Airbus Rudder Pedal AD Inadequate

American Airlines Flight 587 encountered wake turbulence. The pilot countered with rudder inputs. The rudder inputs were excessive, the tail assembly failed, and the aircraft crashed, killing 265 people.

The NTSB determined that the Airbus' rudder controls are unduly sensitive and make it easy for a pilot to overstress the aircraft's structure, causing a catastrophic failure.  Now, eleven years after that crash,the FAA has issued an Airworthiness Directive against the A300 Airbus to remedy what it considers to be a problem with the aircraft’s design. 

Originally, the FAA was going to require that all the A300's be modified to limit the rudder pedals' travel.  The FAA felt that such a modification would make it much more difficult for a pilot to overstress the aircraft. That modification would have cost about $200,000 per aircraft. But Airbus convinced the FAA to allow a cheaper fix.  So for about half of that cost, the FAA will allow the A300’s simply to be equipped with a warning light on the glareshield directly in front of each pilot and an associated "stop rudder inputs" aural warning.

A Warning is a Last Resort

Any engineer will tell you that when a hazard is discovered, the best option is to design out the hazard. If that can’t be done, then the hazard should be guarded against. If that isn’t feasible, the last resort is to warn against the hazard. That’s what’s known as the engineer’s “Safety Hierarchy.”

Here, there was a feasible way to change the design to eliminate the hazard – limit the travel Stress Performance Curveof the rudder pedals. Thus, a warning is the wrong way to go.

A Warning Is Not Always Appropriate.

Warnings work well in some situations. A warning system that alerts a pilot to low fuel is great. A warning system that alerts the pilot that some system is overheating is also useful. But warning systems that activate in emergency situations are often useless.

The problem is that under stressful situations, a pilot’s performance can degrade rapidly. The pilot is unable to comprehend a warning’s meaning in an emergency and respond appropriately. That’s what the BEA (Europe’s NTSB) concluded happened to Air France Flight  447. Confronted with an emergency, the crew could not comprehend and react to the Airbus’ aural warnings. As summed up by Paul Marks:

Despite a stall warning sounding continually, it was ignored and the pilot kept the plane's nose pointing upward - while the plane was in fact plummeting toward the ocean. All the crew needed to do was push the nose down to regain lift - but they didn't.

In the first minute after the autopilot disconnection, the failure of the attempt to understand the situation and the disruption of crew cooperation had a multiplying effect, inducing total loss of cognitive control of the situation," the BEA says.

The combination of the [Airbus] warning system ergonomics, and the conditions under which [Air France] pilots are trained and exposed to stalls during their professional and recurrent training, did not result in reasonably reliable expected behaviour patterns," the BEA adds with massive understatement.

Stress Performance Curve

Why would a trained crew essentially ignore the aircraft’s warning systems in an emergency? A little stress helps people focus, and they tend to perform better.  But after a point, stress makes it difficult, if not impossible, to think.  A Vietnam fighter pilot used to tell me: “The first thing that happens in an emergency is your IQ gauge goes to zero.” 

The Airbus' rudder pedals can be feasibly redesigned to eliminate the hazard. That makes a warning system the wrong solution.

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Flying Donald - December 5, 2012 10:57 PM

Excellently put, Mike. The FAA should be forced to justify their decision. The expense difference is trivial compared to the cost of operating an aircraft, let alone its purchase price. I look forward to hearing from ATPs who may actually have to deal with the proposed "solution."

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