On 1 December 2020, a Viking DHC6-300 (P2-ASM) being operated by Air Sanga on a non-scheduled VFR passenger and cargo flight from Wobagen to nearby Bak in day VMC attempted to begin the takeoff by setting full power and varying the asymmetric power settings to achieve directional control but this failed to prevent the aircraft veering off the runway and into an adjacent drainage ditch. The aircraft sustained substantial damage but only one of the six passengers sustained a minor injury.
The event was reported to the Papua New Guinea Accident Investigation Commission (AIC) by Air Sanga the same day and an Investigation was immediately commenced. The aircraft was not fitted with a crash protected FDR or required to be but relevant recorded flight data were obtained from the SSCVR and from an Appaero V1000 data recorder.
The pilots were both Papua New Guinea nationals and both were Captains on type. The designated aircraft commander, who had been PF for the flight, had a total of 6,748 hours flying experience which included 2,696 hours in command (all types) and 1,450 hours on type. The Co-pilot had a total of 14,865 hours flying experience which included 5,130 hours in command (all types) and 9,550 hours on type and was also a fully current DHC6 Training Captain.
Immediately before taxiing out after a 25 minute turnround following arrival from Kiunga, the crew called Moresby FIS on the radio and advised that they were amending their destination to Bak instead of their initially planned return to Kiunga. They had noted the wet surface of the runway during the earlier landing and stated that for this reason, they had decided to carry out the ‘Before Takeoff Checks’ at the parking area so as to avoid having to stop prior to beginning the takeoff roll. They added that they “had developed this procedure only for their experienced pilots to use for takeoff on unpaved slippery and boggy airstrips”.
The aircraft was then taxied to the threshold of the unpaved and nominally grass runway 11 (which had a 10% downslope) where a 180° left turn was performed using the tiller assisted by asymmetric thrust in order to line up on the runway centreline. During this turn, the PF began to progressively increase power in order to maintain enough momentum to avoid slipping from the top of the central camber of the runway or sinking into the clearly still wet surface. Once lined up, full power was set without stopping and the aircraft began to accelerate.
As the speed increased through 33 knots, the aircraft began to drift to the right and on recognising this, the PF attempted to “steer it back towards the centreline using the asymmetric thrust” but this had no effect and after travelling about 200 metres along the 525 metre-long 30 metre-wide runway and reaching a speed of just over 40 knots, the aircraft began to swerve towards the right. In response, the PF selected the power levers to idle and applied maximum braking but the slide sideways continued and a “soft patchy undulated surface” was encountered and the aircraft subsequently impacted a drainage ditch along the south-western edge of the airstrip.
Once the aircraft had come to a stop, the engines were shut down and the passengers opened the cabin door and exited the aircraft. Both pilots then exited through their respective flight deck external doors and joined the passengers. Damage to the aircraft included to the nose landing gear and flight deck forward bulkhead which were destroyed, the left wing which was severed at the wing root and the left wing tip.
An examination of recorded engine power settings found that once takeoff power was set, the left engine torque was 40 psi and the right engine torque was 24 psi against the maximum permitted torque of 50 psi. Engine temperature, oil temperature and pressure, fuel flow and propeller rpm were all correspondingly different. An exceedence of the right engine ITT (T5) also occurred but was not detected by the crew. The cargo was removed from the wreckage before the investigation team arrived but assuming the aircraft was loaded in accordance with the load and trim sheet, the takeoff weight was within the MTOW.
It was noted that although it had been dry when the accident occurred, it had rained raining earlier in the day prior to the arrival of the aircraft from Kiunga as well as during the previous day and the runway surface was wet.
The annotated ground track of the aircraft. [Reproduced from the Official Report]
Why It Happened
It was clear that the aircraft Operator did not have any documentation on standard characteristics and requirements for the aerodrome category applicable to Wobagen or any special procedures for its use. The crew “stated that they operated into Wobagen based on their experience and knowledge”. The Operator advised that the only information for Wobagen was contained in their ‘Route Guide and Training Manual’ which was found to describe the runway surface as “grassed grey gravel”. An inspection of this surface by the Investigation concluded that it was subject to “undulation” throughout and that although the surface was “short grass”, the surface either side of the centreline was “patchy grass and bare”. No evidence was found that the aircraft operator had carried out a Hazard Identification and Risk Assessment of Wobagen prior to beginning operations there.
It was clear that neither the asymmetric thrust maintained during the takeoff roll nor the exceedance of the right engine ITT limit until the time the accident occurred had been noticed by either pilot with the PM notably stating that he had also been looking outside during the take-off roll. It was also found that the ‘custom and practice’ procedure used by the crew for takeoff was “an unapproved and undocumented procedure” which was significantly different from the procedure contained in the AFM. This particularly applied to the failure of either pilot to monitor the engine parameters when power was applied for takeoff. It was also found that the power lever advance “was relatively rapid” and had caused an abrupt increase in power and although the AFM required that all takeoffs must use full takeoff power, there had been no attempt by the crew to moderate the increase in view of the evident asymmetry and wet runway conditions. Whilst the right engine was not recovered from the wreckage, which meant that the reason for the ITT exceedence could not be conclusively determined, it was concluded that “actions contrary to those specified in the AFM takeoff procedure can be considered a probable factor”.
Whilst the continued use of asymmetric thrust as a means of maintaining directional control was the reason the aircraft eventually started to drift to the right, once the aircraft had done so and left the hardened surface around the runway centreline and encountered the “wet and slippery” clay/silt surface which existed either side of it, manoeuvrability was significantly reduced. The combination of the momentum of the aircraft and the slippery surface led to it continuing past the boundary markers and into the drainage ditch.
The Conclusions of the Investigation were, in summary, as follows:
- No evidence was found to indicate that the engines were not operating normally.
- An unapproved procedure for take-off on unsealed wet runways which was contrary to the procedure in the AFM was used.
- The crew failed to monitor engine parameters during the takeoff.
- Both pilots were qualified and experienced in operations requiring the use of unsealed surface remote highland runways.
- Air Sanga had not risk assessed hazards associated with Wobagen although the crew were aware of a number of such hazards.
- The Air Sanga Route Guide was outdated and did not contain current relevant information for Wobagen.
The Contributing Factors to the accident were summarised as follows:
The crew did not action the appropriate take-off checklist which caused them to miss crucial checks and actions. This caused the engine abnormalities to go unnoticed after take-off power was applied. The abnormal engine parameters remained unnoticed until impact. The indications of the right engine indicated that it was performing at considerably lower power than the left engine, which was operating to the manufacturer’s specifications. This power difference between the right and left engine was what created the tendency of the aircraft to veer right and depart the runway.
This tendency was aggravated by the wet and deteriorated clay/silt surface which did not allow aircraft tyres to gain sufficient traction to follow the control inputs made as asymmetric power was used to try and steer the aircraft back towards the centreline. As the aircraft accelerated towards the right, it ran over an undulating surface and continued veering right because power reduction was unduly delayed and the asymmetric effect continued. Eventually, the PF retarded the power levers to flight idle and applied full braking but this occurred too late to prevent the aircraft from continuing over the slippery surface and reaching the drainage ditch.
The Final Report was approved on 13 May 2022 and subsequently released. No Safety Recommendations were made.