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SEQUENCE OF EVENTS

VH-TQO had been scheduled for an overnight stop in Tamworth where servicing was normally done in preparation for the next day's flying. In addition, the upper torque links were changed on both landing gear legs, as non-scheduled maintenance, to correct main wheel shimmy/vibrations thought to be related to clearances allowed in the links within the service-wear limits.

Immediately after takeoff from Tamworth on the following day, the weight-on-wheels (WOW) caution light illuminated. The crew consulted the abnormal procedures checklist which warned that the landing gear might not retract. As the landing gear retracted normally, the crew decided to continue the flight to Sydney. At about 26 NM from Sydney, passing 8,000 ft on descent for an ILS approach, the ground spoilers deployed when the power levers were retarded to 'flight idle'. Indicated airspeed reduced from 230 kts to 150 kts and rate of descent increased from 1,700 ft/min to 2,800 ft/min. The crew selected the Flight/Taxi switch to the Taxi position, which placed the spoiler control system in the ground operating mode, and the spoilers then retracted. A normal descent profile was regained by 7,500 ft and the aircraft continued for a normal approach and landing.

SPOILER SYSTEM

The spoiler deployment system is arranged so that the ground-spoilers will deploy when all the system input conditions have been met - ie, the throttle levers are retarded to flight idle and the WOW sensors indicate that the aircraft is on the ground. This system has multiple redundancy as there are four WOW sensors (two on each leg) and two separate bus communication channels. With one faulty sensor, or one channel failed, the system will operate normally. Normal operation is still possible despite some multiple failures. However, in this case, all four sensor gaps were incorrect. This would not normally occur with a single system fault or if one torque link was replaced and the WOW sensor was not adjusted.

TORQUE LINKS

The torque links have ramps on the upper surfaces which set a target near/far dimension. This is read by sensors which send digital signals to the Proximity Switch Electronics Unit (PSEU). The PSEU contains the computer which controls landing gear and ground-spoiler logic (among other functions). The torque links fitted to VH-TQO at Tamworth had ramp dimensions which were different to those on the links removed. Variations in torque link ramp height were found to occur across the company fleet. These variations were the result of production specification changes to the same part number torque links.

MAINTENANCE

Maintenance Manual At the time of the occurrence the Maintenance Manual (MM) was the reference for the torque link change. The MM stated that a WOW sensor adjustment was required after a main gear strut change. There was no statement that a WOW sensor adjustment was required when only the torque links were changed. Consequently, the WOW sensors were not adjusted, the sensor gaps were incorrect and the ground spoilers deployed in flight when the power levers were retarded to flight idle. The MM has since been amended by issue of temporary revision no. 32-73, dated November 1993, which specifies sensor adjustment after changing the torque links. Maintenance System and Procedures The company's maintenance system control is located in Sydney.

The aircraft manufacturer's Overhaul Special Inspection Procedures (OSIP) program is used to track the time in service of Control Time Limit (CTL) items across the company's fleet. Aircraft are maintained in accordance with the manufacturer's data, under a system of maintenance approved by the Civil Aviation Authority (CAA). All scheduled maintenance in the company is controlled by a computerised database containing OSIP which then produces task cards with instructions for the work. For scheduled maintenance a work package is generated containing the data for the job which is then issued to the Licensed Aircraft Maintenance Engineer (LAME). The MM is normally the reference for unscheduled maintenance but work packages may be raised. In this case, a work package was not raised. Most of the regular overnight maintenance activity is carried out in Sydney.

Tamworth is retained as a subsidiary facility, available for overnight and longer-term or heavy maintenance. The company's Maintenance Control Manual (MCM) applies to both Sydney and Tamworth which thereby use the same method of controlling maintenance and issuing work instructions (ie: work packages). The maintenance system is audited by the CAA. The torque link change was carried out on the night shift when maintenance control staff in Sydney were not on duty. The LAME who carried out the work in Tamworth attempted to consult control staff to clarify the procedures applicable to the work but no staff were available. Consequently, as a work package had not been raised, the only reference available to the LAME was the unrevised MM.

FINDINGS

1. The WOW sensor was not adjusted following change of both upper torque links as this adjustment was not included in the procedures detailed in the MM.

2. The work was carried out on the night shift which prevented the LAME performing the tasks from consulting maintenance control staff.

3. The MM has since been amended to include a requirement to adjust the WOW sensor after a torque link change.

FACTORS

1. The instructions detailed in the MM were inadequate for the task.

2. No means of clarifying procedures was available.

SAFETY ACTION

The MM has been revised and the CAA requested the company to install an Australian Standard AS3901/3902 type quality assurance system. At the time of the occurrence the company had plans to introduce such a system and is currently implementing the plan.

 
General details
Date: 10 November 1993 Investigation status: Completed 
Time: 7:30 ESuT Investigation type: Occurrence Investigation 
 Occurrence type:Flight control systems 
 Occurrence class: Technical 
Release date: 20 December 1995 Occurrence category: Incident 
Report status: Final  
 
Aircraft details
Aircraft manufacturer: de Havilland Canada 
Aircraft model: DHC-8-102 
Aircraft registration: VH-TQO 
Sector: Turboprop 
Damage to aircraft: Nil 
Departure point:Tamworth NSW
Destination:Sydney NSW
 
 
 
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Last update 24 July 2015