Australian investigators are advising Bombardier Dash 8 pilots to familiarise themselves with the signs of propeller overspeed, following a serious incident involving a QantasLink turboprop on descent.
When the Dash 8-300 hit turbulence, the first officer inadvertently lifted one, or both, of the gates on the throttle levers which are designed to prevent pilots accidentally moving these levers into the below the flight-idle position.
Flight-recorder data shows the propeller speed increased uncontrollably - the left to 1,253rpm and the right to 1,067rpm - exceeding the 900rpm setting for 3s. The aircraft (VH-SBV) had been flying at 11,300ft, descending to Weipa, on 6 December 2011.
Australian Transport Safety Bureau investigators have released a section of the cockpit-voice recording containing the audible propeller-speed increase and the warning horn which alerted the pilots, to aid familiarisation.
The ATSB found that all 57 Australian-registered Dash 8-100s, -200s and -300s were fitted with warning horns but 48 of them had neither of two modifications - a lock-out system and an adapted gate - developed to prevent overspeed incidents.
While damage to the QantasLink aircraft was averted through quick action, the ATSB says the captain had initially thought the horn signalled the autopilot disengaging.
Similar serious incidents have resulted in efforts to mandate a preventative modification. The ATSB says its release of the cockpit recording is an interim measure to "increase awareness" until this work is complete.