MASWings DHC6 at Long Seridan on May 9th 2020, runway excursion on landing
Last Update: September 17, 2021 / 18:13:53 GMT/Zulu time
The airline reported: "MASWings confirms that flight MH3622 from Marudi to Long Seridan skidded off the runway during landing roll at 10:05L (02:05Z) at Long Seridan Airport. The flight was being operated by a Twin Otter (DHC6). Two passengers and two crew disembarked safely and no injuries were reported."
Malaysia's Transport Minister reported the aircraft skidded along the wet runway at 10:02L (02:02Z) following rain in the morning.
No Metars are available, the local weather station reported light rain, 908 hPa, 22 degrees C, dew point 21 degrees C, winds calm at 08:00L. At 11:00L the local weather station reported mostly cloudy, 908 hPa, 24 degrees C, dew point 22 degrees, westerly winds at 4 knots.
Malaysia's AAIB released their final report concluding the probable causes of the serious incident were:
The serious incident was caused by the aircraft wheels hydroplaning during landing on the wet and poorly maintained runway pavement. The poorly maintained runway pavement over a period of time resulted in poor surface friction qualities. The poor mean texture depth of the aggregate and the various type of pavement distresses occurring had drastically reduced the skid resistance characteristics of the runway pavement and the braking efficiency the aircraft. The braking actions by the pilot to maintain directional control when the wheels hydroplane further aggravated the situation.
The contributing factors to this incident are poor runway maintenance practices by the aerodrome operator and pilot not adhering to the braking technique as stated in the SOP DHC6 for adverse weather operations.
The AAIB summarized the sequence of events:
The departure from Marudi to Long Seridan was delayed due to low clouds and light rain in Long Seridan. The aircraft finally departed from Marudi at 0934hrs, 24 minutes behind schedule after they received a report that the weather had improved in Long Seridan with visibility reported to be 6 to 8km with light and variable winds. When the aircraft was visual with Long Seridan they were advised by the operations assistant that it had started drizzling again. However, the drizzle had stopped by the time they were on final approach. The aircraft approached Long Seridan Runway 22 normally and was reported to have been stabilized by 500ft AGL with all checklists completed. This is in keeping with the 300ft stabilization height due to the offset approach to Long Seridan. The flap 37 degrees setting was used with the ‘Vref’ being set to 67kts for the landing weight of 9,863lbs.
The crew was advised by the operations assistant that the visibility was 6 to 10km with light and variable winds with cloud base of 2,500ft during approach. The pilot observed that the runway was damp but not wet. However, the pilot had a limited time to assess the runway surface condition as the aircraft only intercepts the extended centreline at 300ft AGL due to the off-set approach.
The aircraft landed in the normal touchdown zone. At touchdown, the aircraft landed on its main wheels first. After the nose wheel was lowered, the pilot attempted to apply brakes and reversers, however the aircraft veered to the left of the runway while rotating to the starboard, finally impacting an embankment tail-first. It had rotated almost 180 degrees from its original landing path.
The point of touchdown was unable to be determined as there were many other tyre marks left by other aircraft. The most prominent tyre marks were from the starboard main wheel and start at 275m from the start of runway 22. The length of the starboard tyre mark is 167m. The port main wheel tyre mark and nose wheel tyre marks are very faint in comparison, with the port main wheel tyre mark only appearing very late and only for a short distance on the tarmac before disappearing. The port main wheel tyre track mark only reappears later on the grass. The aircraft came to rest approximately 108m from the end of the Runway 22 (440m from start of runway 22).
After the impact the pilots were pinned to their seats by the control column due to the damage sustained on the elevator. The RELA team responded very quickly and managed to evacuate the 2 passengers from the aircraft. During passenger evacuation, it started to rain. They were taken to the terminal by RELA motorcycle.
Both pilots evacuated the aircraft on their own by moving their seats. All 4 (2 pilots, 2 passengers) were taken to the local clinic for a medical check-up. All sustained no physical injuries.
The AAIB analysed:
Preliminary investigation from QAR data did not reveal any abnormalities on the aircraft performance and data shows the pilot approach was stabile till touchdown. Aircraft was rolling straight for about 4 seconds before it started to veered left and subsequently exited the left side of the runway. QAR does not provide data for the aircraft critical system for this investigation ie the brakes and nose-wheel steering system as the aircraft design does not incorporate sensor to these systems.
CVR did not reveal any abnormalities throughout the flight. On approach to Long Seridan, the pilots acknowledge visual with the airfield but low clouds and marginal weather on final. Prior to the flight to Long Seridan, the aircraft had delayed departure at Marudi due to marginal weather in Long Seridan. It was report by the Long Seridan operations assistant that there were intermittent rain and drizzle in the morning prior to the aircraft arrival. Pilot reported runway was damp during landing with wind light and variable.
From on-site preliminary investigation, there were prominent tyre track marks forward of the touchdown area especially for the starboard main wheel which was light initially and became darker till the aircraft exited the runway consistent with heavy braking to steer the aircraft back to centreline. The port main wheel tyre track marks were clear and light but disappear as the marks progress closer to the left edge of the runway consistent with aircraft port tyre skidding initially and regaining traction later before the aircraft exited the runway. The nose wheel tyre marks were clear and light till the aircraft exited the runway with no evidence of scalloping marks which will indicate a nosewheel off-centre (cock) landing. The nosewheel tyre marks indication are consistent with tyre skidding and the loss of traction to steer the aircraft.
On-site investigation also observed that the runway pavement had deteriorated over time and was in poor condition. Preliminary evidence gathered from the QAR, CVR, on-site tyre marks, the runway pavement and the wet landing conditions showed that the aircraft had most probably encountered dynamic hydroplaning which cause it to lose traction and braking efficiency resulting in the incident. To provide evidence that hydroplaning had occurred, AAIB had engaged the Faculty of Engineering UPNM to conduct site investigation on Long Seridan runway which consists of site reconnaissance survey, skid resistance tests and field survey works. The site investigation will provide data and evidence that relevant conditions are met for hydroplaning to occur in this incident.
The wetness on the runway had met the criteria for thickness of water film of 2.5 mm and above or thickness of water film equals (or larger) than the tyre tread depth for hydroplaning to occur. Measurement on the port main wheel tyre shows a thread depth of 4mm. Photographic evidence of the runway condition immediately after the incident shows that this criterion had most probably been met as the weather condition were raining or drizzling intermittently in the morning till the aircraft arrived at Long Seridan.
The site investigation provided evidence that the runway pavement condition had deteriorated over time due to poor maintenance practices. The runway is suffering from various types of distress which had contributed to the poor surface friction qualities. Poor pavement condition had also contributed to the problem of rapid drainage of run-off water from the runway during rain.
To provide evidence that the runway surface friction qualities are poor, skid resistance tests were conducted on Long Seridan runway pavement surface. Data from the test clearly shows that the skid number for various critical phase of flight at various speed measured is 31 and below which corresponded to a pavement mean texture depth of 0.01 inches and below which is below the minimum requirement as stated in ICAO Annex 14 Volume 1. The data also provide evidence that the aircraft is exposed to the risk of skidding even on a dry surface as the skid resistance data results show only minor differences between wet and dry surface readings. These data conclusively show that the runway pavement at Long Seridan had poor surface friction qualities which equates to low skid resistance. The poor runway maintenance practices were the main cause to the above pavement condition problems and was a contributing factor to this incident.
With the physical evidence of the tyre track marks, evidence of pavement distress and analysis of skid resistance test, investigation needs to determine if the touchdown speed of the aircraft was within hydroplaning region to conclude that dynamic hydroplaning had occurred in this incident. Dynamic hydroplaning is subjected to a minimum thickness water film of 2.5 mm on the runway and the aircraft tyre pressure. Analysis of the data shows that the speed of the aircraft when it started to skid (44kts) was in the hydroplaning speed region.
Finally, from all the evidence adduced and to conclusively proof that dynamic hydroplaning had occurred in this incident, the investigation analysed the tyre track marks of the aircraft by combining it with the chronology of events from the QAR data.
The chronology of events matches the tyre track marks that hydroplaning had occurred in this incident.
In conclusion, the poor maintenance practice by the aerodrome operator over a period of time had led to the deteriorating condition of Long Seridan runway pavement. The provision of suitable pavement texture and good surface drainage are the essential requirements to minimize the risk of hydroplaning and to enhance generally the wet surface friction qualities. The pilot incorrect braking input to maintain directional control and stop the aircraft further aggravated the hydroplaning situation. These two factors had contributed to this incident.
In summary, the current Long Seridan poor runway condition is a potential hazard to landing aircraft both in dry and wet conditions. The short-term solution is to temporary prohibit the landing of Twin Otter DHC6 aircraft at Long Seridan in wet conditions until the aerodrome operator completes rehabilitation work on the runway.
The mid-term solution is to pursue for a budget allocation from the Ministry of Transport to upgrade/extend the runway from 548m to 990m to cater for the safe operations of Twin Otter DHC6 aircraft at Long Seridan airfield.
Based on the analysis of physical evidence, QAR and CVR data, evidence and data from site investigation conducted by UPNM researchers, it is concluded that the aircraft experience dynamic hydroplaning when landing on the wet runway surface at Long Seridan which caused it to skid off the runway.
This article is published under license from Avherald.com. © of text by Avherald.com.
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