What happened
On 29 April 2017, an AgustaWestland AW139, registration A6-AWN, was performing a commercial passenger flight from Abu Dhabi International Airport to an oil rig in the Arabian Gulf. Shortly after departing from the Dhabi II oil rig for its next destination, the flight crew received a warning indicating high oil temperature in the main gearbox, which had reached 109° and was climbing.
Following established quick reference handbook procedures, the pilot reduced power and attempted to divert to the nearest heliport at Mubarraz Island. However, as the aircraft descended to 200 feet, the oil temperature rose to 119°C, accompanied by a loud rubbing noise coming from the gearbox area. Faced with an immediate threat, the commander decided to ditch the aircraft in the sea.
While the aircraft's flotation system activated upon impact, the evacuation faced several complications. The left aft float deflated, causing the helicopter to tilt. Although the crew attempted to deploy emergency rafts, only the left raft functioned correctly. The crew and a passenger successfully evacuated through escape windows, but the aircraft eventually overturned in the water. All 3 occupants survived the incident without injuries.
The investigation
Investigators from the GCAA AAIS examined the wreckage and the mechanical components of the cooling system. A forensic examination of the main gearbox oil cooling fan assembly revealed significant internal damage. The investigation found that the fan was not properly attached to its shaft, leading to lateral play of 5.3mm and contact with the outer shroud.
Furthermore, the left bearing showed extreme signs of overheating, with the inner race expanding significantly, while the right shaft bearing was completely destroyed. The investigation is also looking into why the emergency locator transmitter (ELT) failed to deploy automatically and why the flight deck window panel cracked during the evacuation process.
Findings
- The primary cause of the ditching was a mechanical failure within the main gearbox oil cooling fan assembly.
- The cooling fan shaft exhibited excessive lateral movement, causing the fan to strike the outer shroud.
- Severe degradation and overheating occurred within the fan's bearing components, leading to the rise in oil temperature.
- The aircraft was lost to saltwater damage and was deemed beyond repair.