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Investigation leads to unexpected discovery

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An ATSB investigation has uncovered a 27 year-old omission that rendered an aircraft manufacturer’s safety bulletin ineffective. As a result of this discovery, the manufacturer issued a new bulletin to address the issue, which was then made mandatory by Transport Canada and the Civil Aviation Safety Authority of Australia.

On 30 December 2011, a Bombardier DHC-8-102 was being operated on a scheduled passenger service to Cairns, Queensland. During the landing, it seemed to the crew that the aircraft decelerated much more quickly than they expected, given that reverse thrust and landing gear brakes had not been selected.

An inspection of the aircraft found nothing to explain the perceived problem, and a review of the flight recorder data indicated that there had been no abnormal operation of the engines or propellers, and that reverse thrust had not been used. There was little if any risk associated with the event. However, a subsequent inspection of the aircraft discovered a design problem with the aircraft’s power lever controls. 

Once informed of the design problem,
the aircraft manufacturer took prompt action
to address the issue.

The problem related to the friction device within the aircraft’s power levers control quadrant and its interaction with the flight idle gate, which was itself designed to prevent the power levers from going into the ground range while in flight. Operation of the power levers in the ground range slows the aircraft after landing by changing the pitch of the propellers to create aerodynamic drag loads and providing a considerable amount of reverse thrust. The problem with the friction device meant that with power lever friction selected fully off, the flight idle gate could be rendered inoperative.

The design problem only applied to the first 39 DHC-8-100 aircraft that were manufactured; subsequent aircraft were manufactured with a modified design. In 1986, the aircraft manufacturer had introduced a service bulletin requirement to modify these 39 aircraft retrospectively, but the service bulletin omitted a requirement to modify or replace a specific part, which resulted in the bulletin being ineffective.

Once informed of the design problem, the aircraft manufacturer took prompt action to address the issue. They issued a service bulletin to modify the relevant part. 

The results of this investigation show how important it is that crews report occurrences and other perceived problems. Although in this case the actual event reported by the crew was not serious, and no problems relating to the aircraft or crew performance leading to the perceived event were identified, the subsequent investigation did identify a safety issue in the design of the aircraft.

More details of the investigation, along with the findings, can be found in the investigation report AO-2012-005.

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Last update 12 February 2013