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NTSB concerned about rudder sensitivity on Airbus narrowbodies

Similar circumstances between the November 2001 crash of an American Airlines Airbus A300 and a January 2008 in-flight upset of an Air Canada Airbus A319 have prompted US safety officials to urge EASA to modify large aircraft certification specifications to limit rudder pedal sensitivity.

The National Transportation Safety Board (NTSB) determined that the probable cause of the American accident was the in-flight separation of the aircraft's vertical stabiliser due to loads beyond ultimate design created by the first officer's unnecessary and excessive rudder pedal inputs.

In the January 2008 incident the A319 encountered wake turbulence from a Boeing 747 while climbing from 36,000 to 37,000ft. The aircraft's flight data recorder showed several roll and vertical load factor oscillations, and the loss of 1,000ft of altitude. The FDR also recorded a series of three-to-four alternating rudder inputs, and the aircraft reached a maximum roll of 55 degrees. The flight crew declared an emergency and the A319 diverted to Calgary.

"The similarities between the Air Canada Flight 190 and the American Airlines Flight 587 crew members' responses to wake encounters indicate that the Airbus A320 family is also susceptible to potentially hazardous rudder pedal inputs at higher speeds," the NTSB explains in a 6 August letter to EASA.

NTSB noted in the American accident, the pilot applied four full alternating rudder inputs, and after the fourth input aerodynamic loads on the vertical stabiliser exceeded its ultimate design load, roughly twice the maximum load. In the Air Canada incident, the pilot applied three alternating rudder inputs and exceeded the load limit by 29%.

Earlier this year EASA indicated it planned to require a reduced pedal travel limiting unit (PTLU) for the A300 and A310 models, says NTSB. The rudder system for the A320 is similar to the A310/A300, the board explains, with a variable-stop rudder travelling limiter that mechanically limits rudder deflection as airspeed increases.

But NTSB states the American investigation showed that those variable-stop systems "produce dramatically larger aircraft responses to the same rudder input at higher airspeeds than lower airspeeds, which can surprise a pilot and serve to trigger an aircraft-pilot coupling event".

NTSB believes a variable-ratio rudder travel limiter may provide better protection against high loads from sustained rudder inputs at high speeds than variable-stop systems since variable-ratio systems retain a relatively uniform response throughout the airspeed envelope, and require more physical effort from a pilot to produce cyclic full rudder inputs at high speeds.

The board is making three recommendations to EASA regarding rudder pedal sensitivity. The first is to modify its certification specifications for large aircraft to ensure safe handling in the yaw axis throughout the flight envelope, including limits for rudder pedal sensitivity.

Secondly, once those specifications are changed, NTSB believes EASA should review designs of existing aircraft to determine if they meet the new standard, and if they do not, EASA should then determine if they are properly protected from adverse effects of a potential aircraft-pilot coupling event after rudder inputs at all airspeeds. If EASA determines inadequate protection exists, then the agency should require modifications to prevent a coupling event after rudder inputs at high airspeeds.

NTSB's final recommendation to EASA is the review of options for modifying the A300-600 and the A310 to increase protection from hazardous rudder inputs at high airspeeds. Based on the review, if necessary, EASA should require modifications to ensure against dangerous inputs.

In its letter to European regulators, the NTSB points to a previous declaration by EASA that it had reversed a previously-held position that pilot training would be sufficient to prevent against hazardous rudder inputs.

During the NTSB's investigation of the American accident, Airbus argued that American's advanced aircraft manoeuvring programme encouraged dangerously heavy use of the rudder.

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