Skytrans DH8A at Cairns on Dec 30th 2011, pilot perceiption leads to discovery of design issue

Last Update: February 13, 2013 / 14:49:37 GMT/Zulu time

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Incident Facts

Date of incident
Dec 30, 2011

Classification
Report

Airline
Skytrans

Flight number
Q6-53

Aircraft Registration
VH-QQA

ICAO Type Designator
DH8A

A Skytrans de Havilland Dash 8-100, registration VH-QQA performing flight Q6-53 from Normanton,QL to Cairns,QL (Australia), landed safely in Cairns completing an otherwise uneventful flight. During roll out the captain perceived the deceleration faster than expected, especially as no reverse and no brakes had been selected and queried the first officer, pilot flying, who shared that perceiption. The captain believed the propellers may have gone into beta uncommandedly and filed a safety report.

The Australian Transportation Safety Board (ATSB) released their final report concluding:

Contributing safety factors

Based on the available evidence, the investigation could not identify any specific safety factors had contributed to the reported event.

Other safety factors

- The first 39 manufactured DHC-8-100 aircraft had a design problem such that, if the friction control was wound to the full out (friction off) position, the flight idle gate was ineffective in reducing the likelihood of pilots inadvertently moving the power levers below flight idle in flight. [Minor safety issue]

- Although the DHC-8 manufacturer had identified a problem in 1986 with the friction control mechanism that could render the flight idle gate to be ineffective, its service bulletin to fix the first 39 DHC-8-100 aircraft omitted the replacement or modification of the leaf spring, which resulted in the bulletin being ineffective.

Other key findings

- Although the crew were concerned that there may have been an uncommanded reverse pitch of the propellers during the landing roll, an examination of the flight data showed that this did not occur.

- The design issue was identified by the operatorÂ’s engineering personnel during an unscheduled maintenance inspection.

The ATSB reported that the flight data recorder was downloaded, the analysis of the data including comparism with three previous flights did not confirm that the propellers had gone into beta range or reverse thrust had been selected, the deceleration was quite normal.

Maintenance however identified two areas of concern during their post flight inspection, one was a friction lever pad which had become partially unsecured due to a fractured fastener. This was repaired. It was also discovered during the maintenance that if the engine lever friction know was fully wound out (effectively turning friction off), the forward upper edge of the leaf spring would come in contact with the flight idle gate, which could be raised sufficiently for the power levers to pass the flight idle gate without the locks being released, effectively disabling the protection against unintentional commanding the propellers into beta range while in flight, which could result in damage to and the loss of the engine.

Only the first 39 Dash 8-100s showed that issue, the design had been changed subsequenty. The manufacturer had released a service bulletin to udpate the first 39 aircraft too, the service bulletin however had inadvertently omitted the requirement to replace the leaf spring, too. Starting after aircraft number 39 a new leaf spring was installed on new aircraft, the new leaf spring carried the same part number as the old however. As result, the service bulletin did not mention or require the replacement of the leaf spring and effectively became ineffective that way.

An airworthiness directive was released to address the omitted requirements in the original service bulletin in January 2012.

The ATSB stated after assessment that the risk of an inadvertent activation of beta range in flight as result of the flight idle gate becoming ineffective would be very low arguing:

- There is no operational reason for the friction device to be wound to the fully out (friction off) position during flight.

- Some of the aircraft in the serial number range 001 to 039 would have had the leaf spring replaced during unscheduled maintenance. For example, of the five aircraft operating in Australia with a serial number in that range, one had been modified.

- Since 2001, all DHC-8-100 aircraft were required to have a beta warning horn installed. The horn would activate if the flight idle gate release triggers were lifted in flight. In addition, if the friction device had been wound to the fully out position prior to flight, the horn would activate immediately after takeoff.

- Since 2002, all DHC-8-100 aircraft operating in the United States were required to have a beta lockout system fitted, reducing the risk of the consequences of ground beta operation in flight.

The ATSB released following safety message as result of their investigation:

This investigation highlights the importance of crews reporting 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.
Incident Facts

Date of incident
Dec 30, 2011

Classification
Report

Airline
Skytrans

Flight number
Q6-53

Aircraft Registration
VH-QQA

ICAO Type Designator
DH8A

This article is published under license from Avherald.com. © of text by Avherald.com.
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