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ATSB issues recommendations for improved aircraft design tolerance to inadvertent dual control inputs
26 May 2019

The ATSB has issued Safety Recommendations to EASA and aircraft manufacturer ATR seeking improved aircraft system design tolerance to inadvertent dual control inputs by pilots.

The Safety Recommendations are contained in the ATSB’s final investigation report into an inflight upset and inadvertent pitch disconnect experienced by an ATR 72 turboprop airliner on a flight from Canberra to Sydney, Australia, in 2014.

During that flight, as a result of a sudden decrease in tailwind, the ATR’s pilots unintentionally applied opposing control inputs to their control columns while trying to ensure the aircraft remained below its maximum operating speed. These differential forces activated the aircraft’s pitch uncoupling mechanism. Intended for activation in the event of one of the aircraft elevators being jammed, the pitch uncoupling mechanism resulted in a pitch disconnect, where the elevators could operate independently of each other.

With the pilots applying opposing control inputs and built-up tension within the flight control system, the pitch disconnect resulted in transient asymmetric elevator deflections, generating aerodynamic loads that exceeded the strength of the horizontal stabiliser, causing significant damage.
The aircraft landed safely and was inspected by maintenance engineers but the damage was not detected. The aircraft returned to service and operated a further 13 flights before a subsequent inspection after a suspected birdstrike found it had sustained serious structural damage to its horizontal stabiliser, which was subsequently replaced.

As part of its final report, the ATSB has issued a Safety Recommendation to EASA, recommending taking “further action to review the current design standard (CS-25) in consideration of effect that dual control inputs may have on control of aircraft.”

In addition, the ATSB has issued two Safety Recommendations to aircraft manufacturer ATR, recommending that ATR:

  • assess the operational risk associated with limited tactile feedback between left and right control columns in the context of no visual or auditory systems to indicate dual control inputs; and
  • perform a detailed review of the effects of dual control inputs on the aircraft’s longitudinal handling qualities and control dynamics to determine if there are any detrimental effects that could lead to difficulty in controlling the aircraft throughout the approved flight envelope and operational range.

 

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