767 autopilot not available with L & C HYD systems u/s?

[niallcooney]

Guys,

Once again I'm confused. Why in the name of all that's Boeing are the L, C & R autopilots all inop when the L & C HYD systems are inop? Fine, I can understand why the L & C autopilots would be inop in that situation. The only reason I can think of is that the R HYD system doesn't provide enough control authority to maintain flightpath. Although if that were true then manual flying on the R HYD SYS would be difficult. Any comments?

Nial

[comxtek]

It's because if you lose Left and Center hydraulic systems, you lose Stab Trim (although u can get it from the right thru the PTU). In the B757, if you lose C & R you lose all autopilots.

[QAVION]

"It's because if you lose Left and Center hydraulic systems, you lose Stab Trim (although u can get it from the right thru the PTU). "

Is this inability to use the Right A/P applicable to the 767 also, Nial? (As I understand it, the PTU can't be used in association with Autotrim).

My 767 engineering training notes don't specifically tell me what would happen to the Right A/P with the L&C Hydraulics depressurized, but, if I understand what I am reading, loss of Stab Trim (with the Right A/P engaged) would simply generate an A/P Caution message (with no yellow bars through the EADI APFD pitch mode annunication). The elevator would simply be made to work harder than normal.

Although A/P engagement requirements differ on the ground from in the air, yesterday, I was able to engage the R A/P on the ground without it tripping off. I used V/S and HDG HOLD as the active pitch and roll modes. When I commanded a climb, the elevator moved to (presumably) the required position, no flags/messages appeared and the A/P remained engaged (I can't even recall seeing an A/P Caution msg).

Thanks.

Rgds.
Q.

[niallcooney]

Well if you tried it and it works I'm satisfied. I was messing about in the sim and couldn't get it to work. So much for simulation! ;)Much obliged for the information, Q, I was obviously looking at the wrong source of info.

Nial

[mutt]

Is that FS2000 or FS2002?


Mutt.

[niallcooney]

Mutt,

Both, but mainly FS2002 + 767PIC... fantastic. I'm off now to order A320PIC... should be even better... and I'm gonna treat myself to another hour of REAL simtime after I get everything down pat!

[av8er]

"It's because if you lose Left and Center hydraulic systems, you lose Stab Trim (although u can get it from the right thru the PTU). In the B757, if you lose C & R you lose all autopilots."

I am not sure what you understand at the moment, but my manual is quite clear. With the loss of the Left and Ctr Hyd systems the both stab trim systems are lost. A Pitch Enhancement System (PES) uses trpped left trim system fluid to operate the stabiliser. Hence it is possible to use the electric trim at about 1/4 rate, automatic and alternate trim are not available. Thus, the autopilot will not, technically, be available. Testing it on the ground may allow it to work as the elevator can probably put in any demand, only when the stab trim tries to operate will the autopilot decide it is not going to do it and trip the autopilot out. It is actually possible to use the right autopilot as a sort of attitude hold with the left and ctr hyd systems failed, but it will trip out when stab trim is needed (ie any major attitude change).

Anyway, thats how I understand it!

[QAVION]

"Testing it on the ground may allow it to work as the elevator can probably put in any demand, only when the stab trim tries to operate will the autopilot decide it is not going to do it and trip the autopilot out. "

I'm not 100% sure I understand what you're saying here, av8er, but....

(On the ground) I did try operating the L A/P with L hydraulics and the stab moved to cancel the elevator input, so I assume(!) the Right A/P (with only right hydraulics) would also detect elevator movement and (try to) send a command to the SAM to move the stab.

"It is actually possible to use the right autopilot as a sort of attitude hold with the left and ctr hyd systems failed, but it will trip out when stab trim is needed (ie any major attitude change)."

If it states so in your (767?) manuals, then I will accept this (My manuals are not entirely unambiguous).

Thanks.
Rgds.
Q.

[av8er]

OK,

sorry, I don't want to tread on toes, nor let the story get too far from the facts.

The reference to the Manual was ref the Hyd system failures and the PES.

My manual says nothing about using the autopilot as an attitude hold, I was just suggesting that with no stab trims whatever (a 757 is in this situation with Ctr and Rt Hyd systems inop) the autopilot will engage for a while. Occasionally the thing trips out. My (and others) analysis of this is that when a stab trim input is required (attitude change) the thing trips out - thats what the sims do anyway. Just using the autoplit as an attitude hold for a short while can be a great relief when a TRE is breathing down your neck and you can get a respite from hand flying while briefing etc...(but be ready for it to drop out!).

I agree the autopilots do engage on the ground (I tried it today). I would be interested to see what happens if you try your test in a 757 with the Ctr and Rt Hyd systems off and use the autopilot and use the elevator. I suspect the stab will not move to cencel out the elevator input.

Now to the point I was trying to make earlier. The difference between the 757 and the 767 is the PES; which allows stab trim to be used at 1/4 rate when the left and ctr hyd systems are inop. My manual says is only possible to use the electric trim at about 1/4 rate, automatic and alternate trim are not available.

The 757 has a PTU for entirely different reasons.

Hope this is clearer.

[QAVION]

Thanks for the feedback (and clarification), av8er :) I might give Boeing a call regarding this one to see what they have to say/add.

Cheers.
Q.

[mono]

QAVION,

'Tis true the a/p will trip in flight if the stab does not respond to FCC/SAM demands. It is also true that the system appears to work on the ground. I can only think that it is when the elevator i/p is sustained that it trips. Will try it again on 75 and 76 when I am next at work to see if I keep the i/p for a while that it trips.

Cheers.

[QAVION]

"'Tis true the a/p will trip in flight if the stab does not respond to FCC/SAM demands. "

Interesting... The 767 Boeing Maintenance Manual (22-14-00 p14) tells me that:

"The following conditions cause a CAUTION SIGNAL (their emphasis, not mine) prior to multi-channel engage only:

a) Failure of Air Data Computer, Radio Alitimeter, and Insturment Landing System (outer loop) data or interfaces for 4 seconds when the pitch or roll servos are engaged.
b) An elevator or aileron servo cammout (sic) monitor was tripped.
c) An automatic stabilizer trim failure when the servos are engaged
d) etc...."

I'm assuming that an EICAS Caution will not be present at the same time as an autopilot disconnect warning.

Cheers.
Q.

[av8er]

Thanks QAVION and mono.

I haven't gone away!!, but need to get my head around that statement in the 767 manual.

[mono]

Err,

I may have to eat humble pie here. I tried to see if the a/p would trip on both 75 and 76 the other day, (it's nice to be able to play with the a/c from time to time) whilst on the ground with the stab hydraulic systems inop and after 15 mins of operation during which the elevator went to max pitch up (I selected v/s climb), the a/p was happily engaged with EICAS caution for autopilot and stab trim.

So it seems it's back to square one, as I can't see why it should trip in the air and not on the ground.