What happened
On May 4, 2021, an Aircalin De Havilland DHC-6 Twin Otter, registration F-OCQZ, was performing a commercial freight flight from Wallis-Hihifo to Futuna Pointe Vélé. During the final approach to runway 25, the crew increased their approach speed by 10 kt to account for anticipated turbulence.
At approximately 400 ft, the pilot flying reduced the power levers to the idle position. When attempting to increase power to maintain the approach, the pilot found that the power did not increase despite moving the levers to the full power stop. As the airspeed dropped to 62 kt—approaching the stall speed—the pilot applied nose-down input to maintain control. The aircraft landed on the runway using ground effect to compensate for the low speed. Upon reaching the apron, the crew confirmed that the right-hand engine was unresponsive to power lever movements.
The investigation
The BEA examined the aircraft's maintenance history and the condition of the engines. The investigation revealed that the right-hand engine had a history of power malfunctions on the ground and during previous flights. Because the aircraft was not equipped with flight recorders, the investigation relied heavily on crew statements and the Aircraft Technical Log.
Technical examinations of the Fuel Control Units (FCUs) for both engines revealed an unusual level of contamination. This contamination consisted of an aggregation of grease (likely engine oil), environmental elements, and corrosion by-products. The investigation also looked into maintenance practices, specifically the compressor and turbine washing procedures used by the operator.
Findings
- The primary cause of the engine power malfunction was linked to contamination within the Fuel Control Units (FCUs), which likely caused slow or uncontrolled engine acceleration.
- The engines were not equipped with P3 filters, leaving the FCU air section vulnerable to environmental pollutants and salt-laden air.
- Maintenance procedures for compressor washing may have contributed to the contamination, as the P3 air line was not fully removed or protected during the process, potentially allowing contaminants to migrate into the FCU.
- Weaknesses in the management of recurring faults were identified, including a lack of specific alerts for repeated malfunctions and limited technical support for the Wallis-based maintenance team due to pandemic-related travel restrictions.
Safety action
Following the incident, Aircalin implemented several safety improvements, including the installation of P3 filters on the fleet and the use of SD cards in the Garmin 950 interface to better monitor engine parameters. The operator also improved its compressor washing procedures and enhanced communication between the maintenance teams and the technical division.