Why center autopilot in 757/767
Thread Starter
Join Date: Nov 2006
Location: Europe
Age: 58
Posts: 6
Likes: 0
Received 0 Likes
on
0 Posts
Why center autopilot in 757/767
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?
Why don't we use L & R autopilots on a regular basis?
Thanks.
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?
Why don't we use L & R autopilots on a regular basis?
Thanks.
Last edited by Gvendur_Bjalla; 11th Jan 2008 at 22:33.
Join Date: Mar 2002
Location: UK
Age: 47
Posts: 169
Likes: 0
Received 0 Likes
on
0 Posts
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...
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...
Join Date: Feb 2002
Location: The Sandpit
Posts: 555
Likes: 0
Received 0 Likes
on
0 Posts
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.
By using the centre autopilot both pilots are monitoring a different independant flight director which helps them ascertain autopilot performance.
Join Date: Oct 2002
Location: In Frozen Chunks (Cloud Cuckoo Land)
Age: 17
Posts: 1,521
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Transparency International
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.
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.
Last edited by dusk2dawn; 12th Jan 2008 at 10:44.
Join Date: Oct 2002
Location: Heathrow
Posts: 291
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Mar 2007
Location: USofA
Posts: 1,235
Likes: 0
Received 0 Likes
on
0 Posts
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.
Join Date: Oct 2002
Location: Heathrow
Posts: 291
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jun 2006
Location: Canada
Posts: 94
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Mar 2002
Location: UK
Age: 47
Posts: 169
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Aug 2007
Location: The Netherlands
Age: 67
Posts: 288
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Humus Motor
Join Date: Sep 2002
Location: A little place called Samsonite
Posts: 201
Likes: 0
Received 0 Likes
on
0 Posts
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 .....!!
Join Date: Jun 2006
Location: Canada
Posts: 94
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Last edited by Jetdoc; 25th Jan 2008 at 05:52.