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Faro
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Runaway Stabiliser On Airbus Types

Wed Sep 15, 2010 4:08 pm

Boeing and MD types have prominent stabiliser trim cut-out switches in their flight decks, why do Airbus types not have them? Surely there must be a distinct stabiliser trim actuator on the Airbii even if the automatic trim function is layered into the flight control software. What do you do if faced with a runaway stabiliser on an Airbus?

Funny to note that Boeing insist on the auto-throttle moving the thrust levers but omits the moving trim wheel on the 757/767/777/787 whereas Airbus has a moving trim wheel and an auto-throttle that leaves the levers fixed. Design philosophy only goes so far I guess...

Faro
 
tdscanuck
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RE: Runaway Stabiliser On Airbus Types

Wed Sep 15, 2010 5:45 pm

Quoting faro (Thread starter):
Boeing and MD types have prominent stabiliser trim cut-out switches in their flight decks, why do Airbus types not have them?

Mechanical operation of the trim wheel overrides electric operation by the flight control system. Moving the stabilizer with the trim wheel basically cuts the automatic trim off.

Quoting faro (Thread starter):
Surely there must be a distinct stabiliser trim actuator on the Airbii even if the automatic trim function is layered into the flight control software. What do you do if faced with a runaway stabiliser on an Airbus?

As far as I can tell, just manually braking the wheel with your hand would kick the automatic trim out. You could also depower the ELAC's, which would force control of the stabilizer to a different computer and motor, but I'm not sure if that's the right procedure.

Quoting faro (Thread starter):
Funny to note that Boeing insist on the auto-throttle moving the thrust levers but omits the moving trim wheel on the 757/767/777/787

They replaced it with a digital trim indicator. On the 757/767/777 it's on the pedestal right by the manual trim switches. On the 787 it's part of EICAS.

Quoting faro (Thread starter):
Design philosophy only goes so far I guess...

The philosophy isn't that you need a physical moving object, it's that you need to know what the automation is doing. On a 737, the wheel spins. On a 757/767/777/787 the indicator pointer moves...either way, the airplane feeds back to the pilot what it's doing.

Tom.
 
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Faro
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RE: Runaway Stabiliser On Airbus Types

Wed Sep 15, 2010 10:48 pm

Quoting tdscanuck (Reply 1):
As far as I can tell, just manually braking the wheel with your hand would kick the automatic trim out. You could also depower the ELAC's, which would force control of the stabilizer to a different computer and motor, but I'm not sure if that's the right procedur

Is this enough? I was under the impression that the stab trim cut-out was meant to isolate the hydraulics/electrics that power the trim actuator rendering it powerless. Would disabling the automatic trim functionality on an Airbus achieve the same result if the runaway was caused by a defect in the physical actuator and not the flight control computer?

Faro
 
Pihero
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RE: Runaway Stabiliser On Airbus Types

Thu Sep 16, 2010 12:26 pm

On a 'Bus, the pitch trim is part of the general "Flight Controls"' architecture so it is no different in conception from all the other FCS components. The whole system is controlled and monitored by a set of computers (in this case, mainly the ELACs).
As Tom wrote, manual trim has priority over the (electric) auto-trim at all times.
Because there is no mechanical back-up on the A380 (there are always hydraulics and electricity available for flight controls), the need for a trim wheel disappears : we have then two electrical "manual trim" switches on the pedestal and the trim position is on the SD screen.

Quoting tdscanuck (Reply 1):
On a 737, the wheel spins. On a 757/767/777/787 the indicator pointer moves...either way, the airplane feeds back to the pilot what it's doing.

I see that you're sold on the fixed Thrust levers, then   , as you said either it moves or the indicator pointer moves...
I won't forget that sentence.

Regards.
 
mandala499
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RE: Runaway Stabiliser On Airbus Types

Thu Sep 16, 2010 3:24 pm

Just expanding on what Tdscanuck and Pihero mentioned.

Quoting faro (Reply 2):
Would disabling the automatic trim functionality on an Airbus achieve the same result if the runaway was caused by a defect in the physical actuator and not the flight control computer?

With Airbuses are you talking about?
The A310 has the Pitch Trim levers on the top part of the front of column 2 on the overhead panel... Manual handling of the trim wheel trips (one or both of) them to OFF... or, just switch it off.

For the 320, ELAC2 covers the HorStab Autotrim function, If ELAC2 not available, then it's ELAC1, if neither are available, then it'll go to SEC 2 (and if that is not available, SEC1).

Manual intervention of the trim wheel will trip off the autotrim function to off, and you resort to mechanical trimming (as in, move the damn wheel yourself). The QRH covers Stabilizer Jam, but not for runaway stabilizer... but (Pihero is qualified to comment on this and I assume will correct this!), runaway stabilizer will result on a mismatch between the commanded stabilizer position and the sensor reading of the position, and will generate a fault message (most likely through cutting elec power to the stab motors)... and you'd then proceed with stab jam QRH.

When it happens, switch off the autopilot, and check if manual pitch trim is available (move the wheels, if it's a partial elec power problem the the HorStab, it'll cut any further power to the screwed up electric power to the trim)... if available, Zero the trim, and let the FBW deal with the pitch through elevator action alone. If not available, leave it as is, and the FBW will deal with pitch through the elevator action alone. HorStab jam will cause a degradation of the FBW from Normal Law into AlternateLaw where pitch and yaw is in Alternate, and the Roll is direct. Therefore the stick movement pitch will still act on "trajectory" (ie: processed/computed), and not direct (elevator displacement).

For the 330/340, it's the same except that you have different computers doing the job of the ELACs and SECs.

Mechanical trim function is the same as the 737s... it's a wheel, linked to the HorsStab. But, I think you still have HYD power assist to the mechanism???

Mandala499
 
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Faro
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RE: Runaway Stabiliser On Airbus Types

Thu Sep 16, 2010 8:33 pm

Don't quite understand all your replies.

Maybe it's my understand of what constitutes a runaway stabiliser that is erroneous. For me, a runaway is an uncommanded deployment of the trim motor on a trimmable horizontal stabiliser. Perhaps in some cases on an Airbus it may be stopped by arresting the trim wheel by hand but there may conceivably be other cases where the physical actuator itself, sitting back in the tail end, just goes off on a tangent and actuates the THS to, say, a full nose down deflection *irrespective* of what the FCS/trim wheel/control laws are telling it. In that case, what's the use of fiddling with the automatics? You need to kill the power input to the THS real fast; then you can take your time to look at what the FBW gadgetry is or isn't doing.

Isn't this what the stab trim cut-out switches on Boeings do, cut out the power supply to the THS quickly?

Faro

[Edited 2010-09-16 13:41:43]
 
Vmcavmcg
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RE: Runaway Stabiliser On Airbus Types

Thu Sep 16, 2010 9:36 pm

Just my observations. IIRC, on the Boeings the first thing you do is move the control column in the opposite direction of the trim wheel. In theory, the cut out switch should take care of the issue.
Here is the section from my old QRH;

Condition: Uncommanded stabilizer motion detected and
automatic cutout does not occur, or Alternate
Stabilizer Trim switches used with autopilot engaged.
STABILIZER TRIM CUTOUT SWITCHES . . . . . . . . . CUT OUT
Do not accomplish the following checklists:
STAB TRIM 2
STAB TRIM 3
AUTOPILOT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .DISENGAGE
Higher than normal control column force may be required
to prevent unwanted pitch change.
STABILIZER TRIM 2 CUTOUT SWITCH . . . . . . . . . . . . . AUTO
Check for correct stabilizer movement. Trim is available
after a brief delay.
If unscheduled trim occurs :
STABILIZER TRIM 2 CUTOUT SWITCH . . . . . . . . CUT OUT
STABILIZER TRIM 3 CUTOUT SWITCH . . . . . . . . . . AUTO
Check for correct stabilizer movement. Trim is
available after a brief delay.
If unscheduled trim occurs:
STABILIZER TRIM 3 CUTOUT SWITCH. . . . . . CUT OUT
Landing preparation:
Use flaps 25 and VREF30 + 20 for landing.
Do not accomplish the following checklist:
JAMMED STABILIZER LANDING


And the reference to STAB TRIM 2 and STAB TRIM 3 is the auto function.

I know of no cases on the 744 and 747 where there has been a documented case of unscheduled stab trim. As I remember, the issue is a certification issue not a real need issue.

The 320 FCOM states: Automatic pitch trim freezes in the following situations:
1. The pilot enters a manual trim order
2. Radio altitude is below 50 feet (100 feet with autopilot engaged)
3. Load factor goes below 0.5 g
It would appear as if both Airbus and Boeing provide the same protection, just using different methods.

Quoting faro (Thread starter):
Funny to note that Boeing insist on the auto-throttle moving the thrust levers


Actually, it's not Boeing's idea but the customers that have ordered the aircraft. Just like the side stick vs. yoke issue. The side stick was offered during the initial 777 discussions with the airlines. No one wanted it. It's not quite as cut and dry as you portray it to be
 
Pihero
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RE: Runaway Stabiliser On Airbus Types

Thu Sep 16, 2010 11:39 pm

Quoting faro (Reply 5):
a runaway is an uncommanded deployment of the trim motor on a trimmable horizontal stabiliser.

That is correct

Quoting faro (Reply 5):
Perhaps in some cases on an Airbus it may be stopped by arresting the trim wheel by hand

Boeing : Grabbing the trim wheel stops the electric trim movement, be one on the 707, the 727 or the 737. I still remember that runaway trims were so expected that some crews wore gloves on the pedestal hand.
Electric Airbus : in theory, a runaway THS cannot occur because as soon as a fault is detected, first the electric motor is taken out of the loop then if fault persists, the computer in question (see Mandala's post #4 above) is taken out of the system.

Quoting faro (Reply 5):
but there may conceivably be other cases where the physical actuator itself, sitting back in the tail end, just goes off on a tangent and actuates the THS to, say, a full nose down deflection *irrespective* of what the FCS/trim wheel/control laws are telling it.

I honestly cannot think of such a case. Your scenario involves both a breach in the trim wheel - to - THS hydraulic motors link and simultaneous failures of at least the two ELACs.

Quoting faro (Reply 5):

Isn't this what the stab trim cut-out switches on Boeings do, cut out the power supply to the THS quickly?

Two remarks on that question :
1/- The first action, on a runaway trim scenario is not to use the trim cut-out switches but to block the trim movement, either by counter-ing it (if I may use this word) with the control column, or by grabbing the trim wheel.
2/- Secondly, you cannot compare the flight control architecture of - say a 737 with an A320's. More than 20 years apart and one whole new concept.

Quoting mandala499 (Reply 4):
Mechanical trim function is the same as the 737s... it's a wheel, linked to the HorsStab. But, I think you still have HYD power assist to the mechanism???

It's a wheel, linked to two hydraulic motors (green or Yellow) driving a screwjack which in turn drives the THS.
 
tdscanuck
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RE: Runaway Stabiliser On Airbus Types

Fri Sep 17, 2010 1:28 am

Quoting Pihero (Reply 3):
Quoting tdscanuck (Reply 1):
On a 737, the wheel spins. On a 757/767/777/787 the indicator pointer moves...either way, the airplane feeds back to the pilot what it's doing.

I see that you're sold on the fixed Thrust levers, then   , as you said either it moves or the indicator pointer moves...
I won't forget that sentence.

I'm not sure how you got out of there that I'm a fan of fixed thrust levers...if you've got a control, it should track what the airplane is doing.

Tom.
 
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RE: Runaway Stabiliser On Airbus Types

Fri Sep 17, 2010 4:07 am

Quoting mandala499 (Reply 4):
Mechanical trim function is the same as the 737s... it's a wheel, linked to the HorsStab. But, I think you still have HYD power assist to the mechanism???

There is no hydraulics involved with the 737 stabilizer trim. Just a single electric motor or the mechanical (manual) trim wheels.
 
Pihero
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RE: Runaway Stabiliser On Airbus Types

Fri Sep 17, 2010 8:29 am

Quoting tdscanuck (Reply 8):
I'm not sure how you got out of there that I'm a fan of fixed thrust levers...if you've got a control, it should track what the airplane is doing.

Simple : it's in that sentence of yours :"either way, the airplane feeds back to the pilot what'it's doing...", which is exactly what the "pointers" on the thruat gauges (N1 or EPR, depending ) are doing.
Inm other words, what is ok for the gander (Boeing with the trim pos ) should be ok for the goose (Airbus), in a fair world (but, of course, does that sort of woirld really exist ? )  
 
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zeke
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RE: Runaway Stabiliser On Airbus Types

Fri Sep 17, 2010 9:44 pm

Quoting faro (Reply 5):
Perhaps in some cases on an Airbus it may be stopped by arresting the trim wheel by hand but there may conceivably be other cases where the physical actuator itself, sitting back in the tail end, just goes off on a tangent and actuates the THS to, say, a full nose down deflection *irrespective* of what the FCS/trim wheel/control laws are telling it.

The flight control systems on the FBW Airbus are feedback control loops, the pilot tells the FBW computer what it wants, the FBW computer does some fancy maths and converts the pilots demands into control movements. The FBW computer monitors the control movements to make sure they are getting what was ordered.

The trimmable horizontal stabilizer (THS) on the 330/340 has redundant hydraulic and electric circuits, as well as an additional backup, and the aircraft has a number of FBW computers that cascade if the previous one fails.

If the FBW computer detects a problem with a "jack", e.g. not moving as commanded, or moving too much, as part of the feedback loop will take itself and the "jack" offline. The next FBW computer takes over and the, along with different motors. If they have a problem, same again, they go offline and the next ones take over.

On a conventional aircraft, it is the pilot that is doing the fault monitoring and has to override the runaway jack as they do not have automatic feedback control over the flight control surfaces, and the systems will not take themselves offline if they are at fault. That is the reason for the different cockpit controls.
 
tdscanuck
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RE: Runaway Stabiliser On Airbus Types

Sat Sep 18, 2010 1:19 am

Quoting Pihero (Reply 10):
Simple : it's in that sentence of yours :"either way, the airplane feeds back to the pilot what'it's doing...", which is exactly what the "pointers" on the thruat gauges (N1 or EPR, depending ) are doing.
Inm other words, what is ok for the gander (Boeing with the trim pos ) should be ok for the goose (Airbus), in a fair world (but, of course, does that sort of woirld really exist ? )

Yes, but on the Boeing you have a single indicator, and it's tracking what the surface is doing. On the Airbus you have two things (the throttle and the N1/EPR gauge) and only one of them tracks what the airplane is doing.

The Boeing philosophy is that *if* you've got a movable control, it had better track what the airplane is doing. The 757/767/777/787 don't have a moveable trim wheel so there's nothing to backdrive. The equivalent would be if Airbus got rid of the throttle entirely and just used a "faster/slower switch" or pushbuttons or something like that.

Tom.
 
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Faro
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RE: Runaway Stabiliser On Airbus Types

Sat Sep 18, 2010 11:44 am

Quoting Zeke (Reply 11):
The trimmable horizontal stabilizer (THS) on the 330/340 has redundant hydraulic and electric circuits, as well as an additional backup, and the aircraft has a number of FBW computers that cascade if the previous one fails.

If the FBW computer detects a problem with a "jack", e.g. not moving as commanded, or moving too much, as part of the feedback loop will take itself and the "jack" offline. The next FBW computer takes over and the, along with different motors. If they have a problem, same again, they go offline and the next ones take over.

On a conventional aircraft, it is the pilot that is doing the fault monitoring and has to override the runaway jack as they do not have automatic feedback control over the flight control surfaces, and the systems will not take themselves offline if they are at fault. That is the reason for the different cockpit controls.

Ok, thanx Zeke, I better understand now. In an Airbus, the FBW systems cascade from one system to another if the runaway is not stopped whereas other aircraft need someone to manually stop the THS motor. Presumably, the probability of all systems reversions through which the Airbus cycles not stopping the runaway is too remote to take into consideration as Pihero noted in reply 7:

Quoting Pihero (Reply 7):
I honestly cannot think of such a case. Your scenario involves both a breach in the trim wheel - to - THS hydraulic motors link and simultaneous failures of at least the two ELAC's

My impression was that trim runaways were still a significant risk these days, given the extant cut-out switches on the Boeings. Given the reliability of modern flight control systems (and barring freak runaway-like incidents like the 737 rudder hard-on of the 1990's), this is probably no longer the case.

One aspect though, how long would a THS trim runaway take to go from nominal cruise setting to full nose up/down setting? Presumably, the FBW systems can do the full FBW computer/actuator cascade cycle in a fraction of that time.

Faro
 
Pihero
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RE: Runaway Stabiliser On Airbus Types

Sat Sep 18, 2010 11:49 am

Quoting tdscanuck (Reply 12):
The equivalent would be if Airbus got rid of the throttle entirely and just used a "faster/slower switch" or pushbuttons or something like that.

And that would take us into the Boeing philosophy that you advocate, right ?  
 
mandala499
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RE: Runaway Stabiliser On Airbus Types

Sat Sep 18, 2010 1:08 pm

Quoting faro (Reply 13):
In an Airbus, the FBW systems cascade from one system to another if the runaway is not stopped whereas other aircraft need someone to manually stop the THS motor.

Yes, the process of cascading to the end (assuming totally screwed up trim motors), is much quicker than can be done manually.

The electric trim motors take the commands from the FCS only. There is no trim switch for the Buswire. One of the reason for a stab trim cutout in other types is, if the trim switches in one's yoke shorts or goes funny and decides to trim the aircraft up or down and switching the trim up, or down won't change the runaway. Then, hold the wheel and cut it off through the cut off switch. The A310 has this because the pilot or autopilot trims the aircraft, not the computers(no FBW). (the Autopilot merely takes the place of the pilot in terms of trim command... ie: more up, less up, that's enough).

The both the FBW and non-FBW systems have come a long way in making these things rarer than before. But the trim cut off switches are still needed where you have trim switches on the yoke.

Quoting faro (Reply 13):
Presumably, the FBW systems can do the full FBW computer/actuator cascade cycle in a fraction of that time.

Almost instant... otherwise, the cut off switch would still be there.
The cut off by grabbing the wheel, is also instant, otherwise the Buswire would still have the cut off switch.

Once the power is switched off, you can always crank it back up manually to +0.0 ANU or whatever your QRH tells you... and one would probably quickly realize how long it's been since he/she has flown a Cessna.   

Quoting Pihero (Reply 14):
And that would take us into the Boeing philosophy that you advocate, right ?

Moving A/T vs Fixed?    This deserves a separate topic altogether...   
Now, if the A/T is on and one's Buswire suddenly goes "tick tick tick... STALL", you're either in something where no "tactile cues" can help, or have screwed up big time somewhere along the line!!!!

OK, back to the sin bin for me...
 
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zeke
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RE: Runaway Stabiliser On Airbus Types

Sat Sep 18, 2010 2:45 pm

Quoting faro (Reply 13):
My impression was that trim runaways were still a significant risk these days, given the extant cut-out switches on the Boeings.

That is a function of that control system design, with the feedback loop on the FBW system it is not an issue. You also have similar feedback loops with the flaps to prevent unsymmetrical deployment.

Quoting faro (Reply 13):
One aspect though, how long would a THS trim runaway take to go from nominal cruise setting to full nose up/down setting? Presumably, the FBW systems can do the full FBW computer/actuator cascade cycle in a fraction of that time.

I do not think a trim runaway would be possible on a FBW aircraft, once the control surface gets out of tolerance it would be taken offline. Also remember with the FBW, it is outcome driven, i.e. the pilot pull back, that command is given to the FBW computers, and they work out what control surfaces to change. If the THS was out by a little bit, the FBW would compensate for that automatically with elevator to get the desired outcome.
 
474218
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RE: Runaway Stabiliser On Airbus Types

Sat Sep 18, 2010 3:54 pm

Why not a flying stabilizer like the TriStar, that way you will never have a ran-away trim problem.

Maybe, someday the other manufacturers will catch up.
 
mandala499
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RE: Runaway Stabiliser On Airbus Types

Mon Sep 20, 2010 9:19 am

Quoting 474218 (Reply 17):
Why not a flying stabilizer like the TriStar, that way you will never have a ran-away trim problem.

Flying stablizer, you mean use the stabilizer as an elevator, and the elevators as trim tabs?    (j/k on the trim tabs btw)

Look, one can do wonders in a non-reversible flight control system architecture. Tristar's Flying Stabilizer concept was a good one.

But... the L1011 still has a trim wheel right (and gauge) ?    That trim wheel is translated somewhere into stabilizer position on a neutral yoke input.

View Large View Medium
Click here for bigger photo!

Photo © Max Teuber


From what I understand on the Tristar is that trim on the yoke gets connected to the stabilizer position on a no yoke position input. If there are no electronics to deal with the stabilizer actuation, it merely relies on HYD power, and something somewhere, tells the actuators what to do. If there is, then grabbing that trim wheel cuts of the electrical power to it... But I think this is just a different way of setting it up. Perhaps you can describe the system itself to us.   

That something somewhere is probably the PFCES (Primary Flight Control Electronic System), which commands the TAC (Trim Augmentation Computer) and the FCES (Flight Control Electronic System) which covers primary flight control surfaces monitoring, stall warning, stall warning, DLC, and other functions. So much like ELACs and SECs on the Buswire. Stabilizer runaway is prevented through the TAC and FCES, just like Buswire's ELACs and SECs (PRIMs and SECs on the 330/340s). Basically, it's "what did I ask the stabilizer to do?" and checks "is it doing what I'm doing?"

Advanced? Yes, for those days. The L1011 is very much like putting an FBW system into a non-FBW architecture. The current FBW architecture has gone beyond... no doubt taking aspects of the best of previous technologies.

The Tristar's flight control system, is none the less fascinating. Would be good if you can describe it further for us...

Mandala499
 
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Faro
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RE: Runaway Stabiliser On Airbus Types

Mon Sep 20, 2010 1:40 pm

Quoting mandala499 (Reply 18):
From what I understand on the Tristar is that trim on the yoke gets connected to the stabilizer position on a no yoke position input.

Funny that you're commanding a trim position and not a trim up/down speed from the yoke. Wonder how accurately you can trim from the yoke without prolonged thumbulation of a low-gain (and high-precision) trim control...

Faro
 
474218
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RE: Runaway Stabiliser On Airbus Types

Mon Sep 20, 2010 2:54 pm

Quoting mandala499 (Reply 18):
The Tristar's flight control system, is none the less fascinating. Would be good if you can describe it further for us...

If you have a specific question I would try to answer it, but the entire system it would take too much typing.

Here is a schemetic of the L-1011 AFCS:

 
mandala499
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RE: Runaway Stabiliser On Airbus Types

Tue Sep 21, 2010 5:43 am

Quoting 474218 (Reply 20):
Here is a schemetic of the L-1011 AFCS:

Excellent ! Love it !
The whole thing looks like as if it is an analogue FBW system!

Quoting 474218 (Reply 20):
If you have a specific question I would try to answer it,

I'd like to know more on the pitch control mechanism/schematics and system description... in particular how the flying stabilizer works (I read somewhere that the elevators are "geared" to the stabilizer position?)
Anything other than that deserves a separate topic/thread I think!   

Quoting faro (Reply 19):
Funny that you're commanding a trim position and not a trim up/down speed from the yoke.

Ain't that the case with all (but not the L-1011) non-FBW aircraft?
 
tom355uk
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RE: Runaway Stabiliser On Airbus Types

Wed Sep 22, 2010 3:22 pm

Quoting faro (Reply 13):
incidents like the 737 rudder hard-on

I'm sure you meant rudder hard-OVER.

A hard on is quite different....  Things often get lost in translation!

Meanwhile, back on topic, as already stated, on a FBW Airbus trim runaway is virtually impossible - however, if at least two of the AoA sensors are simultaneously providing erroneous readings (as in GXL888T) then it is possible for the autotrim to run to the nose up stop and remain there until stall. The FBW should then revert to Direct Law, the trim will freeze at the last autotrimmed value and USE MAN PITCH TRIM appears on the PFD. If you don't manually trim quickly, then you are in big, big trouble.
 
Tristarsteve
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RE: Runaway Stabiliser On Airbus Types

Wed Sep 22, 2010 7:55 pm

Quoting mandala499 (Reply 21):
in particular how the flying stabilizer works (I read somewhere that the elevators are "geared" to the stabilizer position?)

Correct. what look like elevators are geared tabs on the stab.
The whole stab moves. There is a huge piece inside the aircraft as well. At the leading edge are four linear hydraulic jacks, one on each hyd system. They are about 10 feet long.
The input to the jacks is from two stab control units, connected by rotating rods. Working on it was a nightmare as there were so many parts, and not much space.
If you get near a Tristar, you can see the stab control units through grills about 3ft square on the undersurface fwd of the APU. There is a door on the LHS, and you can climb up and stand on the stab. In the power off condition it is (stab) nose down and you could have a picnic in there its so big.
But turn the hyds on and its a death trap. The stab will move up and fill the space!.

Quoting mandala499 (Reply 18):
So much like ELACs and SECs on the Buswire.

Well not really as in manual flying, it was manual input to the hyd servos.
 
474218
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RE: Runaway Stabiliser On Airbus Types

Wed Sep 22, 2010 10:05 pm

Quoting tristarsteve (Reply 23):
In the power off condition it is (stab) nose down and you could have a picnic in there its so big.
But turn the hyds on and its a death trap. The stab will move up and fill the space!.


I think you have it slightly backwards. When the stabilizer is full down (a/c nose up) there is approximately 4 inches between the lower surface of the stabilizer center box and the APU compartment upper skin and a huge area between the upper skin of the stabilizer center box. However even with the stabilizer is full nose up (a/c nose down) there is a fairly very large gap between the stabilizer center box upper skin and the S-Duct, at least at the forward end!

Additionally, when hydraulic power is removed the stabilizer moves to the full nose up (a/c nose down) position. When hydraulic power is applied the stabilizer will assume the mechanically trimmed or the column position if they differ. The stabilizer position should always be around the blue dot on the pedestal trim wheels, or 4 degrees stabilizer nose down (a/c nose up) when hydraulic power is applied.
 
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RE: Runaway Stabiliser On Airbus Types

Thu Sep 23, 2010 1:36 pm

Quoting 474218 (Reply 17):
Why not a flying stabilizer like the TriStar

Highly interesting thread! But I´m a complete n00b when it comes to all the technical nitty-gritty stuff. Can someone explain to me in layman termsn what a flying stabilizer is?
 
Tristarsteve
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RE: Runaway Stabiliser On Airbus Types

Thu Sep 23, 2010 1:44 pm

Quoting TravelAVNut (Reply 25):
explain to me in layman termsn what a flying stabilizer is?

Most airliner horizontal stabilisors are attached to the aircraft by a screw jack. This moves the stab up and down quite slowly to trim out the elevators. The elevators are at the back of the stab and provide quick motion in pitch.

the Tristar stabilisor has four hydraulic jacks that move the stab fast. It is the primary pitch control for the aircraft. The 'elevators' on a Tristar act like trim tabs.
The flying stab flies the aircraft all the time.
 
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Starlionblue
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RE: Runaway Stabiliser On Airbus Types

Thu Sep 23, 2010 1:56 pm

Flying stabilizer means that primary pitch control is provided by the the entire stabilizer moving.

In other airliners, primary pitch control is the job of the elevators at the back of the stabilizer. The entire stabilizer does move, but only slowly for trim control.

Flying stabilizers are often found on modern fighter planes. You can clearly see the entire stabilizer moving in this video of an F/A-18 http://www.youtube.com/watch?v=Ch2yIRhXzhU&NR=1
 
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Faro
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RE: Runaway Stabiliser On Airbus Types

Thu Sep 23, 2010 2:15 pm

Quoting tom355uk (Reply 22):
Meanwhile, back on topic, as already stated, on a FBW Airbus trim runaway is virtually impossible - however, if at least two of the AoA sensors are simultaneously providing erroneous readings (as in GXL888T) then it is possible for the autotrim to run to the nose up stop and remain there until stall. The FBW should then revert to Direct Law, the trim will freeze at the last autotrimmed value and USE MAN PITCH TRIM appears on the PFD. If you don't manually trim quickly, then you are in big, big trouble.

Does the FBW revert to Direct Law, freeze the trim and flash the PFD instruction:

i) after the autotrim has run to the full nose up position;
ii) after the full autotrim run *and* stall; or
iii) as soon as it somehow determines that there may be a dual AoA sensor anomaly?

Also, is it realistic to expect flight crews to manually trim quickly enough, after all they would be confronted with an anomaly with a very remote probability of occurrence and would take time to understand exactly what is happening to them. How quickly must you trim manually to avoid damage to the structure at cruise speed, within seconds?

BTW, did GXL888T involve a runaway trim too? Don't know if the final accident report has been issued...

Faro
 
travelavnut
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RE: Runaway Stabiliser On Airbus Types

Thu Sep 23, 2010 2:41 pm

Quoting tristarsteve (Reply 26):
It is the primary pitch control for the aircraft. The 'elevators' on a Tristar act like trim tabs.
The flying stab flies the aircraft all the time.

So if I would say that the Tristar has the location of the stabalizer and trim reversed in comparison with more general airliners, would I be correct?
 
474218
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RE: Runaway Stabiliser On Airbus Types

Thu Sep 23, 2010 3:46 pm

Quoting TravelAVNut (Reply 29):
So if I would say that the Tristar has the location of the stabalizer and trim reversed in comparison with more general airliners, would I be correct?

Almost all fighter aircraft since the mid 1950's have used a "flying stabilizer", in lieu elevators for pitch control. The TriStar is unique in its use on a commercial airliner.

To increase the effectively of the "flying stabilizer" at low speed Lockheed added geared elevators to the "flying stabilizer". When the "flying stabilizer" is full nose down (14 degrees) the elevators will be full up (24 degrees). Moving the "flying stabilizer" full nose up (1 degree) moves the elevator to the faired (0 degree) position.

Just like all other commercial airlines the TriStar can be trimmed, manually, electrically or automatically. The difference is that when the TriStar pilot moves the control column fore or aft the entire "flying stabilizer" moves either nose up or nose down. When a pilot in every other commercial airliner moves the control column or side stick fore or aft only the elevators move up or down.

The reason "runaway trim" is critical is there is possibility that the elevators do not have enough authority to overcome the aerodynamic effect of the the mistimed stabilizer. With the TriStar "flying stabilizer" the primary controls (from the columns) overrides any trim in puts so a mistrim can not happen.

However, even the TriStar can have a problem, like the Delta flight out of San Diego and the elevators jammed full up because of a failed bearing in the drive system.
 
travelavnut
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RE: Runaway Stabiliser On Airbus Types

Thu Sep 23, 2010 6:30 pm

Quoting 474218 (Reply 30):

Thanks man! So why did Lockheed chose this method and the other manufacturs did not?
 
474218
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RE: Runaway Stabiliser On Airbus Types

Thu Sep 23, 2010 7:37 pm

Quoting TravelAVNut (Reply 31):
Thanks man! So why did Lockheed chose this method and the other manufacturs did not?


Because when Lockheed designed the L-1011 they started with a clean sheet of paper. Boeing and Douglas did not. They based their new designs, 747 and DC-10, on aircraft they had on the production line at the time.
 
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Starlionblue
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RE: Runaway Stabiliser On Airbus Types

Thu Sep 23, 2010 11:07 pm

Quoting TravelAVNut (Reply 29):
So if I would say that the Tristar has the location of the stabalizer and trim reversed in comparison with more general airliners, would I be correct?

Here's my understanding: The "elevator" on the back of the TriStar tab is a trim tab. However despite the name the trim tab does not trim the aircraft. It is used to help move the stabilizer, much like the system on the DC-9 and MD-80.

Please correct if I got it wrong.
 
474218
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RE: Runaway Stabiliser On Airbus Types

Thu Sep 23, 2010 11:50 pm

Quoting Starlionblue (Reply 33):
The "elevator" on the back of the TriStar tab is a trim tab.


Wrong. As I explained earlier the elevators geared (slaved) to the stabilizer to provided additional pitch authority in slow speed flight, like takeoff.

On every other airliner you trim the stabilizer nose down for takeoff, when takeoff speed is reached you pull back on the column/stick the elevators go up, the nose of the aircraft pitch up and the airplanes is flying. The L-1011 is the same except when you pull back the stabilized nose goes down further and the slaved elevators go up further.
 
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larshjort
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RE: Runaway Stabiliser On Airbus Types

Thu Sep 23, 2010 11:56 pm

Quoting Starlionblue (Reply 33):
Here's my understanding: The "elevator" on the back of the TriStar tab is a trim tab. However despite the name the trim tab does not trim the aircraft. It is used to help move the stabilizer, much like the system on the DC-9 and MD-80.

IT's not like the MD80. on the MD80 you have servo tabs, elevators and a trimable horizontal stabilizer. The control column moves the servotabs which then moves the elevators aerodynamically. On the Tristar you move the horizontal stabilizer which is mechanically linked with the elevators and therefore moves with the stabilizer. Think of a Piper stabilator, and instead of the trim tabs you have elevators givig ekstra deflection with the movement of the stabilator.

View Large View Medium
Click here for bigger photo!

Photo © Robert Beaver


/Lars
 
474218
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RE: Runaway Stabiliser On Airbus Types

Fri Sep 24, 2010 12:33 am

Quoting Larshjort (Reply 35):
On the Tristar you move the horizontal stabilizer which is mechanically linked with the elevators and therefore moves with the stabilizer.


It may be a translation thing but. The TriStar's elevators are attached to the stabilizer and positioned by movement of the stabilizer. In the sketch below you can see the cables, quadrant and push rod that positions the elevators as the stabilizer is moved by the hydraulic actuators. Shown is the left hand there is identical installation for the right hand elevator

The cable that positions the elevators up is 3/8" in diameter.


 
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larshjort
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RE: Runaway Stabiliser On Airbus Types

Fri Sep 24, 2010 1:35 am

Quoting 474218 (Reply 36):

It may be a translation thing but. The TriStar's elevators are attached to the stabilizer and positioned by movement of the stabilizer. In the sketch below you can see the cables, quadrant and push rod that positions the elevators as the stabilizer is moved by the hydraulic actuators. Shown is the left hand there is identical installation for the right hand elevator

The cable that positions the elevators up is 3/8" in diameter.

Thank you for the explanation, I wasn't 100% sure how it was connected but that drawing made a few things clearer.

/Lars
 
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Starlionblue
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RE: Runaway Stabiliser On Airbus Types

Fri Sep 24, 2010 10:18 am

Quoting 474218 (Reply 34):
Wrong. As I explained earlier the elevators geared (slaved) to the stabilizer to provided additional pitch authority in slow speed flight, like takeoff.

I see. So it is a hinged section a bit like the rudder on the 747SP?
 
tom355uk
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RE: Runaway Stabiliser On Airbus Types

Fri Sep 24, 2010 12:02 pm

GXL888T did not suffer from a trim runaway - as stated, this is virtually impossible. The full report is available on the BEA website,

http://www.bea.aero/docspa/2008/d-la081127.en/pdf/d-la081127.en.pdf

but basically this is what happened:


- The aircraft was washed without the AoA sensors being covered, allowing water ingress.
- During cruise flight, the water in two of the AoA sensors froze while they were at identical values - voting out the third (correctly functioning) AoA sensor.
- During the approach, the crew decided to perform a low speed protection test (which relies on the AoA vanes to trigger A-Floor). During a low speed protection test, the sidestick is moved aft to provide an increasing pitch up and resultant decrease in speed. As the aircraft is in normal law at this point, the autotrim will follow the sidestick order (because the control laws are based on a load factor) until it reaches it maximum deflection.
- The vanes obviously could not move in sync with the aircraft, so the pitch was allowed to increase past the critical AoA of the wings, resulting in a stall.
- The response to the stall (pitch down and TOGA thrust) caused the aircraft to nose down and gain speed, but during this maneuver the aircraft entered direct law. Instananeously, the automatic pitch trim is frozen at it last value - in this case, maximum nose up. The USE MAN PITCH TRIM caption illuminates right in the middle of the FMA in amber text and a master caution sounds. you can't miss it!
- Because the crew did not trim manually, when the speed increased the full nose up elevator trim command still applied and the aircraft rapidly pitched up to over 50 degrees.

The aircraft then stalled for a second time and nosedived, unfortunately there was insufficient height to recover.

In fact, IIRC there was a similar incident a few years ago with a 737 belonging to Thomson at Bournemouth, except in that case the crew recovered.
 
474218
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RE: Runaway Stabiliser On Airbus Types

Fri Sep 24, 2010 12:51 pm

Quoting Starlionblue (Reply 38):
I see. So it is a hinged section a bit like the rudder on the 747SP?


Yes or the DC-10. To provide additional authority the 747 and DC-10 (MD-11) have a double hinged rudder. Forward section moves some many degrees and the aft section more a few degrees more.

That is the theory behind the L-1011 "flying stabilizer" with geared (slaved) elevators.

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