). All phases of autoland are possible with LAND 3 or LAND 2 displayed.
• Autoland configurations are not active until each FCC recognizes LOC and G/S capture; the airplane is below 1500 feet radio altitude; and other autoland logic requirements are met. App can be anytime below 1500 feet but i can'tremeber the lower limit for engagement.
• The dot-bar matrix on the lower half lights when the FCC has accepted the associated mode request only. It does not indicate engagement.
• If the right bus were to fail, the static inverter and battery still power the C FCC. The battery will not discharge in the few remaining minutes of flight.

I have not worked on a B757 or B767 since 1992 and last taught a course on a B757 in 2004 but it sure feels good to talk about these aircraft again.

[NSEU]

Jetdoc..
There still doesn't seem to be any timelines. When does the dc signal to the BCU happen (when the APP button is pushed?). I've been led to believe that the 767 and 744 are different (the 744 definitely bus-isolates at 1500).

"Autoland configurations" could refer to triple engage, etc. Not necessarily bus isolation.

Can you explain the VHF comm glitch?

I believe that a bus failure will only de-isolate the busses above 200' (below 200', the isolation remains and no annunciations are given regarding A/L status).

Rgds.
NSEU

[Spanner Turner]

For the 767 only.

General
The purpose of the power isolation logic is to cause the power
sources to separate during a triple channel approach.
The standby electrical system can be used as an independant power
source to supply the center channel during a triple channel autoland.
During autoland, the center dc bus receives power directly from the
hot battery bus and the center ac bus is supplied from the standby
inverter. The BTBs are locked in their current state as at bus
isolation.
.

The following will cause the power isolation logic to be set:

• Triple channel arm - all three channels either engaged or armed.
• None of the three channels generating NO LAND 3 logic.
• Approach mode armed.
• Airplane above alert height (200 feet RA).
.

Any of the following will cause the power isolation logic to reset:

• Total autopilot disconnect.
• Any channel generating NO LAND 3 logic above alert height.
• Go-around initiated.
.

Single power source failure above alert height after LAND 3 annunciated:

Above alert height, a single power source failure will lead to the
respective autopilot disengaging. The autoland status annunciator will
display LAND 2/NO LAND 3. As a consequence of NO LAND 3
condition, the autopilot system will drop the request for bus isolation.
If the APU generator is available, the failed electrical bus will
automatically be transferred to the APU generator.
.

Single power source failure below alert height after LAND 3 annunciated:

Below alert height, a single power source failure will not affect
LAND 3 status. The power isolation logic will not reset until the
autopilots are disengaged or go-around is initiated. The affected FCC
will disengage due to the failed power source.
.

NOTE:
NO LAND 3 status will cause power isolation logic to be
reset but the re-establishment of fail-operational capability
will not re-establish power isolation unless the approach
mode or triple arm is deactivated and re-established.
Through the power isolation circuits, NO LAND 3 logic
will be generated if isolation does not occur within 4
seconds of being requested by the FCC and isolation will
be reset and not requested again.

Keep the q's coming. keeps the brain working!

[Jetdoc]

The following will cause the power isolation logic to be set:


• Triple channel arm - all three channels either engaged or armed.
• None of the three channels generating NO LAND 3 logic.
• Approach mode armed.
• Airplane above alert height (200 feet RA)

I borrowed this from Spanner Turner. Its important to remember that the bus isolation request is given only when the 3 A/Ps agree that they are good to go for autoland. I highlighted this yesterday when I mentioned CMDENG. What is written above makes it a little more clear.
The approach logic requires that all 3 A/Ps capture LOC and GS and are below 1500 feet but above 200 feet RA. No bus isolation request occurs before this point.

As far as the VHF glitch goes, I suspect that its the L VHF that has the power interuption when the main battery transfer relay drops out and the main battery relay kicks in. Even the slightest power interuption could cause a problem for the electronics. Thats why the modern aircraft have the no break power transfer.

[NSEU]

None of the three channels generating NO LAND 3 logic.

Jetdoc... Why would they put in the line "• Triple channel arm - all three channels either engaged or armed." if

"No Land 3 logic" could simply be a fault in any of the channels at any height.

The Boeing Maintenance Manual (22-21-00) simply says...

4) Bus isolation discrete is issued when at ground is generated at the FCC's A/L BUS ISOL inputs. The ground is generated when:

a) The approach (APP) mode is selected
b) (Effectivity XXX, etc) The A/P is single channel engaged, multi-channel armed (off line channels are automatically armed by the FCC's when APP is selected) in the CMD mode.
c) (Effectivity ZZZ, etc) the A/P is single channel engaged, multi-channel armed (off line channels are manually armed by pressing the CMD switch/lights) in the CMD mode.
d) Autoland Status Annunciator (ASA) is not displaying NO LAND 3."

Anyway, I don't think we're going to resolve this issue (to our level of satisfaction) without input from Rockwell-Collins or Boeing, but my original quiz has been answered: There will be no drain on the battery under normal circumstances, because (in a very roundabout way), the Right AC Bus provides two FCC's with power.... that is, until something breaks. Under normal circumstances, independent power sources for each FCC is an illusion

As far as the VHF glitch goes, I suspect that its the L VHF that has the power interuption when the main battery transfer relay drops out and the main battery relay kicks in.

I agree... but pilots are telling me this happens when they push APP... not at 1500'

Rgds.
NSEU

[misd-agin]

Why doesn't someone admit that it was decided by a committee, and after numerous arguements about left or right A/P, depending upon who was flying the aircraft, odd or even flight number, east or west bound, northern or southern hemisphere, the committee did what committees always do....they punted and settled on 'center a/p'.

[Jetdoc]

Quote:
None of the three channels generating NO LAND 3 logic.
Jetdoc... Why would they put in the line "• Triple channel arm - all three channels either engaged or armed." if

"No Land 3 logic" could simply be a fault in any of the channels at any height.

I'm not quite sure what you are getting at here. At or below 1500ft RA with the APP sw pressed, LOC and GS CAP, the 3 FCCs bite check themselves and if all is ok, there will be a bus isolation request. I can't say much more. I have been away from the airplane for a long time and don't have anyone here to discuss this with.
Perhaps a B757 pilot could comment on this and also the VHF power interuption with the APP pressed. If you are out on the flight line, maybe you could talk to some of the pilots and see if they can do some checking on these things for you.

misd-agin

I think I highlighted the reason for the C A/P being used in post #20.

[misd-agin]

Our a/c had a problem with electrical power isolation on 3 A/P coupled approachs if L A/P was selected first. Procedure was 'do not select L A/P first for autolands'. So everyone just got used to selecting the C A/P.

That problem no longer exists. Either it's been fixed, didn't exist in the first place, or the powers that be realize it doesn't matter. So we use any A/P we want.

At what point is using the C A/P because of switching logic to the trim getting so nitnoid to the point of being ridiculous?

We do the same with switching 121.5 to the #1 radio for oceanic crossings. Never mind that it was already on radio #2 so there's radio frequency and mike selector switching being done needlessly, 2X, to satisfy some decision maker's fear that if the one in a billion total electrical failure occurs in oceanic waters the crew would save about 5 secs by already having 121.5 preset on the remaining power.