AW139 A6-AWN Ditching off UAE, 29 April 2017

AW139 A6-AWN Ditching off UAE, 29 April 2017

Leonardo AW139 A6-AWN, operated by Abu Dhabi Aviation (ADA), made a precautionary ditching off the coast of the UAE near Mubarraz Island on 29 April 2017.  Details are limited but early reports suggest that the ditching was in response to apparent indications of a mechanical failure.  All persons on board were recovered safely.  The aircraft (MSN41213, a long-nose 139 built in the US in 2009) capsized before salvage.

ADA Leonardo A6-AWN (Credit: Unknown)

ADA Leonardo AW139 A6-AWN (Credit: Unknown)

Common themes of the limited amount of (mostly 3rd hand) information available in the public domain describe MGB oil temperature rise and mechanical noise/vibration.  MGB pressure loss or oil loss are not mentioned.

Leonardo issued a statement to customers and other stakeholders on 1 May 2017.

This occurrence is subject to investigation by the Air Accident Investigation Sector (AAIS) of the UAE General Civil Aviation Authority (GCAA).

UPDATE 7 May 2017: It is being reported that the investigation is focusing on the oil cooler fan.

In the EASA certification standard for large rotorcraft, CS 29.917 on design of rotor drive systems states (emphasis added):

CS 29.917 (a) General. The rotor drive system includes any part necessary to transmit power from the engines to the rotor hubs. This includes gearboxes, shafting, universal joints, couplings, rotor brake assemblies, clutches, supporting bearings for shafting, any attendant accessory pads or drives, and any cooling fans that are a part of, attached to, or mounted on the rotor drive system.

Hence the following rule applies:

CS 29.917 (b) Design assessment. A design assessment must be performed to ensure that the rotor drive system functions safely over the full range of conditions for which certification is sought. The design assessment must include a detailed failure analysis to identify all failures that will prevent continued safe flight or safe landing, and must identify the means to minimise the likelihood of their occurrence.

‘Minimise’ is defined as applying ‘technically feasible and economically justifiable’ provisions.

UPDATE 9 May 2017: The local press report that the helicopter had departed Abu Dhabi International Airport at 12.05, landed on one offshore installation at 12.31 before departing for a second. All POB are reported to have boarded the liferafts.

UPDATE 12 June 2017: The GCAA Air Accident Investigation Sector (AAIS) has issued their preliminary report with commendable promptness:

At 1233, the Aircraft departed from Dhabi II for BUNDUQ with a selected climb altitude of 2500 feet (ft). One minute into the climb, at approximately 490 feet (ft), the flight crew received a high oil temperature warning for the main gearbox. The observed oil temperature was 109°C.

A review of data downloaded from the flight recorder shows that the oil temperature started to increase from the normal operation temperature of approximately 85°C, 1 minute 40 seconds prior to landing on the helipad of Dhabi II. The Aircraft landed with an MGB oil temperature of 102°C. By the time the Aircraft left Dhabi II, 1 minute 52 seconds later, the oil temperature had increased to 103°C. One minute 5 seconds after departing from Dhabi II, the MGB oil temperature reached to 109°C, at which point the MGB OIL TEMP warning alerted the flight crew to the exceedance in the MGB oil temperature.

The flight crew decided to follow the QRH and divert to the nearest heliport on Mubarras Island. When the new destination was entered into the flight management system, the heliport was 14.9 NM away and the MGB oil temperature had increased to 114°C. Three minutes 56 seconds later, when the temperature had reached 119°C, a rubbing noise was heard by the flight crew, who then decided to ditch the Aircraft in accordance with the QRH.

The AW139 QRH, Emergency – Malfunction, states: “Transmission System Failures In general a single failure indication dictates that the helicopter Land as soon as practicable while a double failure dictates Land as soon as possible. If multiple failures, including abnormal noise and/or vibration are present LAND IMMEDIATELY.”

The Aircraft conducted a controlled ditching 40 seconds later [at 1240] with 119°C MGB oil temperature.

The flight crew [had] deployed the emergency floats during the controlled descend at 150 ft prior to contact with the water.  All four floatation devices inflated fully and provided sufficient buoyancy to keep the Aircraft upright and afloat.

During the evacuation, the Commander noticed that the Aircraft tended to tilt towards the aft left float. It was identified that part of the aft pillow chamber had delaminated, releasing air from the aft floatation chamber through the pillow inflation valve. A second area of delamination was found at a main chamber seam which was slowly releasing air.  An inspection of the Aircraft identified that the shear bolts holding the upper edge of the float covers in place had not sheared causing fracturing of the float covers with the remains attached to the Aircraft.

 The flight crew pulled the emergency raft deployment handles in the flight deck, but only the left raft deployed successfully.

An operational test of the right raft deployment system confirmed operational functionality of the handle and cable mechanism. The raft was subsequently removed to conduct an inspection of the system, confirming the operational serviceability of the mechanism. A functional check of the pressure cylinder confirmed that the raft was serviceable at the time of the Accident. An inspection of the right raft deployment handle revealed that the safety clip was loose and that the handle installation showed excessive lateral play. The failure of the raft deployment is subject to further investigation.

The Copilot reported that when the left [cockpit] window panel handle was pulled as described in the emergency evacuation procedures, the window panel cracked in the area of the pull handle creating a hole in the panel. The Copilot was able to pull the panel inwards by using this hole as a grip point. [This window was not recovered.]

After initial difficulties in opening the left flight deck escape window, the flight crew evacuated the Aircraft into the life raft. The passenger successfully opened the cabin escape window and evacuated the Aircraft into the same life raft.

The raft floated away from the Aircraft as it became apparent that the Aircraft was not remaining in the upright position [so the commander cut the raft's tether].

Subsequently the aircraft capsized.

The Crash Position Indicator (CPI), or Automatically Deployed Emergency Locator Transmitter (ADELT) did not deploy.  ADELTs have not had a stunning service experience as the UK Civil Aviation Authority (CAAreported in CAP1144.  In the case of the last AW139 ditching (B-MHJ in Hong Kong Harbour in 2010) the ADELT did activate.

AW139 A6-AWN Being Salvaged (note: the EFS bags were deflated during salvage) (Credit: GCAA)

AW139 A6-AWN Being Salvaged (note: the EFS bags were deflated during salvage) (Credit: GCAA)

The GCAA say that after salvage of the aircraft damage was found to the MGB cooling system:

The MGB oil is cooled by ambient air from the top of the open gearbox fairing, which is forced through an oil cooler by the attached oil cooling fan. The cooling fan is encased in a housing as an assembly and is driven by a drive shaft from the accessory gearbox, which is attached to the MGB.

The MGB oil cooling fan assembly was removed from the Aircraft and send to Leonardo Helicopters for a forensic examination.

The manufacturer (Technofan) disassembled it in the presence of the IIC, ANSV, Leonardo Helicopters, and Technofan representatives. It was found that the fan shaft had lateral play of 5.3mm. The fan was not attached to the shaft and was touching the outer shroud.

AW139 A6-AWN Oil Cooler Shaft and Bearing Debris (Credit: GCAA)

AW139 A6-AWN Oil Cooler Shaft and Bearing Debris (Credit: GCAA)

The left bearing was completely degraded, showing signs of overheating, with the inner bearing race physically expanding from 11mm to 18mm. The right shaft bearing was destroyed and, together with its seal package, extended from the housing by 4.8mm. The shaft thread was stripped and showed signs of extensive heating near the lower end. The balls and ball cages from both bearings were destroyed.

The Investigation is ongoing and the investigators say it will include further examination and analysis of:

  • The root cause and sequences of the MGB cooling fan assembly failure
  • The MGB cooling fan assembly reliability
  • Floatation device damage
  • Raft deployment failure Flight deck escape window condition
  • The Operator’s ditching procedures and training
  • The activation of the ELT
  • The MGB oil temperature indication system
  • The MPFR water resistance
  • Any other safety aspects that may arise during the course of this Investigation.

The Investigation will also conduct an in-depth analysis of the “contextual factors, human factors and organizational factors”.

EASA published NPA 2016-01 Helicopter ditching and water impact occupant survivability in March 2016, seeking comments on improvements in the certification of new helicopters that would address several of the potential concerns here.


We have previously published this article in 2014: Hong Kong Harbour AW139 Ditching – HKCAD Report Issued

UPDATE 22 December 2017: The AW139 fleet globally has now exceeded 2 million hours.  With the AW139 fleet leader having exceeded 12,000 hours, nearly 900 units are today in service out of over 1000 ordered by more than 300 customers in 80 countries.  The second million was achieved in only 2.5 years.

Aerossurance has extensive air safety, operations, airworthiness, human factors, aviation regulation and safety analysis experience.  For practical aviation advice you can trust, contact us at: enquiries@aerossurance.com