ATR and Virgin Australia have taken steps to prevent pilots from accidentally overstressing the ATR 72’s tail and to help maintenance engineers identify a rare but significant condition when the aircraft’s pitch-control system separates pilot and copilot inputs.
The changes are the result of an Australian Transport Safety Bureau’s (ATSB) investigation into a February 2014 incident with a Virgin Australia Regional Airlines ATR 72 and a number of similar incidents that came to light as part of the investigation.
The common factor is the ATR pitch-control system design. It features a “pitch disconnect” that, in cases of jammed controls, gives the captain control of the left-side elevator and the first officer control of the right-side elevator. In normal mode, each pilot controls both elevators.
In the February 2014 incident, the pitch split occurred when the pilots applied opposite inputs to their respective control wheels, an action that split the controls and resulted in a high-speed pull-up that injured a flight attendant and damaged the aircraft. But the damage was not discovered until five days later, after 13 flights, because technicians followed turbulence-inspection procedures that did not require a detailed view of the elevator and tail assembly.
Further inspection discovered substantial damage, including external and internal damage to the horizontal and vertical stabilizers. According to ATR analysis of flight recorder data, the ultimate load, in terms of asymmetric movement on the horizontal stabilizer, was exceeded by 47%.
ATR was initially unaware of other inflight pitch disconnects. In an investigation requested by the ATSB, the manufacturer discovered 11 inflight pitch-disconnect incidents. ATR initially thought the problem was limited to ground operations. In those cases, certain reverse-thrust, wind and pilot actions can cause a pitch disconnect with no damage to the aircraft.
Some of the inflight incidents had been caused by encounters with strong turbulence or mechanical failures, but at least three had occurred when pilots made opposite pitch-control inputs on the control wheel. ATR said it was not aware of any incidents in which a pitch disconnect had occurred due to the primary reason for its design—a jammed pitch-control system.
Based on the ongoing investigation, the ATSB put out a “safety issue” alert to ATR about potential problems with a pitch disconnect, including “catastrophic damage” to an aircraft structure before crews can act.
ATR separately conducted a review of both maintenance and operational documentation related to pitch disconnects, making changes to flight and maintenance procedures. These include a new dedicated job-instruction card for post-pitch-disconnect incidents and a revision to pilot procedures explaining the reasons for pitch disconnects.