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NTSB flags concern about Airbus rudder sensitivity to EASA

Similar circumstances between the November 2001 crash of an American Airlines Airbus A300-600 and a January 2008 in-flight upset of an Air Canada Airbus A319 have prompted US safety officials to call for changes to the European Aviation Safety Agency's large aircraft certification specifications to limit rudder pedal sensitivity.

The National Transportation Safety Board determined that the probable cause of the American Flight 587 accident was the in-flight separation of the aircraft's fin 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 36,000ft (10,980m) to 37,000ft. The aircraft's flight data recorder showed several roll and vertical load factor oscillations, and the 1,000ft loss of altitude. The FDR also recorded a series of three-to-four alternating rudder inputs, and the aircraft reached a maximum roll of 55e_SDgr. The flightcrew declared an emergency and the A319 diverted to Calgary.

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

The NTSB noted that in the American accident, the pilot applied four full alternating rudder inputs. 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 that it planned to require a reduced pedal travel limiting unit for the A300/A310, says the NTSB. The rudder system for the A320 is similar to the A300/A100, the board says, with a variable-stop rudder travelling limiter that mechanically limits rudder deflection as airspeed increases.

But the NTSB says 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".

The 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.

Among NTSB recommendations to EASA about rudder pedal sensitivity is the advice to modify certification specifications for large aircraft to ensure safe handling in the yaw axis throughout the flight envelope, including limits for rudder pedal sensitivity.

Once those specifications are changed, 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 speeds. If EASA determines inadequate protection exists, then the agency should require modifications to prevent a coupling event after rudder inputs at high airspeeds.

It also says that EASA should review options for modifying the A300-600/A310 to increase protection from hazardous rudder inputs at high speed. 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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