What happened
On 6 December 2011, a Bombardier DHC-8-315 aircraft, registered VH-SBV and operated by QantasLink, was on a scheduled flight from Cairns to Weipa, Queensland. The aircraft was on descent with the power levers in the flight idle position and the first officer’s hand was on the power levers. When the aircraft encountered turbulence, the first officer inadvertently lifted one or both of the flight idle gate release triggers and moved the power levers below the flight idle gate. During the short time that the power levers were in the ground beta range, both propeller speeds increased uncontrollably by over 300 revolutions per minute (rpm). Realising the situation, the first officer immediately moved the power levers back above the flight idle gate and the propellers returned to the normal controlled operating rpm.
What the ATSB found
The aircraft design included features to reduce the likelihood of the power levers being moved below flight idle and into the ground beta mode during flight. However, the ATSB found that many DHC-8-100, -200 and -300 series aircraft did not have a means of preventing inadvertent or intentional movement of power levers below the flight idle gate in flight, or a means to prevent such movement resulting in a loss of propeller speed control. This design limitation has been associated with several safety occurrences.
The ATSB also concluded that the beta warning horn sounded as designed; however, the pilots were not acutely aware of the purpose of the warning horn due to a lack of previous exposure to the sound.
What has been done as a result
The aircraft manufacturer has advised that it will be releasing a Service Bulletin modification to rectify the propeller speed control issue. That bulletin will be mandated by an Airworthiness Directive (AD) from the airworthiness authority of the State of Design (Canada) to ensure that the bulletin is incorporated into all the aircraft affected by the design issue worldwide, including those in Australia. In addition, the aircraft operator has introduced a series of actions to reduce the risk of such occurrences. The ATSB has released an extract from the cockpit voice recorder with the beta warning horn and the audible rise in propeller speed to all Australian operators of the aircraft type and it is also available here in an effort to increase awareness of the issue.
Safety message
Until appropriate modifications are made to DHC-8 aircraft, pilots and operators of DHC-8-100, -200 and -300 series aircraft should familiarise themselves with the circumstances surrounding this occurrence and take the appropriate steps to minimise the possibility of propeller overspeed due to ground beta selection in flight.