View Full Version : Why center autopilot in 757/767

11th Jan 2008, 23:18
Being fairly ignorant I've never known why we always use the center autopilot on the 757/767, given all remain operative (and of course they all kick in on ILS).
I was asked this; "why", but have to confess I'm not sure. Has it anything to do with the C-autopilot answering to "hidden" center systems not visible on either side (FCC), has it "just always been like this" or is there an educated answer to this riddle?:uhoh:

Why don't we use L & R autopilots on a regular basis?

12th Jan 2008, 00:08
On some aircraft, not all by any means, but depending on the mod state, each FCC can either provide an autopilot in command, or a flight director. For example, on some of the aircraft I fly when all 3 autopilots kick in on an ILS approach you simultaneously lose the FDs on both sides. So using centre command saves faffing about with the flight director source selector...

Not saying that this is necessarily the reason why (maybe some people just like to keep things symmetrical! :)) but it's one possibility that occured...

12th Jan 2008, 04:21
Try it. Most, tho not all 757/767's are configured to blank the flight director if the command is coming from the same source.

By using the centre autopilot both pilots are monitoring a different independant flight director which helps them ascertain autopilot performance.

12th Jan 2008, 05:02
I understand that the C Autopilot is the least likely to get knocked out by system failure.

Obvioulsy various exceptions apply.

Any sims where you are going to get electrical and hydraulic failures, the C, seems to withstand the most before dropping offline.

I think the L autopilot is next favoured, followed by the R.

I am sure others will have differing views. Some airlines alternate between A/Ps - Captains sector - use L or C, F/Os use the R.

The other thing which may impact on the decision, is transponder system usage vs Autopilot - depending on which transponder is used, it may not correspond to the altitude the autopilot is flying - ie L Transponder, comes off Capts altimiter which may be 40' diff to F/Os Altimiter - which is what the R transponder uses and the R autopilot.

Of course that depends on your airlines config too.

12th Jan 2008, 09:59
blueloo, yes - no - maybe...
I understand die Lufthansa SOP is as you describe and that it is based on

JAR-OPS 1.872 Equipment for operation in RVSM....
(4) A secondary surveillance radar (SSR) transponder with altitude reporting system that can be connected to the altitude measurement system in use for altitude keeping.

Hovewer, "can" is just an option, or?

(we also did this topic in http://www.pprune.org/forums/showthread.php?t=284493)

BTW, Gvendur_Bjalla, the ASA is monitoring all 3 systems. Even a C RA failure in FL 430 will trigger an NO LAND 3 announciation - no "hidden" systems here.

Jetstream Rider
12th Jan 2008, 11:44
We have both source selectors set to C, and the favoured autopilot to use is the R. If we were to use the C, the F/D's would blank as described above. I know most 75 operators use L source on the left side and R source on the right side, so using the C autopilot prevents the F/D's on either side from blanking.

We are not supposed to manually tune VOR's in the FMC when the L autopilot is engaged as "dual FMC reboot may result" (!). That probably depends on your FMC fit though.

Our order of preference for the A/Ps is R, L, C on the 757.

On the 767, the L or R may be used as a first preference, the source selectors are the same as for the 757, both in C.

Spooky 2
12th Jan 2008, 15:32
Sorry guys as I don't have any Boeing manuals here at the moment but I believe it has to do with a C system hyd failure and the fact that the AP will remain engaged in spite of this event. Maybe someone can jump in here a refresh my memory regarding how this works. Also, one needs to be cautious when discussing AP on the various 757/767 as there are several configurations out there and the one your operating with may not use the same logic as the next guys.

12th Jan 2008, 15:46
Long time since I flew Boeing but as far as I recall the C autopilot was used for autolands to give the required redundancy on cat 3.

Jetstream Rider
12th Jan 2008, 17:09
All three are needed for a Cat3b/c Autoland.

2 are needed for Cat 3a.

I'm sure you can't autoland on 1 autopilot, although as Spooky 2 says, there are very different mod states out there. Our autopilots don't automatically engage for instance, each has to be manually engaged with a paddle - airline difference, most 75's have push buttons I believe.

12th Jan 2008, 17:31
I believe its due to the fact that the C FCC inputs to both SAM'S for stab trim. If one fails, the other takes over no problem.

Spooky 2
12th Jan 2008, 17:38
BINGO! You get the cuppie doll as that is the reason which goes back the the hyd system mentioned earlier.

24th Jan 2008, 13:41
Jetdoc, wut kind of thingy is SAM ?

24th Jan 2008, 14:13
Stab trim and elevator Assymetry Module.

24th Jan 2008, 15:15
The overall function of the stabilizer trim system is to maintain the airplane in a trimmed condition and to provide automatic mach and speed stability. The system consists of: (1) several alternative sources of trim command signals; (2) two independent actuation devices which position the stabilizer in response to source commands; and (3) an electronic module which selects a particular trim signal source and also energizes one or the other actuation device.
The three sources of electric trim command signals are: (1) the flight control computer (FCC); (2) the mach/speed trim function within the stabilizer/elevator asymmetry module (SAM); and (3) the manual-electric trim switches on the control wheels.
The selection of the active trim source is done by logic circuits in the SAMs.
The SAMs accept input signals from the FCC, the mach/speed trim function in the SAM, and the control wheel switches. The output signals from the SAM are sent to two STCMs.
There are two independent channels of trim command and actuation. Each channel consists of a SAM, an Stabilizer Trim Control Module, and a hydraulic motor and brake on the stabilizer ballscrew actuator. The two channels are known as trim channel left and trim channel right. When an FCC or mach/speed function is the active trim source, only one trim channel is activated, providing half-rate trim. When the electric trim switches are energized, both trim channels are activated simultaneously, and the stabilizer is driven at full-rate trim. The trim rate varies with airplane speed and horizontal stabilizer position as determined by hydraulic pressure output from the elevator feel computer. The variable hydraulic pressure output from the elevator feel computer is fed to each STCM. In each STCM a rate control valve regulates actual hydraulic pressure available to drive the horizontal stabilizer.

24th Jan 2008, 17:59
Excuse my ignorance here, but I don't see why that explains the reason for mainly using the C autopilot (and hence FCC). Unless you're saying that:-

The left FCC can only provide inputs to the left trim channel.
The right FCC can only provide inputs to the right trim channel.
The centre FCC can provide inputs to EITHER the left or right trim channel.

We could then see what the effect of a trim channel failure would be on autopilot trim.

24th Jan 2008, 19:24
It is a matter of checking - there are 3 Flight Control Computors (FCC's).

Each FCC can deliver its output in the form of Flight Director (FD) guidance, or in the form of AutoPilot (AP) commands.

Normally, Left FCC supplies Left FD, Right FCC supplies Right FD and now if you use Centre FCC to fly the a/c (through Centre A/P), you have a constant check that all FCC's agree.

24th Jan 2008, 21:16
Yes, that was my original solution to the original query (see above), but then this whole thing about stab trim came up...

25th Jan 2008, 00:13
Emit - that's the reason given to me by the guys who made the aeroplane .... I've never questioned it because they generally know what they're talking about I find .....!!

25th Jan 2008, 02:21
"stabilizer/elevator asymmetry module (SAM); "

"elevator asymmetry"? :confused:

Who writes these crazy pilots' manuals???

SAM = Stab Trim and Aileron Lockout Module


25th Jan 2008, 06:19
On the B757, there are only two ailerons. The lockout function is not required. On the B767, there is a lockout function.
Automatic Stabilizer Trim
The purpose of the Flight Control Computers (FCCs) in the stabilizer trim system is to provide trim commands when at least one channel of the autopilot is engaged. All three FCCs may be used; any one when in the cruise mode, or two or more when in the autoland mode. The left and right FCCs provide inputs to their respective SAMs. The center FCC provides inputs to both SAMs. Only one SAM is engaged at any time, with the first FCC engaged selected as the autotrim source. Priority for simultaneous FCC engagement is left, center, right.
The FCCs generate trim commands in response to an elevator deflection exceeding a preset authority threshold. Trim commands applied to the SAM cause the stabilizer to assume the elevator trim load. Stabilizer position signals are fed back to the FCCs from the associated Stabilizer Position Modules (SPM). Trim commands stop when the elevator is within the elevator authority threshold limits.
The FCC generates a digital trim engage discrete and a separate analog autotrim discrete. These cause the SAM to select the FCC as the active trim source. With any one FCC controlling, a single STCM hydraulic output drives the stabilizer motor at half-rate speed (0.1 degrees/sec to 0.25 degrees/sec). The elevator feel computer determines the pressure applied at the STCM, which determines stabilizer rate.
The three FCCS communicate with the SAM via ARINC 429 buses. Protocol logic within each FCC determines which computer trims the stabilizer. The FCCS monitor each other. In the autotrim mode, the command response monitor in FCC performs loop monitoring to detect autotrim failures. Autotrim failure occurs when stabilizer does not respond to computed commands, stabilizer movement is inconsistent with computed commands or when analog AUTOTRIM VALID from SAM becomes invalid. When a single autopilot is engaged, a detected autotrim failure which cannot be cleared by internal autosequencing (center FCC only) disables the active trim control path by disengaging the appropriate SAM. Since the center FCC interfaces with both SAMs, sequencing to another trim channel following a trim failure can occur only if the center FCC is engaged. When the center autopilot is engaged, the left SAM is always selected. A subsequent trim failure requires the FCC to inititate sequencing to the right SAM. When more than one autopilot is engaged, sequencing to another trim channel is attempted when the current SAM-driving autopilot disconnects, or the active trim control path fails as detected by the local command-response monitor. The FCC autosequencing cycle is left - Right - Center - Left.
As stated above from the B757 AMM, the C autopilot is the only FCC that can switch to the other stab trim channel automatically. Using L or R requires disengaging the A/P and engaging a new channel.

25th Jan 2008, 13:43

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.

25th Jan 2008, 20:41
Thanks, mono... I'd ask what its function is, but I don't want to hijack this thread ;)

27th Jan 2008, 03:08

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 (http://www.iasa.com.au/folders/RoboLander_files/AutolandFailmodes.htm).


chemical alli
27th Jan 2008, 03:48
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.

27th Jan 2008, 16:15
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
28th Jan 2008, 03:05
go jetdoc your the man.anymore 767 questions and i am coming to you

28th Jan 2008, 08:09
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
28th Jan 2008, 10:05
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 !

:ok: :ok:

28th Jan 2008, 12:41
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.

29th Jan 2008, 07:56
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).


29th Jan 2008, 16:58
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.

29th Jan 2008, 21:32
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).


Spanner Turner
30th Jan 2008, 10:47
For the 767 only.

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

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.

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.

:ok: :ok:

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

30th Jan 2008, 12:35
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.

30th Jan 2008, 23:48
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' ;)


31st Jan 2008, 02:03
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'.

31st Jan 2008, 17:40
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.


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

1st Feb 2008, 01:50
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. :ugh::ugh: