Created Saturday, Jul 24th 2021 07:01Z, last updated Friday, Aug 8th 2025 16:06Z
An Air Caledonie International de Havilland Dash 6-300 Twin Otter, registration F-OCQZ performing a freight flight from Wallis Hihifo to Futuna (France), was on final approach to Futuna when both engines lost power. The aircraft as able to land safely nonetheless.
On Jul 23rd 2021 the French BEA reported the occurrence stating the cargo flight went between Wallis and Futuna Islands in Oceania, belonging to France, when both engines lost power on final approach to Futuna. The occurrence was rated a serious incident and is being investigated by French BEA.
The BEA released their final report concluding the probable causes of the serious incident were:
The examinations carried out on the FCUs of both engines revealed an unusual level of contamination, made up of environmental elements and grease (probably engine oil), which may have caused malfunctions such as a lack of engine power increase, although this was not established with certainty.
The investigation was unable to determine with certainty the cause of this contamination.
The investigation led to the identification of weak factors that may have contributed to the contamination of the FCUs:
- operation in salt-laden air with engines configured without filters before the P3 port. The presence of a filter limits the contamination without eliminating all the contamination sources;
- washing processes that differed from those recommended by the manufacturer, potentially including weaknesses, in particular with regard to the protection of the P3 line.
The investigation resulted in the identification of weaknesses in the processing of recurring faults:
- the absence of an alert or of specific processing in the event of repeated faults;
- limited exchange and support opportunities offered to the maintenance teams at Wallis, due in particular to the geographical distance and restricted travel during the pandemic period;
- difficulties in confirming some malfunctions in the absence of recorders;
- reduced cooperation with the engine manufacturer.
The BEA summarized the sequence of events:
At the start of the descent, with the autopilot engaged in vertical speed mode, the aeroplane suddenly adopted a steep nose-up attitude. The PF disconnected the autopilot and pushed the stick to correct the aeroplane’s attitude. He noticed that a substantial force was required to make this correction and realised that the elevator trim was at its nose-up limit.
The PF cut off the electric trim and continued the flight in manual mode.
As he was approaching the destination airport, given the weather conditions observed, he decided to make a visual approach to runway 25. As the AFIS agent had informed them that there was a north-westerly wind, the crew anticipated possible turbulence on final due to the wind passing over the island’s terrain. The approach speed was increased by 10 kt.
According to the crew, at 500 ft, the approach was stabilised.
At approximately 400 ft, the speed increased to 90 kt. The PF moved the power levers to the idle position. The speed decreased and the PF returned the levers to the “final power” position. As the speed continued to decrease, the PF made repeated inputs on the power levers until the “full power” stop was reached. During this phase, the PF held the approach slope and aiming point.
The successive inputs on the power levers up to the full power stop seemed to have no effect and the speed continued to decrease.
When the speed reached 62 kt, the PF made a nose-down input on the stick to maintain a margin in relation to the stall speed (around 56 kt). The aiming point shifted to the area located between the shoreline and the runway threshold. The speed increased to approximately 66 kt.
He flared over this area and used the ground effect to land on the runway.
The PF managed the landing run without any particular difficulty, using the power levers between the idle and reverse positions (beta range).
On the apron, the crew moved the RH engine (#2)4 power lever to the forward limit and observed that this action had no effect.
The report analysed that there had been multiple reports on engine problems between Jan 1st 2021 and May 3rd 2021 on both engines. The fuel control units for the both engines were replaced in response on Jan 1st and Jan 19th, the propeller overspeed governor was replaced for the right hand engine was replaced. In other occurrences no fault was detected on the ground.
On Apr 9th 2021 the engine remained in idle despite the power lever was into maximum power. The BEA wrote: "Tests carried out on RH engine #2 did not reveal any anomaly. The fuel system of RH engine #2 was checked and cleaned, no anomaly was detected. The maintenance actions carried out in the meantime concerned the autopilot (fault detected on 30 April during the daily check, replacement of the elevator trim servo). The aeroplane did not fly until the training flights conducted the day before the event."
Following the occurrence flight both fuel control units were sent to the manufacturer and checked. The BEA wrote:
The FCUs were examined in several steps: an initial naked-eye and borescope inspection, bench testing, disassembly and new visual inspection, analysis of the elements found. The FCU was tested in accordance with the Honeywell manual (CMM 73-20-31).
During the physical inspection of the two FCUs, debris was found at the P3 and Py ports.
The borescope inspection carried out on the P3 and PY air inlets of LH FCU #1 revealed the presence of flaky debris, black granular deposits, fibres, yellowish particles and signs of corrosion.
The borescope inspection carried out on the P3 and PY air inlets of RH FCU #2 revealed the presence of black granular deposits, yellowish crystals and fibres.
Bench testing of the two FCUs did not reveal any operating anomalies that could explain the event.
Black debris was found in the Px and Py air passages, as well as black, flaky debris in the Px bleed cavity, comparable to the debris found in the Py orifice inlet during the borescope inspection.
The Py port was contaminated with black debris.
The chemical analysis showed that the residues observed were an aggregation of grease (probably engine oil), different types of environmental elements and corrosion by-products. The environmental elements detected were chlorine, sodium, carbon, sulphur, calcium, silicon, magnesium, fluorine and cotton fibres.
According to PRATT & WHITNEY, the debris in the Py orifice inlet of LH FCU #1 could have affected the operation of the FCU and led to engine acceleration malfunctions (slow or no engine acceleration).
According to PRATT & WHITNEY, the debris found during the examination of RH FCU #2, in the location where it was found, would not have had a direct impact on the operation of the FCU.
However, PRATT & WHITNEY highlighted the fact that this debris could have migrated into the air section and could have momentarily altered the operation of the FCU.
