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