Coking Causes Power Loss: Australian AS350BA

Coking Causes Power Loss: Australian AS350BA

On 2 November 2015, Airbus Helicopters AS350BA VH-SFX of GBR Helicopters was performing a low-altitude aerial weed spotting operation over dense forest in the Whyanbeel Valley, QLD. On board the pilot, a navigator and two aerial spotters.  The Australian Transport Safety Bureau (ATSB) explain in their safety investigation report that:

…during the fourth flight of the day, the helicopter momentarily yawed twice within a short period in an uncommanded and unusual way. The pilot, concerned with the uncommanded movements, ceased the operation, climbed and increased the helicopter’s forward airspeed. The pilot then elected to head back towards the base of operations (approximately 11km away) and, if required, land along the way if a suitably safe area along the flight path presented.

The helicopter’s GPS track, arriving in the area of operations, its flight path around the forested terrain in the Whynabeel valley, and the accident site (Credit: ATSB)

Shortly after, the chip detector light illuminated…prompting the pilot to search for a suitable landing area. As the helicopter continued to climb through approximately 200 ft, the engine stopped producing power… which required the pilot to conduct an autorotation and emergency landing.

…the emergency landing was handled in a competent and proficient manner.  …the helicopter landed heavily with the skids digging into the uneven terrain and breaking off.

AS350BA VH-SFX at the accident site (Credit: ATSB)

The navigator in the front seat received minor injuries and the pilot received serious back injuries from the impact forces.

The ATSB comment that:

The pre-departure briefing gave the passengers the necessary knowledge to prepare for the emergency by adopting the brace position and exiting the helicopter only when it was safe to do so.

The Engine Failure

The helicopter was powered by a Turbomeca (now Safran Helicopter Engines) Arriel 1B.  The ATSB say:

…the aircraft lost power due to a front bearing failure in the turbine module.

Cross-section through the power turbine assembly from the engine showing that the disc had separated from the shaft and the blades had fractured (Credit: ATSB)

The failure was due to an accumulation of coke particles in an oil jet. Coking is an artefact from exposure to abnormally high temperatures that leads to oxidation and chemical breakdown of the oil.

Close up of the blockage (circled) at the oil jet to the front bearing, as positioned in the general arrangement view (Credit: ATSB)

The observed coking of the front bearing and its oil jet duct was likely a result of the engine oil exposure to abnormally high temperatures in the area. While there were clogging inspection procedures of the power turbine rear bearings, no preventative maintenance actions existed that allowed for the identification of coking within the front bearing.

There is…

…published guidance for the thermal stabilisation of engines at shutdown. The guidance involved throttling the engine back until the engine was at ground idle for at least 30 seconds prior to shutdown. [This] allows for the temperature to reduce and thermally balance, while maintaining sufficient oil-scavenging capability and oil flow rates to minimise the potential for coke formation. Non-compliance with the manufacturer’s stabilisation recommendations may lead to coking.

In the period January 2000 to September 2015, there were 13 cases of Arriel engine deterioration in the power turbine shaft front bearing due to oil jet clogging. However, this is the only accident that has resulted in the failure of the turbine shaft.

The ATSB was unable to specifically determine why the coke particles had formed.

The severity of the engine failure was increased through the fracture of the power turbine shaft and the subsequent separation of the turbine disc. This was due to a lack of adhesive on the splined nut that was threaded to the rear of the power turbine shaft.

The engine manufacturer had intended for an adhesive to be used in order to secure the rear nut into position.

The adhesive should be applied during power turbine overhaul.

Safety Actions

The engine manufacturer has amended their practices to include:

  • Periodic cleaning of the power turbine front bearing assembly oil jet and oil jet supply pipe. This reduces the occurrence probability for oil jet clogging by removing any accumulated deposits from these locations.
  • Degreasing of the threaded surfaces prior to application of the adhesive bonding and assembly of the parts. Maintenance and overhaul personnel have been informed of the importance of degreasing the surfaces before bonding the nut to the power turbine shaft.  They are also studying the use of heating equipment to obtain a more repeatable polymerization.

Aerossurance will be presenting at EASA‘s 11th Rotorcraft Symposium on Cologne on 5 December 2017 on human centred design (HCD) for rotorcraft.  This presentation will be live streamed.

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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