As the aircraft was levelling at FL390 the left engine appeared to suffer a compressor stall. The crew reduced engine power to idle as a precaution. The engine was running normally at idle therefore a descent clearance was requested and the aircraft continued at a lower altitude to Melbourne for an uneventful landing.

The engine was removed after a borescope inspection revealed damage to the compressor. The engine was disassembled. The damage was found to have caused by the liberation of the inner shroud from one 8th stage compressor stator vane segment. Secondary damage had occurred to both the 7th and 8th stages of the compressor.

The liberation of compressor stator inner shrouds has previously been recorded by overseas operators and is under active investigation by the manufacturer.

FACTUAL INFORMATION

Sequence of Events

During a scheduled transit stop, the number 1 engine required three engine starts for the rectification of a minor oil leak and troubleshooting of other defects before the aircraft was subsequently released for service. Soon after departure, whilst climbing through 8,500 ft at 263 kts, the pilot in command noticed that the number 1 engine start valve OPEN light was illuminated and he requested the engine shutdown checklist. Almost immediately, whilst disconnecting the auto-throttle, there was a bang, a slight yaw to the left and vibration for a very brief duration. During the engine shutdown sequence, the engine fire detection light on the centre panel illuminated momentarily. After securing the engine, 30 tonnes of fuel was jettisoned before the aircraft returned to Sydney for an uneventful landing.

Damage to the Aircraft

Inspection on the ground revealed that only the top sections of the fan cowls, containing the hinge fittings, remained on the number 1 engine. The forward section of the pylon, and the wing leading edge outboard of the pylon, were holed and dented. The paint on the starter case was blistered and there were detached turbine blades in the starter exit screen.

Additional Information

After repairs to the aircraft were completed, a fuel leak was detected from a loose connection at the highest point in the engine environmental drain system. During engine run testing, the leak only became apparent after the third engine shutdown and resulted in the leaked fuel pooling on top of the engine gearbox, immediately above the starter.

ANALYSIS

Examination of the fan cowl latches revealed they had all failed in tension overload. This, together with the nature of the deformations found on the recovered cowl sections, indicated that an explosion had occurred within the cowled area in the vicinity of the starter. The explosion was of sufficient magnitude to deform and weaken the integrity of the cowling in the area of the starter pressure relief panel and to deflect it into the airflow. The high-speed airflow then tore the cowls from the engine. Damage to the pylon and wing leading edge resulted from collision with the separated cowl sections in the airflow.

It is considered likely that fuel had leaked from the environmental drain system as a result of the four engine shutdowns during the transit stopover prior to the flight. Fuel pooled on the gearbox above the starter and evaporated as the unvented under cowl temperature rose during the climb. When the start valve opened, the starter oversped and disintegrated, liberating hot debris in the immediate vicinity which ignited the explosive fuel/air mixture. The force of the explosion deformed the cowl to such an extent that the airstream tore the cowlings apart.

Investigation of the start valve and its associated wiring and control circuitry failed to determine a reason for the uncommanded opening. The aircraft maintenance records revealed a history of starter problems and failures involving uncommanded start valve openings in flight on both number 1 and 2 engines.

There is other documented evidence to show that uncommanded opening of the start valve in flight is not an uncommon event.

SIGNIFICANT FACTORS

The following significant factors were identified as contributing to the accident.

1. A fuel leak occurred from a loose connection in the environmental drain system of the number 1 engine which pooled in the area above the starter.
2. The fuel evaporated to form an explosive mixture in a contained, unvented area within the fan cowls during flight.
3. The engine start valve opened, uncommanded, in flight.
4. The starter oversped and disintegrated, having no engine load to contain its speed.
5. The hot liberated debris from the starter ignited the explosive fuel-air mixture.
6. The subsequent explosive force deformed the cowlings into the high-speed airstream, resulting in overload failure of the cowl latches and loss of the cowl panels.

SAFETY ACTION

As a result of the investigation the Bureau made Interim Recommendation 960026 to the Civil Aviation Safety Authority:

The Bureau of Air Safety Investigation recommends that the Civil Aviation Safety Authority bring this occurrence to the attention of the Federal Aviation Administration and the equipment manufacturers. In the light of the history of this problem, these organisations should consider further research into the possible causes of uncommanded start valve openings in flight.

Similarly, the development of an appropriate engineering modification to the relevant start valve should be considered. This would ensure that if a start valve opened in flight, and a starter overspeed occurred, the potential for an ignition source is minimised.

The Civil Aviation Safety Authority responded on 17 July 1996 as follows:

'I refer to Interim Recommendation IR960026 regarding the incident involving US registered Boeing 747 200, N942PR on 21 April 1995 which recommended that CASA notify both the Federal Aviation Administration and Boeing Aircraft Company of the incident.

I have attached copies of the Authority's correspondence with the FAA and Boeing for your information. In each case a copy of the Interim Recommendation was attached to the facsimile.

The Authority will keep you informed of any relevant responses from either the FAA or Boeing.'

The strip direction at Riddell is 15/33. The intention was to fly circuits. There was a northerly wind blowing so take-off was on the 330 degree strip. A storm was approaching from the south and during the circuit the pilot listened to the Essendon ATIS which indicated that the wind was going around to the south.

On final approach the pilot noted that the windsock was indicating that the wind had swung around the southwest and was about 12 to 15 knots. He had flown a fairly tight circuit. Carburettor heat was selected on base when power was reduced and deselected on final approach. The pilot estimated that carburettor heat was probably only on for about 10 seconds.

Approach to the 330 degree strip is over a gully. On short final the aircraft encountered some wind shear and began to sink below the glide path. The pilot pushed the throttle forward but there was no response from the engine. Further sink was encountered, and it became obvious that the aircraft was going to touch down before the airfield boundary fence. The aircraft touched down heavily, ran into the fence and slowly went over onto its back.

Post accident inspection of the engine did not reveal any mechanical reason for the lack of response to throttle application. Information from the Bureau of Meteorology showed that conditions were conducive to the formation of serious carburettor icing at any power setting. The pilot thought that because carby heat was only applied for about 10 seconds, carburettor ice was the only reasonable explanation for the loss of power.

The pilot reported that he and his 13 year old son were going for a short flight from Maryborough airport. All necessary preflight checks were carried out and sufficient fuel was on board for the intended flight. After take-off from runway 17 at between 100 and 200 ft the engine stopped completely. Witnesses saw the aircraft enter a left turn at a low height. The aircraft then appeared to stall and hit the ground inverted. The landing gear was in the retracted position.

The pilot stated that the engine failed because the magneto switches were bumped to the off position. Both switches are mounted close together in the centre of the instrument panel with other switches below them. The pilot stated that he was told in hospital that his son was waving to a person on the ground and had bumped the magneto switches off.

VH-NJG departed unrestricted on the Runway 19 Kilcoy Standard Instrument Departure (SID). The next departure off Runway 19 was VH-XFZ departing on a heading of 220 degrees. The departure restriction was to accommodate an aircraft arriving from the west for runway 14.

The Departures Controller intended to turn the trailing aircraft (VH-XFZ) further right shortly after contact was made. However, there was a Secondary Surveillance Radar (SSR) correlation problem with VH-XFZ in that its transponder did not operate until the crew recycled the selector. When VH-XFZ showed up on the SSR, it was less than 2 NM behind VH-NJG. However, the crew of VH-XFZ reported that they had VH-NJG in sight since take-off.

The Departures Controller had assumed that the ADC would provide separation of two minutes or 3 NM for departing aircraft if their tracks diverged by less than 30 degrees.

As the crew of the trailing aircraft had visual contact with the preceding aircraft throughout their take-off and initial climb, there was no risk of collision.

Analysis

Under the circumstances, the Departures Controller's operational technique did not provide separation assurance. His assumption that the ADC would apply separation was the more significant factor in the breakdown of separation.

Safety Result

Following the incident Local Operating Instructions were amended to define more clearly the responsibilities of both the ADC and Departure Controller in relation to initial departures.

On return from the training area the pilot advised that he had an unsafe indication from the nose landing gear. The tower confirmed that the nose gear was in the retracted position, so the pilot elected to return to the training area to attempt to rectify the problem. All attempts to lower the nose gear using normal and emergency systems were unsuccessful. Emergency services were placed on standby, and the aircraft subsequently landed with the nose gear retracted.

Investigation revealed that the nose gear door actuating system was worn excessively and out of rigging tolerance. This allowed the system to go to an over centre position and hold the doors in the closed position, thus preventing the nose gear from extending. The door actuating rods were incorrectly attached to the rear side of the door brackets, and the door hinges were stiff from lack of lubrication.

The pilot made a slightly heavier than normal landing. During the landing roll the nose gear down light extinguished, and the nose gear collapsed.

An investigation revealed the nose gear retraction ram mounting bracket had failed, allowing the nose gear to fold rearwards.

The aircraft had been involved in a similar occurrence 70 hours previously when the original factory installed aluminium mounting bracket failed. The bracket used to effect the repair was made from the same type of material, and had failed in a similar manner

Later models of this type of aircraft are fitted with a steel bracket.

Research of the Bureau's occurrences database, and contact with the manufacturer did not indicate this type of aluminium bracket was unduly prone to failure. Because the new stronger steel bracket is now available from the manufacturer no further safety action is considered necessary.

Approaching the Mangalore area the pilot commenced descent and had reached a height of about 1000 feet when the engine suddenly started to vibrate severely. When the throttle was closed the engine stopped completely. The pilot selected a nearby paddock and landed with a slight tailwind. During the ground roll the right wheel entered a rut, which dislodged the right main gear leg and caused the right wing and the propeller to contact the ground.

The reason for the engine vibration and failure has not been determined.

Significant Factors

The following factors were considered relevant to the development of the accident:

1. Engine power loss, reason undetermined.

2. Forced landing on unsuitable terrain.

The pilot intended to fly the balloon to Jacobs Well, due east of his take-off point at the Coomera Golf course. When the balloon rose to about 100 ft above ground level, he saw that extensive areas of fog covered the ground in the vicinity of Jacobs Well. He rejected the flight and landed back on the golf course in the middle of a fairway, only 70 m from his take-off point.

He called his ground crew assistant over to tow the balloon by a trailing rope to the edge of the fairway as he did not want to bring the recovery vehicle onto the fairway. His passenger had left the basket, but the ground crew member had difficulty moving the balloon. The pilot decided to swap places with her. The ground crew member operated the balloon's burners as required and the pilot towed the balloon to the side of the fairway. When the balloon touched trees, the gas was turned off at the tanks and the envelope deflated. As it did so, the basket rolled onto one side on sloping ground. The ground crew member struck her head on the burner controls lighting both burners for several seconds as the gas burnt out of the supply lines.

The ground crew member received burns to the upper part of her body, including inhalation damage to her oesophagus. The pilot received minor burns.

A witness reported that, after turning base for runway 12, the aircraft began oscillating in pitch as it continued a shallow descent. The aircraft crossed the extended centreline of the runway and struck a tree at the edge of a public car park. This caused the aircraft to break up, part of it falling on an unoccupied vehicle.

The pilot is reported to have indicated that the aircraft stalled and that he was unable to recover to normal flight before colliding with the tree.