Sanga DHC6 at Wobagen on Dec 1st 2020, runway excursion during rejected takeoff
Last Update: May 27, 2022 / 15:29:23 GMT/Zulu time
Incident Facts
Date of incident
Dec 1, 2020
Classification
Accident
Airline
Air Sanga
Departure
Wobagen, Papua New Guinea
Destination
Bak, Papua New Guinea
Aircraft Registration
P2-ASM
Aircraft Type
De Havilland DHC-6 Twin Otter
ICAO Type Designator
DHC6
Papua New Guinea's AIC reported: "Information provided to the AIC indicates that as a result of the occurrence, a child resulted injured and the aircraft sustained damage. The child was later airlifted to a hospital in Kiunga to receive medical attention." The AIC was informed the same day and opened an investigation.
On Dec 30th 2020 PNG's AIC released their preliminary report reporting the aircraft was flown by the captain (48, ATPL, 6,748 hours total, 1,450 hours on type) sa pilot flying and first officer (51, ATPL, 14,856 hours total, 9,550 hours on type) as pilot monitoring. The AIC summarized the sequence of events:
Data obtained from the aircraft recording systems showed that the crew completed the ‘Before Take-off’ checklist at the parking area and then began taxiing west towards the threshold of strip 12.
At about 09:13, upon reaching the threshold of strip 12, the PIC conducted a 180° turn and applied maximum power, transitioning from taxi into take-off roll.
During an interview with the AIC, the crew stated that after applying maximum power, the aircraft had a tendency to veer to the right. The PIC tried to maintain the aircraft on centreline by using asymmetric thrust, however, as the aircraft accelerated down the strip, the tendency to veer right increased. About 154m down the strip, the aircraft began veering towards the right and tracked away from the centreline (see Figure 2). As the aircraft passed the strip boundary (cone markers), with a recorded groundspeed of 40kt the crew aborted the take-off; power to idle and applied full brakes. The aircraft continued forward and veering right. Four seconds later, the aircraft impacted a drain.
The nosewheel subsequently separated from the fork causing the nose to drop and the nose gear subsequently collapsed. The momentum of the aircraft caused it to continue another 8m with the nose dragging across the ground before it impacted the right edge drain.
The crew stated during interview that after the aircraft came to rest, the flight crew carried out the procedure for shutting the engines down. In the meantime, the passengers opened the door of the cabin and exited the aircraft. Both pilots exited through their respective doors. One passenger reported to have suffered minor injuries.
In May 2022 the AIC released their final report concluding the probable causes of the accident were:
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 parameters engine parameters remained unnoticed until impact. The parameters of right engine indicate that it was performing at considerably lower power than the left engine, which was operating to the manufacturer’s specifications. The power difference between the right and left engine created the tendency of the aircraft to veer right. The right engine T5 Turbine temperature exceeded the limit which shows that the overtemperature condition was sustained by the right engine.
The wet deteriorated clay/silt surface did not allow aircrafts tires to gain sufficient traction to follow the control inputs of the PIC as he attempted to steer the aircraft back towards the centreline. As the aircraft accelerated towards the right, it ran over undulated surface. The aircraft continued veering right because the power was not reduced, and the asymmetric effect continued. With the asymmetric thrust, the undulated slippery surface, the aircraft swerved right. This prompted the PIC to pull power into idle and apply full brakes. After pulling power to idle, and applying full brakes, the aircraft continued with momentum over the significantly slippery surface and impacted the drainage ditch at the edge.
The AIC analysed:
AIC determined from the available evidence that there was asymmetric thrust sustained during the take-off roll immediately following take-off power selection. There was a significant variation between engine parameter readings of the left and right engine. It was apparent that neither crew member noticed the variations, including the right engine exceedance of the intake turbine temperature limits for after selecting take-off power until the time the accident occurred.
During the PIC’s interview, he confirmed that the aircraft was deviating right of his intended take-off track, against his steering and control inputs. However, he was not able to identify what was causing this deviation. Furthermore, the copilot, during his interview, could not explain what had caused the uncommanded deviation. He confirmed that he was looking outside during the take-off roll.
The investigation determined that the procedure used by the crew for take-off was an unapproved and undocumented procedure. The crew did action the procedure according to the Appereo data, however, the procedure did not include certain key considerations and from the standard approved Aircraft Flight Manual. The requirement for the crew to observe engine parameters when applying power for take-off, was the appropriate steps should have been taken. The crew, especially the PIC, would not have been able to carry out this observation effectively as their procedure is meant to transition directly from taxi to the takeoff roll. The investigation determined that the PIC’s advancement of the throttles was relatively rapid and the engine parameters abruptly increased.
Although the Manufacturer’s AFM Take-Off procedure contains a warning requiring that all take-off’s be conducted with full take-off power, the Crew custom procedure did not incorporate a check for power correspondence. The AFM further adds that a 5 s delay is mandatory at 85% Ng for engine parameters to settle before take-off power can be applied. The crew did not conduct this step to ensure power must be present for take-off and also the crew customized procedure did not include this requirement. The crew must check that full take-off power has been achieved by delaying to allow engine parameters to settle was not catered for as the crew had planned to begin the take-off, transitioning directly from taxi. The right engine’s exceedance of the T5 temperature limits could not be determined as the engine was not recovered and inaccessible. However, the AIC considered that actions contrary to those specified in the AFM Take-Off procedure can be considered a probable factor to the abnormal parameters sustained by the right engine.
Asymmetric thrust
The AIC, through the Appareo 1000 video data, observed a disparity between the Propeller RPM gauges for both left and right engine. The left propeller RPM, during the take-off roll was about 5 to as high as 10% higher than the right propeller RPM. This indicates that there was asymmetric thrust sustained during the take-off roll causing a righthand veering tendency. The AIC believes that the asymmetric thrust sustained would not be manageable over normal paved or dry surface conditions. However, because the aircraft was accelerating down a wet slippery clay/silt surface, the maneuverability was reduced significantly. Furthermore, as the aircraft diverged further from the center path, it signified that there was thrust asymmetry, which is likely to have contributed to the aircrafts tendency to veer right. The video also shows that the thrust asymmetry was not corrected in time to maintain the aircrafts intended center path.
Because the aircraft had veered off the hardened surface around the narrow center take-off path onto the wet and slippery clay/silt surface right of the centre path of the airstrip, the maneuverability was significantly reduced. The momentum of the aircraft and the lack of traction of due to the slippery surface caused the aircraft to continue past the boundary markers and into the drainage ditch.
The pilot records showed that the crew had significant experience operating into airstrips. The crew confirmed that they had operated on airstrips with similar conditions many times using the taxi turn transition technique. The video data shows that they had briefed on the procedure prior to take-off. As this was a take-off the PIC kept his eyes outside as is the appropriate practice. However, the monitoring pilot was also looking outside and did not notice the RPM.
The investigation parameters were for the engines reflected the engine control settings by the PIC. It was therefore deduced that the thrust asymmetry was due to thrust lever positions set by the PIC.
From the time between the take-off and accident, the AIC believes, the PIC did not have sufficient time to identify the cause of the aircrafts tendency to veer towards the right.
Airstrip Conditions
The investigation identified certain obvious hazards during its visit to the accident site. During interview with the crew, the investigators also understood that the crew were aware of some of those significant hazards, and through experience, took certain steps to mitigate the associated risks.
Due to unpredictable weather conditions at Wobagen Airstrip, the important weather and airstrip condition information from source at Wabogen was vital. However, that information was not gathered on the day of the accident. The investigation also identified that having a reliable source at each strip providing reliable information would be beneficial to the crew, to fully aware them of risks at a particular strip and how to mitigate those risks.
Hazard Identification
Although not directly causal to the accident, the AIC found that a risk assessment was not carried out at Wabogen Airstrip before operating there. According to the investigation, the last flight into Wabogen was a month prior to the accident so conditions of the strip was unknown at the time of the occurrence.
Although the Operator had established Hazard Identification and Risk Management procedures in its SMS Manual, the operator did not identify hazards at Wobagen Airstrip and associated risks to properly develop special or specific procedures for Wabogen Airstrip according to the data gathered from the risk assessment. This would have ensured the crew were trained in these special procedures to avoid accident
Take-off technique
The take-off procedure described by the crew could not be found in the aircraft manufacturer flight manual or other operational manuals. The crew also confirmed that they did not have the procedure documented. However, they trained and permitted PNG airstrip experienced pilots to conduct this procedure.
Understanding the challenges posed by airstrips in Papua New Guinea, and the number of unconventional techniques that pilots operating into those airstrips use to allow services, it is still important to develop procedure and/ or deviations from procedures and submit to the appropriate authorities any deviation from standard operating procedures from approved procedure developed or procedures must be by Operators for only used by highly experienced pilots. They confirmed that they also did not have a written procedure for the take-off procedure.’
Incident Facts
Date of incident
Dec 1, 2020
Classification
Accident
Airline
Air Sanga
Departure
Wobagen, Papua New Guinea
Destination
Bak, Papua New Guinea
Aircraft Registration
P2-ASM
Aircraft Type
De Havilland DHC-6 Twin Otter
ICAO Type Designator
DHC6
This article is published under license from Avherald.com. © of text by Avherald.com.
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