On 7 January 2020, a DHC 6-400 Twin Otter (9M-SSE) being operated by MASwings on a scheduled domestic passenger flight from Lawas to Miri as MH3517 in day VMC with light and variable surface wind reported departed the left side of runway 02 soon after touchdown and came to a stop approximately 40 metres from the edge of the runway. None of the 16 occupants were injured and the aircraft structure was undamaged although once excavated from the mud, the nose wheel was found to have “heavy spot wear and some heavy wear marks” consistent with marks on the runway.
The accident aircraft where it stopped after leaving the runway. [Reproduced from the Official Report]
The Air Accident Investigation Bureau, Malaysia (AAIB) was notified of the Accident by the aircraft operator and AAIB Inspectors were dispatched to Miri the following day to begin an Investigation. The CVR was removed and its data successfully downloaded and relevant QAR data was also available.
The 30 year-old Captain, who was acting as PF for the landing involved, held an ATPL (airline transport pilot licence) and had a total of 4,351 hours flying experience including 98 hours on type. He had recently joined the operator and, after training and assessment, had been released for unsupervised line flying after passing a Line Check just over a month earlier. The 30 year-old First Officer held a CPL (commercial pilot licence) and 807 hours flying experience including 633 hours on type.
It was established that the visual approach to the 2,745 metre-long 60 metre-wide runway at Miri had been conducted normally and had been stabilised. However, CVR data showed that Normal Checklists were not carried out in accordance with operator’s DHC-6 Series 400 SOP and it was not clear whether the electronic or paper Checklists were being used. It was also found from these data that the Captain had not called for checklists and that the PM had completed them by reading “in a mumbling tone which was barely audible” and that the Take-off and Departure brief were not recorded and the Approach brief was not carried out in accordance with SOP. It was also noted from the CVR data that in the absence of any SOP guidance to pilots in respect of the need for “sterile cockpit procedures” there had been “unnecessary chattering between the pilots when aircraft was on final”.
Touchdown on the main landing gear was normal but after the nose gear had been lowered to the runway, the aircraft soon began to veer towards the left side of the runway. It continued on the runway for 175 metres whilst following a slight curve towards the left edge of the runway which it then exited before continuing on the grass for another 84 metres which brought the total distance travelled in a continuous curve to 259 metres. By the time it stopped, the aircraft was approximately 40 metres from the left hand edge (i.e. 70 metres from the runway centreline).
The Captain reported having initially applied full rudder and brake to counteract the veering but this had no effect. Nearing the runway edge and “upon glancing down”, he stated that he had noticed that the nose wheel steering lever was fully deflected to the left. He then tried to re-centre the lever but by then the aircraft was already on the grass and soon stopped completely in the soft ground. He then shut down and secured the engines before coordinating with the TWR controller for the emergency services and evacuated the passengers who were then taken to the terminal.
Why it happened
Testing of the steering actuator confirmed that it had been fully serviceable. Touchdown occurred with the nosewheel steering not centred. It was noted that the paper checklist used by the pilots in preference to the electronic alternative had not been updated to reflect temporary amendments TA-31 & TA-32 issued in October 2017 which made changes affecting the Nose Wheel Steering Lever (see the illustration below) in, respectively, the After Takeoff and Descent/Approach Checks required the pilot to “apply slight upward and downward pressure to the nose wheel steering lever to confirm that the nose wheel is centred and locked” which was slightly different from the previous confirmation procedure which was to “wiggle” the steering lever to the centre and align it with the index mark.
The Nosewheel Steering Lever. [Reproduced from the Official Report]
The Investigation found that during both type conversion and recurrent training, only a brief explanation was provided to pilots on the nose wheel steering system with “insufficient emphasis” placed on explaining its working principles and the operation, in particular in respect of the nose wheel centring mechanism which had clearly set up the scenario which had led to the runway excursion being investigated.
More generally, it was considered that the context of poor routine operating practices detected from the CVR data had their origin in the fact that the POH/AFM states that the aircraft can be operated by a single pilot and that this has resulted in procedures which are predicated on such operations and have not been adapted for the two pilot operation used by MAS Wings.
A number of previous actual or almost DHC6 veer-off events were found to have occurred at the operator including two left side veer offs, one in 2015 and another in 2016 and several other incidents where unanticipated veering also occurred but recovery was achieved before the aircraft departed the runway.
In summary, the Conclusion of the Investigation was that poor Checklist use in confirming the centred status of the nosewheel steering lever, especially but not only after takeoff were the direct cause of the failure to centre the nosewheel steering after which the veer off after touchdown was all but inevitable. However, the context for this was considered to have been inadequate emphasis on nose wheel steering system knowledge during the Captain’s recent type conversion training including the lack of practical explanation on the function and operation of the steering system during training visit to the aircraft. This situation was compounded by the longstanding failure to update the poor wording in the steering lever centring procedure. Finally, the wider context was assessed as the inadequacy of checklists and procedures for two pilot operation of the aircraft type which the operator had not addressed.
Nine Safety Recommendations were made as a result of the findings of the Investigation as follows:
MAS Wings should amend its DHC-6 Series 400 Standard Operating Procedures as follows:
- by implementing a challenge and response system for checklist reading procedures.
- by implementing the sterile cockpit principle to support the use of standard terminology during the critical stages of flight i.e. during takeoff and after stabilising height until touchdown.
- by implementing takeoff and departure briefs when an aircraft is powered up either by battery or aircraft generator to ensure these briefs are recorded by the aircraft CVR.
MAS Wings should improve the conversion and recurrent training syllabus in the DHC6-400 Training Manual as follows:
- so as to enhance pilots’ subject knowledge on the landing gear and brake systems contained in the Computer Based Training syllabus.
- by introducing an additional subject on nose wheel steering operations and the locking mechanism to the aircraft visit syllabus.
- by including teaching of the correct and standardised technique on how to check and ensure the nose wheel steering lever is in the centred and locked position in both the initial type conversion and the initial operating experience syllabus.
MAS Wings should ensure that standard procedures and coordination between the pilots are emphasised and practiced correctly in the following phases of flight described in the DHC6-400 SOP Chapter 3 ‘Procedures and Policies during Conversion and Recurrent Training’:
- Take-off, departure and approach briefing.
- Take-off and after take-off procedures and coordination.
- Approach and landing procedures and coordination.
- Practice of Checklist procedures.
MAS Wings should amend the POH/AFM content on Nosewheel Steering Lever instructions (Temporary Amendments Nos TA-31 & TA-32) in the After Take Off and Descent Approach Checks.
MAS Wings should update and use the paper checklist as the primary reference for all normal checklist purposes in the Twin Otter aircraft fleet.
MAS Wings should revamp publication update, maintenance and distribution processes to ensure all amendment of revised procedures related to aircraft operations reached the end-users promptly.
MAS Wings should study the option of using a flight simulator as a training platform for the Twin Otter fleet.
CAA Malaysia should conduct pilot assessments which ensure compliance with the DHC-6 Series 400 SOP contained in Chapter 3 ‘Procedures and Policies during Proficiency and Line Check’ on:
- Takeoff, departure and approach briefing.
- Takeoff and after takeoff procedures and coordination.
- Approach and landing procedures and coordination.
- Use of Checklist procedures.
Viking Air should liaise with MAS Wings to study the implementation of a visual indicating or monitoring system to allow a pilot to confirm that the nosewheel is centred in flight.
The Final Report was issued on 7 October 2020.