Rudder trim control on your aircraft?

[Hardy Heinlin]

Good evening,

this is something I've asked before and still wonder ...

On the 747, the rudder trim control is powered by DC bus 3. This bus cannot be powered by batteries.

In uncomfortable situations, this may lead to even more uncomfortable situations.

Classic volcanic ash scenario: Two or three engines fail on one side. You apply rudder trim. Eventually the last engine fails and all DC busses are unpowered. Thrust is now zero, i.e. symmetric again. Rudder trim is stuck out of neutral. It cannot be centered because DC bus 3 is now unpowered.

You're gliding with limited hydraulic resources, in severe flight conditions, with reduced instrumentation, incl. clogged pitot tubes, and on top of that you'll be busy kicking the pedal to compensate the stuck rudder trim.

I wonder why they didn't put it on a battery bus.

Just curious: For the pilots on the forum: On the aircraft type you fly, can your rudder trim operate by battery power?


Cheers,

|-|ardy


OK, in a volcanic ash cloud you should set idle thrust and don't need much rudder trim. Then let's take any other engine-out scenario. Asymmetric thrust. Rudder trim applied. All engines fail.  Symmetric (zero) thrust. Rudder trim stuck out of neutral.

[torrence]

I guess this case falls under your extreme failure rubric:  "Exceeding the scope of standard emergency training. Creativity required".  Required action: "Display Creativity!"

[Hardy Heinlin]

Creativity ... -- One could put a broomstick between the seat and the rudder pedal. But broomsticks are seldom on the deck nowadays.

[Pandy280677]

Hardy, one could always go straight on the pax PA and ask if there's a wicked witch of the west aboard who might want to help save the day?!

[Jamie]

Well on the Fokker 70/100 it's possible

[Hardy Heinlin]

Well on the Fokker 70/100 it's possible

You mean you can command the trim to neutral with battery power?


|-|

[Garry Richards]

What if, with the power off, the hydraulics are not providing differential pressure so the airflow pushes the rudder to neutral?

[Hardy Heinlin]

As far as I've learned it, on the 747 the rudder trim actuator is driven by a DC motor. The trim actuator varies the rudder's neutral angle; the rudder is at this neutral angle when the rudder hydraulics are relaxed or symmetric.


|-|

[Jamie]

On the F100: the rudder trim is a closed loop cable system. No fancy electrical/hydraulic stuff. The rudder itself is powered hydraulically and backed up by an electrical system and cable/mechanical system as well.

[John H Watson]

With no hydraulics, the wind/airflow is still going to push the rudder to wherever it wants. Just look at parked 744's: the wind pushes the rudder to one side (even though the rudder trim is usually centred). In flight, the rudder should move to the aircraft centreline (assuming no significant crosswind).

Rgds
JHW

[Hardy Heinlin]

Right, I just edited my previous post. It's similar to the ailerons and elevators drooping down by gravity when parking without hydraulics pushing them up -- which is simulated in PSX.

Garry, now I see what you mean.

This is simulated in PSX also; without hydraulic power, the rudder trim is ineffective and the rudder aligns with the aircraft centerline.

However, as long as the windmilling engines provide hydraulic pressure, the rudder will be kept at the trimmed angle, I assume. (This is what happens in the PSX Alpha.)

I can depower the rudder hydraulics only if I depressurize all hydraulic systems -- which makes the aircraft eventually completely unflyable.


Cheers,

|-|ardy

[John Golin]

I can depower the rudder hydraulics only if I depressurize all hydraulic systems -- which makes the aircraft eventually completely unflyable.

Challenge accepted!

[Hardy Heinlin]

Challenge accepted!

Some tips ...

Pitch control:
- Tell the pax to walk back or forward (by moving the CG slider)
- Lower the gear if required and if you won't ever need it up again
- Move the flaps electrically if you have some minutes left

Bank control:
- Start jettison and set an asymmetric configuration of the pumps and valves
Edit: Damn, you need a running generator for this. Call Scotty ...


|-|

[Will]

How significant is the pitch moment with thrust?

[Blake H]

As far as I've learned it, on the 747 the rudder trim actuator is driven by a DC motor. The trim actuator varies the rudder's neutral angle; the rudder is at this neutral angle when the rudder hydraulics are relaxed or symmetric.


|-|

Hi Hardy, Does the rudder pedals change position with Rudder trim movement, or just the rudder itself?

[John H Watson]

Rudder pedals move with trim on the real aircraft. I've seen it happen. The DC motor backdrives the rudder pedal mechanism (cables).

In theory, the rudder trim should steer the aircraft on the ground (I've been told that there is some ground steering effect on the 767, but it's been harder to confirm on the 744).

The yaw damper system is different. There is no feedback to the pilot through the pedals.

Outputs from the Feel, Centring and Trim Mechanism utilizes three
output rods. Two rods are connected to the Upper and Lower Rudder
Ratio Changers respectively. These outputs result in Rudder
deflection.
The third output backdrives the Input Rod and Aft Quadrant to move
the Rudder Cables and Pedals during Rudder trim and rollout
operation.

[Blake H]

Thanks John for the answer looks like a big sim update.

I am thinking for my sim a linear actuator with a pot connected to the two centre springs of the rudder pedals cam this will off-centre the rudder pedals position. During rudder trim conditions. The offset of the linear actuator position (trim position) is sent to phidgets then to aerowinx rudder trim gauge.

A number I pulled from a book the rudder trim at unit 10 if 80% of the totally rudder pedal travel.

I think because the tiller is connected to rudder pedals mechanically you will transfer pedal position into the tiller. I can visually see the movement in my real cockpit sim. Same must be true during taxi, if you use the trim to steer.

I will let you know. Sounds like fun.

Cheers Blake

[IefCooreman]

To be honest I wouldn't know the answer about the power resource. There are situations in which we (737) lose the trim indication. But the idea of losing trim does not seem "problematic" to me. I'm not sure if I get the "scenario" right, but...

But from a pilot's perspective: rudder trim does indeed work through the rudder pedals which is a very logical way of working. It's to offload the force required by the pilot (on the 737, Boeing actually uses this wording "use trim to offload force". If it's stuck, you simply can't offload and you need to apply a continues force on the pedals in the opposite way. Which is a very logical reaction for a pilot. Annoying in the long term, but logical.

Same applies to the horizontal stabilizer, which has a much larger authority compared to a rudder or aileron trim. Even for a completely stuck horizontal stabilizer (secondary flight control), the elevator itself (primary flight control) should provide sufficient authority to counter. Yet the flying will not be comfy... (but part of basic training on the 737)

[John H Watson]

I think because the tiller is connected to rudder pedals mechanically you will transfer pedal position into the tiller. I can visually see the movement in my real cockpit sim. Same must be true during taxi, if you use the trim to steer.

The tiller is a different kettle of fish. At the gate, I haven't noticed any movement of the tiller with rudder trim use. It may be a gearing ratio issue or something else (trim motor strength?). Perhaps the tiller wants to move. We probably need pilot or steering specialist input on this issue. Does rudder pedal input (with feet) move the tiller? In my engineering career, I've only operated the rudder pedals a few times with hydraulics operating and my focus wasn't on the tillers.

The operation is muddied by quirks of the rudder pedal steering system. e.g. if the tiller or rudder pedals are moved with the aircraft stationary, this doesn't equate to nose wheel movement. The steering hydraulic actuators can't overcome ground friction (at least on a dry tarmac). However, you can disconnect the scissor link between the upper and lower halves of the nose gear and the hydraulic rams will move the upper half of the scissor link.
Also, we know that holding the tiller stationary during ground flight control checks will stop the rudder pedal nosewheel steering operating (but, of course, not the rudder pedals themselves)

[emerydc8]

In theory, the rudder trim should steer the aircraft on the ground (I've been told that there is some ground steering effect on the 767, but it's been harder to confirm on the 744).

I have found that the rudder pedals on the 767 have little effect when taxiing. Everyone has his own technique, but I prefer not to use rudder pedal steering at all on taxi because in order to make the steering input substantial enough to return to centerline (it's only 7 degrees max anyway) you either have to concentrate a lot on keeping it right on centerline (taking away some of your ability to look around the cockpit), or you have to use a lot of rudder pedal -- usually full -- to get it back on centerline. Plus, I'm usually tired and I don't feel like concentrating that hard and pushing that much on the rudders just to taxi.

Another reason I don't like to taxi using rudders is that, to be comfortable for me, my feet would have to be on the bottoms of the pedals with heels on the floor, but when you do this, you give up quick (and smooth) access to the brakes because you have to shift your toes up to get to the brakes, unless you have a huge foot. So, I use a combination of brakes (including asymmetric) and tiller input to control the speed and direction on taxi, almost exclusively. Coming out of a 90-degree turn, I'll usually push up the engine on the side of the direction I'm turning because it has a tendency to start the nose back to neutral such that I don't have to be as qucik on the tiller input in that direction (It makes it easier to be smooth.) Plus, in a sharp turn, I am only doing 10 knots and pushing the engine up helps to accelerate it back to taxi speed.

On the flight control rudder check, I haven't ever looked to see whether the tiller actually moves with rudder pedal deflection such that I have to hold the tiller in position when pressing the rudders, or whether the tiller stays neutral and I am actually having to put pressure on the tiller in the opposite direction to counter the nosewheel steering. In other words, does the tiller stay centered on the rudder check and only the nosewheel moves, or does the tiller move with rudder deflection and I have to counter it with opposite tiller input? I don't know, probably because my hand is always on the tiller and the tiller overrides rudder input anyway. In any case, it's hardly noticeable on the 767 and that's with full rudder pedal deflection on the control check.

As for taxiing with trim. I would assume that full rudder trim would be equal to, or less than, full rudder for purposes of nosewheel deflection on taxi. But I have never tried to taxi using rudder trim. I think it would be pretty difficult because of the time it takes to move the trim to full-scale deflection and the minimal effect that rudder nosewheel steering has to begin with. If you let it get more than about 10' off centerline, you don't really have time to input full rudder trim to get it back. In this case, you would probably want to use the tiller because you don't know what you might hit cruising along that far off the centerline -- and you don't want to find out either. A similar comparison to steering with rudder trim might be the idea of controlling pitch with power -- You can do it, but there is a significant lag in returning it back to altitude, just as there would be a lag to getting back to centerline using rudder pedal steering, and even more lag using rudder trim. It takes a lot of work and it's much easier just to use the tiller on the taxi and be smooth about it. Just my 2 cents.

Jon