Rudder limit based on airspeed

[Dont Hang Up]

Notwithstanding the AF incident, can I ask the more general question:

Why limit rudder deflection based on airspeed?

If the issue is one of potential damaging loading on the rudder and fin, then surely the load can be measured more directly at the load bearing structures as the pilot input is being applied. The pedal deflection thus becomes a proportion of the total load the rudder can stand under the prevailing conditions.

Creating a dependency on an airspeed that may be incorrect seems to asking for problems.

[vapilot2004]

Large aircraft must have an assist between the controls and the control surfaces due to the forces needed for deflection. Flight controls that are hydraulically actuated require either some programmed limitations or accurate force feedback to the pilot.

This is the difference between fly by wire aircraft and the previous analog loop group. One might suggest the earlier iteration was superior, but the apparent advantages would be mitigated by accuracy, redundancy and predictibility in all but the most extreme environments.

A powered flight control surface that not only does what it is told according to a program, but feeds back into the system the forces that it exerts to maintain the requested position will be the next step in FBW.

[Clandestino]

It's because, everything else being equal, dynamic pressure is proportional to TAS squared. Consequently, the large rudder deflection needed to bring Vmca and Vmcg down to usable values might cause structural failure or control problems when applied at Vmo.

Not every transport category aeroplane needs speed referenced rudder travel limiters. ATR 42-300 is a bit short on power so it has no limiter at all and DHC-8 400 uses mechanical limiter connected to flap handle. However, I don't think that such a workaround would be practical on jets.

I'm not sure how rudder works on other FBW types, but, apart from sophisticated yaw damper and travel limiter, there's nothing FBW about A320's rudder. It's controlled via cables and hydraulically operated.

[Mad (Flt) Scientist]

In some cases the limits on the rudder deflection may also be to address unacceptable handling issues, in which case a load-based devive would be useless. For aircraft where there are configuration dependencies you still need a configuration input.

Load cells are also not that reliable, and especially for hingemoments. One concern I might have is that the hingemoment on a rudder is a powerful function of sideslip as well as rudder angle. It might be very difficult to device a limiting scheme based on load which provide appropriate protection (which can be against sideslip loads on other components, so a low rudder load doesn't mean everything is ok) while also ensuring adequate rudder was available for the various OEI and controllability cases.

Limiting based on airspeed is simple and easily understood, and reversionary modes fairly straightforward.

[hikoushi]

A correction regarding the DHC-8-400:

(From the Q400 FCOM)

"Hydraulic pressure supplied to both rudder PCUs is regulated by the FCECU (Flight Control Electronic Control Unit) in response to airspeed inputs it receives. Rudder authority is limited as a function of airspeed to reduce excessive yaw rate. As airspeed increases, the FCECU reduces hydraulic pressure to the PCU's. This results in a reduction of rudder deflection in response to rudder pedal inputs. As airspeed decreases, the FCECU increases hydraulic pressure to the PCU's. This results in an increase of rudder deflection in response to rudder pedal inputs."

The earlier models (-100, -200, never flew the -300 so can't say about that one) had a similar system whereby the Rudder Pressure Regulator steped rudder actuator pressure up and down in response to TAS signals from the DADC passing through 140 KIAS.

These airspeed-based rudder limits are in addition to the previously-mentioned rudder input restrictor mechanism which limits rudder pedal travel mechanically based on flap lever position.

[LimaFoxTango]

Regarding the 100/300, (never flew the 200, but im sure its the same), the rudder is limited to half its normal travel with the flap handle at 0 degrees. Normal rudder pressure of 1500psi is reduced to 900psi at airspeeds above 150KIAS. Failure of one actuator results in the remaining actuator receiving full 3000psi.

[Dont Hang Up]

Thank you to mad (Flt) Scientist for a clear explanation of the gap in my undertanding.

So if I now understand correctly, it is not applying excessive load to the rudder which is necessarily the problem. Rather it can be the secondary effect of the rudder input creating excessive yaw or sideslip at high airspeed and thus overloading the whole vertical structure.

[WeekendFlyer]

DHU,

spot on. Remember the rudder's sole function in life is to impart or counter sideslip. When the sideslip gets very large the forces on the vertical fin can also become very large, and this problem gets worse as IAS increases (dynamic pressure, qc = 1/2 x sea-level density x IAS^2). Many large aircraft have a sideslip limit that reduces rapidly with IAS.

So the risks of too much rudder input at speed are:
1. Exceeding the sideslip limit
2. Exceeding the structural limit of rudder and/or hinges
3. Exceeding the structural limit of the fin
4. A combination of the above.

And the problem gets worse with rudder reversals (doublets), particularly if they are applied at the natural yawing frequency of the aircraft. There was a nasty accident a few years ago where a test pilot tore the fin off a S-3 Viking by applying rapid opposite inputs of rudder; sadly the aircraft and crew were lost.

So to solve all of these problems a mechanical rudder limiter is usually used. On some aircraft it is operated manually or by flap selection, on others it is electrically actuated by a switch in the cockpit, and on most large aircraft it is commanded by a speed switch, linked to the air data computer(s).