Pilots and operators of Bombardier DHC-8-100, -200 and -300 series aircraft are being urged to take steps to reduce the risk of propeller overspeed following an incident involving a DHC-8-315 aircraft.
The incident occurred on 6 December 2011 when the aircraft, operated by QantasLink, was on a scheduled flight from Cairns to Weipa in Queensland. The first officer was the pilot flying. The aircraft was descending, with the power levers in the flight idle position and the first officer’s hand on the power levers, when it entered a layer of cumulus cloud. There were a few small bumps from the turbulence. A few seconds later, the aircraft encountered a strong updraft followed by a downdraft, which caused the first officer to inadvertently lift one or both of the idle gate release triggers and move the power levers below the flight idle gate
The flight idle gate is a mechanical stop that assists with preventing the power levers from going into ground beta mode. Ground beta is a control mode used to control the pitch of the propeller blades during ground operations. Because ground beta mode inhibits propeller speed governing, pilots are prohibited from using it while in the air.
The ATSB found that while the beta warning horn sounded as designed, the pilots were not acutely aware of the purpose of the warning horn due to a lack of previous exposure to the sound.
In the cockpit, the beta warning sounded, and the propeller noise increased. The captain initially thought that the warning horn was the sound of the autopilot disengaging, but realised that was not the case as soon as he heard the propeller noise increase. The first officer realised the situation, and immediately moved the power levers forwards of the flight idle gate.
During the short time that the aircraft’s power levers were in the ground beta range, both propeller speeds increased uncontrollably by over 300 revolutions per minute.
The crew found no abnormal engine indications as a result of the occurrence and the flight proceeded normally to Weipa. Subsequent maintenance checks after the flight did not reveal any engine or propeller system damage.
The aircraft design included features to reduce the likelihood of the power levers being moved into the ground beta mode during flight. However, the ATSB found that many DHC‑8-100, -200 and -300 series aircraft did not have any way of preventing movement of the 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.
As of March 2012, there were 57 DHC-8-100, -200 and -300 series aircraft registered in Australia. Some of the aircraft were acquired second-hand from the US and had protective measures previously installed as required by US airworthiness requirements. The ATSB contacted the DHC-8 operators and determined that, at the time of this occurrence, there were 52 aircraft in Australia that were not protected against the consequences of a flight crew moving the power levers below flight idle in flight.
The aircraft manufacturer has advised that it will be releasing a Service Bulletin modification to fix the propeller speed control issue. That bulletin will be mandated by an Airworthiness Directive 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 found that while the beta warning horn sounded as designed, the pilots were not acutely aware of the purpose of the warning horn due to a lack of previous exposure to the sound. In response, the ATSB has released an extract from the cockpit voice recorder with the beta warning horn sounding and the audible rise in propeller speed evident to all Australian operators of the aircraft type. The audio file is also available on the ATSB website to help increase awareness of the issue.
Read the report: Double propeller overspeed involving Bombardier DHC-8, VH-SBV, near Weipa, Queensland, on 6 December 2011, which provides the full report and the sample of the cockpit sounds.