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​ATSB warns of “catastrophic” potential of ATR pitch disconnect

The Australian Transportation Safety Board (ATSB) has revealed that opposite control column inputs caused a “pitch disconnect” on a Virgin Australia ATR 72-600, resulting in overstress damage to the aircraft’s horizontal stabiliser.

In its June 2014 preliminary report, the ATSB said that the aircraft (VH-FVR) suffered overstress damage to its tailplane during a flight on 20 February 2014, but an overnight inspection failed to detect the damage. The aircraft then flew 13 additional sectors before pitch control anomalies were detected on 25 February, after which an inspection following a suspected bird strike discovered structural damage.

A fresh interim report, using flight data recorder information, reveals a crucial new detail: at one point during the 20 February incident the captain made a nose-up input, while the first officer made a nose-down input. This caused a “pitch disconnect” that damaged the aircraft’s tailplane.

The ATSB summarises the safety issue as such: “Inadvertent application of opposing pitch control inputs by flight crew can activate the pitch uncoupling mechanism which, in certain high-energy situations, can result in catastrophic damage to the aircraft structure before crews are able to react.”

The incident occurred as VH-FVR operated a Canberra-Sydney service. At 16.40 the aircraft was at 8,500 feet on approach to runway 16 Left when the crew noticed airspeed rising quickly. Several control inputs followed, eventually leading to both crew’s making “pitch up” inputs.

“Shortly after, with both flight crew making simultaneous nose up pitch inputs on the controls, the aircraft rapidly pitched up with an associated increase in the g load,” says the interim report.

“The first officer responded by immediately reversing the control input to nose down. Both flight crew noticed that the controls suddenly felt different and ‘spongy’. The crew verified that the aircraft was under control, stable, and in “level or slight descent.”

Several warnings had occurred, including “pitch disconnect,” indicating that left and right elevator control systems had uncoupled. The crew worked through the pitch disconnect checklist to determine which control column was working normally. Both were found to be normal, and the captain landed the aircraft.

“The aerodynamic loads generated during the pitch disconnect resulted in serious injury to the senior cabin crew member and significant damage to the aircraft’s horizontal stabiliser,” says the ATSB. “Although the aircraft was inspected after the pitch disconnect, the damage was not identified until 25 February 2014.”

ATR, in a load analysis, found that several were exceeded. ATR also provided data about several incidents involving pitch disconnects, including three resulting from different pitch control inputs.

“During examination of the aircraft, the pitch uncoupling mechanism was tested in accordance with the aircraft’s maintenance instructions,” adds ATSB. “The load applied to the control column to activate the pitch uncoupling mechanism was found to be at a value marginally greater than the manufacturer’s required value. The reason for this greater value was not determined, but may be related to the damage sustained during the pitch disconnect event.”

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