The left engine suffered a 5 second power interruption while the aircraft was cruising in moderately heavy rain. Following the power interruption the engine continued to operate normally. This was one of a series of similar events that led to the manufacturer undertaking instrumented and filmed flight tests. These disclosed that ice was building on the engine inlet and when released was being ingested into the engine.

Further testing found that the inlet heating cycle was being interrupted by electro-magnetic interference in the de-icing controller. This allowed ice to build up. Two modifications were introduced. SB F50-71-037 rerouted the bonding cable for the oil cooling flap actuator to reduce EMF transmission and SB F50-71-041 enlarged the lower cowl bypass duct to allow any displaced ice to eject overboard rather than through the engine. The operator has embodied both modifications and to date has not had a recurrence of power interruptions.

The pilot landed on the bush strip after a short local flight. After the aircraft had touched down and was decelerating along the strip, its nosewheel struck a small hole and a rock, causing the wheel to break up and the nosewheel support structure to collapse.

The pilot was conducting a test flight as part of the certification process required by the Civil Aviation Authority. Positive gear down and locked indications were obtained during the downwind leg of the circuit. As the aircraft touched down the pilot detected a settling of the right main landing gear and elected to go around. Inspection from the ground confirmed that there was a fault with the right landing gear. The pilot adjusted his approach so that the right gear touched down at a low speed. The gear leg collapsed on touch down and the aircraft slewed off the runway. It has not been determined why the landing gear collapsed.

The aircraft was engaged in spraying cotton crops. At the commencement of the third run, the aircraft contacted the ground with the main landing gear and bounced back into the air. The spray run was completed after which the pilot was advised by his marker that the landing gear appeared to be damaged. This damage was confirmed by the pilot of another company aircraft, which was operating in the near vicinity.

The pilot continued with the treatment of the cotton crop until his load of chemical was exhausted, upon which the aircraft was flown to Narromine where, after a further assessment of the damage by a ground observer, emergency services were called out for the landing. During the subsequent landing on a grass strip, both main landing gear legs collapsed.

The aircraft was in cruise at an altitude of 8,000 ft. A sudden severe shower of hail was encountered, lasting about 5 seconds. Hail impact damage was sustained by the aircraft which continued the flight to Moorabbin.

The helicopter was being used for yarding cattle, operating in a gusty 20-25 knot wind, when the pilot inadvertently turned downwind. As the helicopter began to sink the pilot made every attempt to correct the situation but was unable to regain control at the low operating height. The helicopter struck the ground with the front of the left hand skid, cartwheeled and came to a stop lying on its right hand side. The main rotor was probably in an over-pitched condition prior to impact. The pilot, who was the only occupant and wearing a flying helmet, was uninjured.

During a crop spraying operation, the pilot noticed oil on the windscreen. Because the amount of oil was increasing, he decided to proceed to a private airstrip to make a precautionary landing. As the aircraft climbed away from the cotton field, smoke was observed issuing from the engine cowl. The pilot advised his markers he would attempt an emergency landing at Cudgewa airstrip. Shortly after there was a loud noise as the propeller separated from the engine.

The aircraft was landed in a wheat field but overturned during the landing roll due to the soft surface. The hopper contained 150 litres of chemical at the time of the accident, some of which splashed over the pilot. The propeller was found to have separated from the engine drive shaft after the retaining nut which secured the propeller to the shaft had come off. The bolt designed to secure the retaining nut on the propeller shaft was missing for reasons which were not determined.

The pilot was returning to Mackay after a business flight to Brampton Island. During the pre-flight checks at Mackay there had been an excessive drop in engine revolutions during the magneto checks, but this had cleared. A similar check conducted by the pilot when taxiing at Brampton Island indicated satisfactory magneto operation.

During the take-off run, just before lift-off, the right engine surged. The pilot rejected the take-off. The rate of deceleration was reduced when the pilot used excessive braking, causing the mainwheels to lock. The aircraft overran the runway and was damaged when it crossed a concrete drain.

The pilot reported that the fuel flow indication had surged during the take-off roll but was unable to recall whether the surge was an increase or a decrease. Spark plugs and engine fuel system components were examined. The condition of the spark plugs removed from the no.6 cylinder indicated that the fuel mixture to that cylinder was excessively rich. However, the reason for the engine surge was not determined.

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 take-off 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 - i.e., 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 (i.e.: 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.

After the flight, the captain reported that he momentarily lapsed into sleep during approach to runway 05 at Adelaide. A subsequent investigation revealed that the captain, who was a dedicated night freight pilot, was suffering from a sleep disorder. He had not been subjected to excessive duty periods. He was relieved from flying duties pending a medical assessment.