The FAA has amended an earlier Airworthiness Directive for all Airbus A320-series aircraft, requiring operators to modify elevator control systems to prevent vibration problems caused by excessive freeplay. The AD applies to the A319 (which is the basis for the Airbus Corporate Jetliner), the A320 and the A321.
The amendment was issued as a final rule in August and supersedes an existing AD that currently requires repetitive measurements of the deflection of the elevator trailing edge, inspections of the elevator servo controls and their attachments and replacement of worn or damaged parts, if necessary. The version became effective on September 24.
The amendment requires periodic inspection of the elevators for excessive freeplay, repair of worn parts if excessive freeplay is detected and modification of the elevator neutral setting. According to the FAA, the amendment was prompted by additional reports of severe vibration in the aft cabin of A320-series airplanes and studies that indicate that the primary cause is excessive freeplay in the elevator attachments.
Approximately 352 airplanes of U.S. registry will be affected by the revised AD, the FAA said, and compliance will take approximately two work hours at an average labor rate of $60 per work hour. Based on these figures, the cost effect of the initial inspection required by the AD on U.S. operators is estimated to be $42,240, or $120 per airplane.
Approximately 112 airplanes will require adjustment of the elevator neutral setting, which will take approximately 12 work hours. Based on these figures, the cost effect of the required adjustment on U.S. operators is estimated to be $80,640, or $720 per airplane.
The FAA noted that several commentators, including the NTSB, support the proposed AD. But Airbus Industries requested that it be withdrawn, asserting that there is no unsafe condition due to limit cycle oscillation (LCO) of the elevator. It also disagreed with the FAA’s conclusion that elevator LCO could result in reduced structural integrity and reduced controllability of the airplane.
Airbus argued that because LCO is a fixed-frequency vibration with a constant amplitude, it is not a stability problem. It contended that “such a phenomenon is well detectable, and the flight crew can determine the significance of the airframe vibration and initiate appropriate corrective action.”
The manufacturer further claimed that during the period between LCO initiation and uncomfortable vibration, there is no structural concern. Airbus added that it conducted extensive flight tests with “representative backlash configurations combined with low hinge moment, and no adverse effect on handling qualities was found.”
The company maintained that adherence to the aircraft maintenance manual (AMM) coupled with Service Bulletins (SBs) would be sufficient to address any possible LCO phenomenon, and Airbus expressed doubt that there would be any benefit from making corrections on an airplane with no vibration reported.
But the FAA determined that the A320 elevator LCO, as defined by Airbus, is actually an aeroelastic stability problem–self-excited and not damped with time–which, if not addressed, could result in reduced structural integrity and reduced controllability of the airplane. The agency disagreed with the manufacturer that the vibration will be felt by the flight crew, pointing out that the modification of the elevator neutral setting would tend to mask the presence of freeplay and associated vibration, and make the freeplay checks even more critical.
The FAA further spurned requests by two operators to extend the compliance time for the freeplay inspection, saying that it determined that 18 months is the maximum amount of time allowable for these airplanes to continue to operate safely between inspections.