Trans Maldivian DHC6 at Medhafushi Island on Oct 22nd 2020, ship strike

Last Update: January 4, 2022 / 20:39:30 GMT/Zulu time

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Incident Facts

Date of incident
Oct 22, 2020

Classification
Accident

Airline
TMA Trans Maldivian Airways

Departure
Male, Maldives

Destination
Medhafushi Island, Maldives

Aircraft Registration
8Q-TMR

Aircraft Type
De Havilland DHC-6 Twin Otter

ICAO Type Designator
DHC6

A Trans Maldivian de Havilland DHC-6-300, registration 8Q-TMR performing a flight from Male to Medhafushi Island (Maldives) with 14 passengers and 3 crew, landed on the water inside the Island Lagoon normally, when after applying reverse thrust the engines and propeller developed asymmetric reverse power causing the aircraft to veer left. The captain took control of the aircraft and attempted to control the situation, but was unsuccessful. The aircraft collided with an anchored vessel causing damage to the left hand wing and left hand propeller as well as damage to the vessel (nobody on board of the vessel). Following the collision the captain taxied the aircraft to a platform where passengers and crew disembarked safely. No injuries occurred, the aircraft sustained substantial damage.

On Nov 23rd 2020 the Maldives Accident Investigation Coordination Committee (AICC) released their preliminary report listing following facts out of the initial investigation:

- The Engine Performance check (Task Card number 202457-363) requiring engine performance check and operation of engine instruments prior to and post EMMA check, was carried out on 19 October 2020 at 11:05 hrs and 20 October 2020 at 17:00 hrs, respectively. The recorded engine ground run readings show some parameters were out of the limits, but the ‘EGR Troubleshooting and Analysis Form’ has no entries made suggesting there were no corrective actions taken to rectify the variations, prior to releasing the aircraft back to service.

- The Crew from previous day reported a rejected take off at MLE due to weak reverse power observed, and also reported ‘very weak’ forward power as well

- There was no wind direction indicator at the water aerodrome.

The Maldives AICC released their final report concluding the probable causes were:

The AICC determines that the causes / contributing factors of this incident as:

a. The landing line is too close to the anchored boats;
b. Abnormal differential spooling of the engines during reverse;
c. The prevailing left cross wind.

The AICC analysed:

The aircraft had no known maintenance overruns or open deferred defects when released after Daily Maintenance check on 21 October 2020 evening, and no defects were noted by the operating crew at the time of pre-flight checks in the morning of next day, 22 October 2020. The analysis therefore focusses on crew qualification, performance, operating procedures and conditions.

Both crew members held valid licenses, medicals and had satisfactorily completed the required proficiency checks and were current to operate the type.

The PIC held a valid ATPL with a total of over 13,000 hrs including 7,848.5 hrs as PIC on DHC-6 floatplanes. The captain is a senior Line Training Captain (LTC) who is an experienced and qualified captain actively involved in training and checking activities (including LIFUS, route familiarization and line checks) for newly appointed and existing first officers and captains.

The FO held a CPL with a total of 626.7 hrs which includes 376 hours on type. According to him, he had previously landed inside the lagoon at IRU and did not land outside, hence he was comfortable to land inside considering the favourable water and wind conditions.

The PIC was unfamiliar with the FO’s experience. No appropriate pre-flight crew briefing was done; as laid down in the Operations Manual (OM) Part B Chapter 2.2.5, to discuss about the flight, weather conditions, operational restrictions, passengers and other relevant factors.

The two flight crew members and the cabin crew adequately rested prior to the flight and there was no exceedance of duty time and sectors. Hence, it is unlikely that crew fatigue contributed to this incident.

TMA holds Water Aerodrome License number AP/O/92 issued to Sun Siyam IruFushi Water Aerodrome (IRU), by the MCAA, dated 28 November 2010.

The VFR approach charts issued by TMA for IRU dated 10 June 2018 shows 4 different possible landing lines: three outside the reef and one inside the lagoon.

The Caution column on the aerodrome chart states that “cross swell can be present inside lagoon at high tides, and hard to see during overhead inspection when considering inside lagoon ops. Inside lagoon operations can be conducted at all tides. Boat activity and boats moored in close proximity to operating area inside lagoon.’’

In the remarks column it is stated “Noise Abatement: please take-off and land outside lagoon if water and weather conditions permit. Deep water inside lagoon free of coral heads. Beware of possible coral heads in the shallower water when back tracking for take-off at low tides.”

VR-IRU inside lagoon is considered as a challenging area for take-off and landing as the area is very confined and if the pilot made an error, it may be difficult to recover.

According to the PIC, the reason why he allowed the FO to perform the landing was to provide FO with a practice opportunity to land cross wind on confined areas. However, considering the limited experience of FO, the judgement to let him take the risk of landing inside the lagoon while the weather and water condition allowed to land outside of lagoon, was not the most appropriate decision, considering the captain’s experience as a senior LTC.

The crew did not notice any discrepancies or abnormalities with the aircraft during the flight, till reverse was applied. As per the crew, when reverse was applied the aircraft behaved unexpectedly and there was a delay in activating the reverse and/or there may have been a difference in reverse power generated (asymmetrical reverse) on both sides.

There have been incidents involving aircraft in the operator’s fleet installed with the MT Propeller type, where the crew has complained of the performance of the type of propeller. These initial problems with the change of propeller type from 3 bladed Hartzell propellers were found to be related to engine rigging after the installation of the MT Propeller type.

One previous incident similar to this occurred on another aircraft of the same operator’s fleet during docking where the crew also complained of producing differential reverse thrust on the engines. At the time of that incident there was a strong left cross wind prevailing.

Veering to the left can be caused by:

1. Inadvertently applying left rudder after touchdown, however, rudder is not sufficiently effective to make a sharp turn on a decelerating floatplane;
2. Not releasing the rudder input on landing for the crab applied for the cross-wind correction;
3. Strong cross wind during landing;
4. Application of differential reverse power or a difference in spooling of the engines during the application of reverse power;
5. When propeller controls are incorrectly rigged, the possibility of forward thrust being produced on the engine can occur when reverse is selected. In this situation the Propeller RPM reaches more than the CSU pneumatic maximum set RPM (91%, ± 1%). If the propeller RPM reaches more than the CSU pneumatic maximum set RPM (91% +/-1%) in reverse, then CSU or OSG can enter in governing RPM and dump the servo oil from the propeller which cause the propeller to go to a forward angle.

The last scheduled inspection of the aircraft was EMMA number 04 carried out on 20 October 2020, at 44,384.27 TAT and 92,576 landings. During the EMMA, several airframe, engine, propeller and float related tasks were carried out. Additionally, engine ground runs were carried out before and after the EMMA check. No maintenance overruns were noted.

During the pre-EMMA check requiring engine performance check and operation of engine instruments, some recorded engine parameters were out of the limits, but no entries were made relating to any corrective actions taken, prior to releasing the aircraft back into service.

The Torque value entered in the Engine Ground Run sheets post EMMA for “Max Reverse check”, was found to be incorrect. This was confirmed by the maintenance staff involved.

After repairing the LH engine and replacing the LH propeller for ferrying the aircraft to base, the ferry Captain carried out engine runs and high speed taxiing.

Following were his observations:

- RH engine start-up was within the starting parameters and normal;
- Auto feathering test was found satisfactory;
- RH engine spooling on forward was normal;
- RH engine spooling on reverse was slow;
- Total of three high speed taxiing, followed by reversing to stop the aircraft was made. RH engine reverse was significantly slow in comparison to the LH engine. Based on the ferry captain’s experience he found the RH engine too slow to respond.