Rudder reversal

[timmcat]

Was up in the night (nasty cold and all that!) and turned on the TV. Caught a repeat of the Horizon documentary regarding the AA587 (I think) accident.

Is it really correct that pilots should not apply immediate rudder reversal? Surely in such as wake turbulance, or other avoidance / weather situations, it is normal to apply rudder to counteract. Are you supposed to centralise the control briefly before applying rudder to the opposite direction.

Sorry if some of my terminology proves I am not a pilot (!), but it just seems odd.. bit like fighting a rear skid on a car but having to restrict your movement of the wheel.

Cheers

Tim

[witchdoctor]

Not seen the programme, but I think a lot of the debate at the time of the incident centered around the differences in technique between light aircraft and heavy transport aircraft for the use of rudder or aileron in recovering from stalls/maintaining directional control. Light aircraft pilots are taught to use rudder not aileron to maintain directional control in a stall, and there was some doubt as to how deeply ingrained this habit had become in the F/O of the airliner due to the large amount of rudder input prior to the crash. In a heavy transport aircraft such as the Airbus involved, using rudder would have been incorrect, but as events turned out any recovery technique would likely have proved futile.

[Notso Fantastic]

Witch, you've missed the point of a program you didn't see. It's arguable, but if rudder had been left alone and aileron only used to counteract the and roll, and the yaw left to wash itself out, it is more than likely the fin would not have departed.
Rather than rerun the discussion in a new thread, I would encourage you to refer to the extensive discussion already in Rumours and News.

[atpcliff]

Hi!

The basic problem was that Boeing, the FAA, FAA officials, airline officials, airline policy, pilot trainers and pilots all assumed it was OK to make full and abrupt control movements when below a certain speed.

When the very old FAA certification requirements were studied in depth, it turned out that transport category aircraft were NOT certified to withstand a full rudder movment, followed by a complete reversal of the rudder. No one knew this (there might have been a couple of people, but none of the people that mattered) prior to the crash.

Basically, ALL US airline pilots (and I assume all airline pilots) were trained to use the rudder in high angle of attack situations to control the aircraft. At high angles of attack the ailerons become less effective, and the rudder moreso. This is what I learned in the US Air Force.

Emergency notifications were put out after the accident explaining the technical details of the control deflection problem, and pilots are now being trained differently in their simulator situations than they were before the accident.

Cliff
KGRB

[411A]

atpcliff,

You may be right, many of the new(er) trainers have never heard of 'problems' with full rudder travel at higher speeds.

To their companies I can only say...SHAME on you all, as these lessons were learned a very long time ago with jet transport aircraft.

The more things change, the more they remain the same.
Lessons learned the hard way.

[KATLPAX]

Ive heard the term "Rudder Reversal" in connection with the Boeing 737 Rudder hard over issue in that the acuator would in fact create a rudder reversal whereby the rudder pedal inputs were reversed. This was a possible explanation as why pilot input was ineffective in recovery from full hard over.

I take it this is not the same thing. Is this the correct terminology for the AA587 issue? Just curious if there might be a connection?

Any thoughts or clarifications?

CattlePax/KATLPax

[Genghis the Engineer]

As I understand it, the whole thing is distrssingly straightforward.

The certification requirements (worldwide, or at least as much of the certification world as I've seen, which is pretty much all of it except for the FSU) are straightforward. The rudder, fin and supporting structure must be designed for full rudder deflection at Va and 1/3 deflection at Vne. What this requirement has never included has been any sideslip component.

So we've always interpreted that requirement as rudder deflection from balanced flight. I don't think anybody has included in their assumptions that the aircraft might be already fully yawed the other way. If it is, then the force applied by the rudder, and associated stresses on the fin are much greater.

On a light aircraft, which is what most abrupt use of controls training is done on this doesn't really matter. Firstly because the aircraft has so much less yaw inertia to react the rudder force so it tends to swing with rudder input almost instantly, whereas in an airliner there's an appreciable time delay. Secondly because the calculations tend to be less accurate and also weight is slightly less critical, stress engineers use larger safety factors. So, we have been doing this sort of rudder reversal in light aircraft for at-least half a century without incident.

But in an airliner, with smaller structural safety factors and huge inertia reacting the rudder input you can get far greater stresses in the fin supporting structure than it was ever designed for. And it comes off.

I imagine that we'll be seeing changes to both pilot training / operating data, and to airliner certification requirements with regard to static strength of the vertical stabiliser before long.

G