On 4 August 2017, a de Havilland DHC8-200 (P2-ANK) being operated by Air Niugini subsidiary Link PNG on a scheduled passenger flight from Port Moresby to Tari as flight PX 713 was en route and climbing through 20,000 feet in day VMC when there was what was described as a loud bang and the aircraft shuddered. They were initially uncertain of the origin of this but having observed that indications for both engines were normal, they then saw that an unsafe left main gear door caution had appeared and had been confirmed open by the cabin crew. The decision to return to Port Moresby was advised to ATC with the qualification that a normal approach and landing was expected. Prior to the approach and an uneventful landing, the aircraft entered a holding pattern to reduce the aircraft weight to MLW. Only on the ground was it realised that panels were missing from the left engine exhaust nacelle and that the inner tyre on the left main gear was deflated and damaged and deduced that the tyre had exploded in flight.
The event was reported to the Papua New Guinea Accident Investigation Commission (AIC) by Air Niugini the same day but they “did not reveal the full extent of aircraft damage”. Further enquiries by the AIC two days later established that the aircraft had sustained significant damage and that panels had been lost at sea and an Investigation was immediately commenced. Recorded data from the FDR and CVR were successfully downloaded.
It was noted that the Captain, who had been PF for the flight, had 24,381 hours total flying time including 5,217 hours on type and the First Officer had 1,257 hours total flying time, all but 242 of them on type.
It was found that after the explosion, the crew had reduced airspeed to not more than 130 knots and descended to remain below 15,000 feet in accordance with the applicable QRH drill for open main gear doors. A full examination of the consequences of the tyre explosion within the closed wheel well disclosed that some panels had been lost from the engine at the time of the explosion and that the nacelle area had been extensively damaged as a result of the tyre exploding up through the top of it (see the illustration below). The inner left main wheel was disassembled and the tyre removed and sent to the tyre manufacturer in Thailand for further examination to determine the cause of the failure. The NDT of the wheel hub found that it was undamaged.
A detailed examination of the tyre found no sign of any manufacturing anomaly but the nature of the damage provided clear evidence of hard impact damage attributable to contact with FOD at specific locations on the outer side wall of the tyre. It was considered that the casing plies in the impacted area of the affected tyre had fractured after departure from Port Moresby and the fracture had then propagated from the outer-facing tyre sidewall along the cord direction to the tread area and then to the inner-facing sidewall. The significantly damaged and weakened tyre casing and the increasing pressure differential between the inside of the tyre and the steadily reducing atmospheric pressure had then caused the tyre to blow out explosively.
The damaged area of the left engine nacelle. [Reproduced from the Official Report]
The Investigation noted that the tyre had exploded during the second sector of the day and that the aircraft had arrived at Port Moresby from Bulolo where the aircraft had night-stopped. Prior to departure from Port Moresby, the aircraft had been inspected and released to service by the duty licensed engineer and an external inspection had been performed by the flight crew. Both inspections required examination of the external condition of the wheels and tyres to confirm their airworthiness.
Given the finding that FOD had been the cause of loss of tyre integrity and that the Port Moresby runway had been routinely subjected to inspection during the morning of the takeoff including soon after it, attention had turned to the 1460 metre-long gravel surface strip at Bulolo. A detailed inspection of the condition of this strip six days after the event found that just under a year earlier, a major part but not all of the runway had been compacted with coarse gravel, but that since then, continuous exposure to the weather elements had led to the erosion of the gravel surface which had exposed “large embedded stones at several locations along the runway”. In several places, depressions in the runway surface were also evident.
Jagged rocks found during inspection of the Bulolo airstrip on 10 August 2017. [Reproduced from the Official Report]
It was noted that the Bulolo aerodrome was operated under a 5 year licence which was due to expire in June 2018. Under this arrangement, the Licensor retained responsibility for administrative matters and “support” of the Licensee and was expected to “maintain and comply with all relevant safety regulations under Part 139 of the Civil Aviation Safety Authority (CASA) Civil Aviation Rules (CARs)”. For its part, the Licensee was obliged to carry out and pay for maintenance of the airport facility and its runway and to carry out and complete “specified airport works”. The latter had included the removal of vegetation identified (by the Licensee or CASA) as hazardous or potentially hazardous to aircraft using the airport and in particular “grading and compacting of the runway surface” so that it was “maintained to a standard in accordance with CASA requirements and the CARs”.
It was determined that:
- the FOD impact damage which had weakened the left inner main wheel tyre occurred at Bulolo.
- it is likely that this FOD damage occurred when a jagged rock impacted the non-serial sidewall of the No 2 tyre during the landing at Bulolo on 3 August 2017.
- the Bulolo airport licensee had neglected its primary responsibility of properly maintaining the airstrip to provide for safe landings and takeoffs of aircraft, in particular those aircraft operated by Link PNG aircraft.
Safety Action taken during the Investigation and known to it was noted to have included the non-renewal by the CASA of the Aerodrome Operating Certificate required under Part 139 of the CARs following its lapse soon after the investigated event.
The Final Report was approved on 4 January 2018 and subsequently published. No Safety Recommendations were made.