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Over Controlling the Rudder Still Possible After 2001 Crash

Tue, Aug 10, 2010 — David Evans

Articles, Featured

The smoldering wreckage in Belle Harbor, a residential area in New York City, casts a shadow to this day. The tailfin of an Airbus A300-600, operated as Flight 587 by American Airlines, separated in flight, causing the pilots to lose control. The separated tailfin fell into the adjacent waters and the plane plummeted into the neighborhood, killing all 260 aboard and five persons on the ground. That was in 2001. Now, in 2010, the National Transportation Safety Board (NTSB) recommends yet again that the super-sensitive rudder control system be redesigned.

It seems that the fatal flaw exists in other Airbus models, as exemplified by a similar — thankfully nonfatal — incident involving an Air Canada A319 on 10 January 2008.

The Transportation Safety Board (TSB) of Canada investigated this incident, and last modified its report in May of this year. Based on the TSB’s findings, and the NTSB’s evident frustration following its investigation into the Flight 587 disaster, the Board sent a letter to the Federal Aviation Administration (FAA) 4 August basically venting its frustration with the lack of corrective progress since the crash in Belle Harbor – especially since the latest hazard evidently lurks in other Airbus models.

The two cases have a number of parallels.

The American Airlines A300 had just taken off from JFK International Airport when it encountered wake turbulence from preceding aircraft. First Officer Sten Molin, the pilot flying, applied a series of rudder corrections, in opposite directions, that put tremendous strain on the tailfin – so much stress that the fin ripped out of its mounting brackets.

The tailfin mounting bracks; note piece of tailfin in the left bracket.

The tailfin mounting bracks; note piece of tailfin in the left bracket.

Molin’s inputs were inappropriate and may have been linked to American’s advanced aircraft maneuvering program (AAMP), which he had completed. He was a sharp, aggressive pilot, perhaps too aggressive in his foot movements on the rudder pedal.

But NTSB investigators were also surprised at how little movement was required on the pedals to achieve full deflection of the rudder. Only 32 pounds of foot force (just 10 lbs above the 32 lb breakout pressure) were needed, and the pedal moves just 1.2 inches to achieve maximum rudder deflection.

Apply that pedal force in opposite directions (known as a rudder reversal), and the aerodynamic force on the tailfin builds rapidly. In this case, the wrenching force was strong enough to rip the tailfin out of its mounting brackets. In certification testing, the rudder reversal features a stop at center, then deflection to the opposite side. That pause results in less force on the tailfin.

In the 2004 final hearing on the Flight 587 disaster, an exceedingly trenchant exchange occurred between Member Deborah Hersman and NTSB professional staff member (and former airline pilot) David Ivey:

Hersman: If you are a line pilot, how likely would it be that you would get the full amount [of rudder]? Or get 1.2 inches of the pedal at 250 [knots airspeed]?

Ivey: If I were to put in rudder? And knowing what I had found that … there was a very good chance you could put in full rudder [with 1.2 inches of travel]?

Hersman: If you put in full rudder to the right, how likely is it that you’re going to have to come back with the left rudder?

Ivey: I think I could speak for most pilots that if I had any input that had sent me to the right, for example, I’m not going to do what certification says and put my rudder to neutral. I am going to counter the effects that I have just experienced in my body or what I have seen and I’m going to put in opposite rudder to try to correct the problem … But to answer your question, if I had a big yaw to the right I would put in left rudder. I certainly wouldn’t put it in neutral.

Hersman: And once that happened, is it your belief that this pilot was in APC? [APC: aircraft pilot coupling, where one action reinforces the other.]

Ivey: It is my belief.

As a result of its inquiry, the NTSB issued safety recommendations regarding the A300-600 and the A310 rudder control systems, the two being similar in design:

A-04-56: To the FAA, modify certification standards to ensure safe handling qualities throughout the flight envelope, including limits for rudder pedal sensitivity. The FAA promised a study on the issue and the NTSB classified its recommendation “OPEN – Acceptable Response.” As of 2010, no such study has been sighted.

A-04-57: To the FAA, once the certification standards in A-04-56 are upgraded, review existing transport designs and modify rudder controls to provide increased protection from rudder input-induced APC. Again, the FAA assured the NTSB it would study the issue, and the recommendation was classified “OPEN – Acceptable Response.” As of 2010, rudder pedals remain sensitive with just 1.2 inches of travel from full deflection on one side to full rudder deflection on the other.

A-04-63: Addressed to the French certification authority, the Direction General de l’Aviation Civile (DGAC), to modify the A300-600 and A310 for increased protection from potentially hazardous rudder pedal inputs. The European Aviation Safety Agency (EASA) indicated that a reduced pedal travel limiting unit (PTLU) seems a likely fix. Noting that the PTLU has not yet been retrofitted, the NTSB has classified the recommendation “OPEN – Acceptable Response.” One notes that this characterization is for a six year old recommendation in which only a “promissory note” has been received.

Basically, the FAA, the DGAC and EASA have “slow rolled” the NTSB recommendations, despite the fact that there are 265 deaths pointing starkly to the problem, not to mention a history of rudder reversals before the Flight 587 crash.

Enter the TSB. Its investigation of the incident involving the Air Canada A319 indicates striking similarities to the Flight 587 disaster. On 10 January 2008 the Air Canada A319, climbing to cruise altitude 9 NM south of the U.S.-Canadian border in Washington state was following a United 747. The A319 experienced three sharp jolts, with the aircraft rolling four times, once to a maximum of 55º before the pilots restored control.

Air Canada Flight ACA190

Air Canada Flight ACA190

The TSB investigative report indicated:

“Rudder control inputs, which were coordinated with the out of phase roll inputs, had a direct relationship with vertical stabilizer loads. Abnormal accelerations in the normal or vertical axis were correlated with changes in angle of attack, and sidestick pitch control inputs opposed these angle of attack excursions.”

Even less movement is required on the A319 to whap the rudder from full deflection on one side to full displacement on the other side: 1.14 inches at 36.5 pounds of force.

Fortunately, the pilots recovered the aircraft and made an emergency landing at Calgary. The tailfin was inspected and found undamaged, but the attachment fittings sustained 29 percent higher loads than the maximum load expected in service – the design limit load in technical parlance.

In its analysis, the TSB said:

“The most risk of vertical stabilizer structural overload would occur on a side slipping aircraft when full rudder displacement is effected on the side of the aircraft toward the relative wind vector. The A319 rudder system does not reduce the available travel on the upwind side of the sideslip compared with the lee side. The existing rudder limiting system does not prevent potentially hazardous loads from being applied to aircraft structure as a result of full, alternating rudder pedal inputs.”

So, once again, the NTSB reiterates its previous recommendations and, in its recent letter to the FAA, issued two new recommendations:

A-10-119: Modify certification standards to ensure safe handling qualities, including limits for rudder pedal sensitivity.

And…

A-10-120: Require modifications of existing airplanes to assure greater protection against APC following rudder inputs.

Meanwhile, as Ivey indicated, the tendency of a pilot in extreme yaw is to apply full rudder deflection to one side, then to the other. Pilots are not going to halt the rudder at neutral before moving it to the opposite full deflection. Yet nine years after the crash of Flight 587, that remains the falsely optimistic certification standard.

Whatever the FAA is studying, it would seem that rudder movement through the centerline to the opposite full deflection ought to be implemented immediately as a standard, and all transport category airplane designs should be tested to determine if they meet it.

If they don’t, structural modifications should be made.

For new airplanes like Boeing’s B787, the new structural standard should be applied, and the pedal movement should not be so sensitive that the pilot over controls the rudder into a dangerous reversal.


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