Eastern Australia DH8C near Weipa on Dec 6th 2011, turbulence causes engine problems
Last Update: February 25, 2013 / 15:01:21 GMT/Zulu time
Incident Facts
Date of incident
Dec 6, 2011
Classification
Incident
Cause
Turbulence
Airline
Eastern Australia Airlines
Aircraft Type
De Havilland Dash 8 (300)
ICAO Type Designator
DH8C
Contributing safety factors
- The pilot flying inadvertently lifted the flight idle gate release triggers and moved the power levers below the flight idle gate during turbulence, which led to ground beta operation in flight and loss of propeller speed control.
- A significant number of 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. [Significant safety issue]
Other safety factors
- The operatorÂ’s DHC-8 pilots had not had the beta warning horn demonstrated to them during their training. A similar situation applied to other Australian operators. [Minor safety issue]
Other key findings
- The first officer realised that he had inadvertently selected the power levers to below flight idle and immediately rectified the situation, avoiding damage to the propellers and engines.
- The investigation identified several incidents where a DHC-8 pilot inadvertently moved one or both power levers behind the flight idle gate in flight that led to a loss of propeller speed control. Those events collectively indicate a systemic design issue within the aircraft's propeller control system.
- Other transport category aircraft types in use in Australia have systems that either mechanically prevent in-flight selection of the power levers below flight idle in flight or prevent propeller overspeed when the power levers are in the ground beta range.
- The US Federal Aviation Administration and the Civil Aviation Safety Authority of Papua New Guinea had mandated the fitment of a system that prevents DHC-8-100, -200 and -300 series aircraft propeller overspeed if the power levers are selected into the ground beta range while in flight.
The first officer (CPL, 2,430 hours total, 500 on type) was pilot flying, the captain (ATPL, about 4,000 hours total, about 2,000 hours on type) was pilot monitoring. The aircraft was descending through FL120 towards Weipa when the aircraft entered a layer of cumulus clouds. The engines were at idle at that point at 900 rpm, the first officer had his hand on the power levers. After entering the cloud the aircraft experienced a few bumps, then a strong updraft followed by a downdraft which caused the first officer to rise out of his seat against his restraints (seat belts), at that point the first officer inadvertently moved the thrust levers below flight idle and thus into beta range. A warning horn activated, the propellers accelerated uncontrolled to about 1200 rpm as result before the first officer realized the situation and moved the power levers back to the flight idle gate, after which the engines and propellers normalized again. The captain inquired what had happened, the first officer explained that he had inadvertently tripped the mechanical flight idle release trigger due to the turbulence. The flight continued for a safe landing in Weipa, subsequent maintenance inspection found no damage.
The ATSB reported that while the rpm is being controlled between 900 and 1200 rpm in flight by its governor, an additional overspeed governor prevents the rpm to increase beyond 1309 rpm. Both systems however are disabled if the power levers are moved 13 degrees or more past the flight idle gate permitting an overspeed condition that could easily destroy the engine.
The ATSB analysed: "The manufacturer had installed a beta warning horn to alert pilots when the flight idle gate release triggers had been lifted. Audible warnings can be very effective at attracting attention, although experience has shown that they are not always heard or comprehended in sufficient time to make an effective response, particularly in times of high workload or distraction. The potential effectiveness of the DHC-8 beta warning in achieving a rapid response was further limited by the fact that pilots had not had the horn demonstrated to them during training. In summary, the DHC-8-100, -200, -300 power lever design had features that significantly reduced the likelihood that flight crew would pull the power levers below flight idle in flight. However, there have been several documented occurrences where flight crews have bypassed the flight idle gate during flight. Although the likelihood of any such occurrence on each flight is very low, the potential for any such event to result in engine damage and a more adverse outcome is significant. No other transport category turboprop aircraft in use in Australia were associated with a similar design issue."
The Canadian TSB had already indicated an airworthiness directive requiring a beta lockout would be issued.
Incident Facts
Date of incident
Dec 6, 2011
Classification
Incident
Cause
Turbulence
Airline
Eastern Australia Airlines
Aircraft Type
De Havilland Dash 8 (300)
ICAO Type Designator
DH8C
This article is published under license from Avherald.com. © of text by Avherald.com.
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