A change in the formulation of avgas delivered to Northern Australia is suspected of causing widespread and significant engine maintenance events in the fleet of hundreds of Lycoming-powered Robinson helicopters based there, according to a study published the Australia Helicopter Industry Association (AHIA). The problem is being linked to the failure of more than 2,000 engine cylinder barrels on Australia-based Robinson R22s and R44s since 2013. Some cylinders failed within the first 100 hours in service.
The AHIA determined that significant changes were made in avgas delivered to the region from 2012 to 2018 after the region’s terminal supplier was switched. Specifically, the lead content was more than halved while the aromatics content was increased to preserve octane levels. Increasing aromatics slows and significantly increases the exhaust gas temperature (EGT) during the burn process. The result was significant damage to engine cylinders, pistons, and valves. The AHIA study also found that “fuels containing a greater proportion of aromatic hydrocarbons have also been shown to have a greater soot and deposit-forming tendency.”
Trouble usually appeared during cylinder compression checks as part of routine maintenance. According to the AHIA, “Data from the Civil Aviation Safety Authority’s Defect Reporting Service (DRS) database showed compression loss was attributable to inlet or exhaust valve degradation and sealing loss. Follow-up investigation by the engine manufacturer showed related mechanisms of failure—both attributable to extended periods of high-temperature exposure and accelerated valve guide wear.”
Compounding the damage was how R22s and R44s are used in the region, typically for agricultural application or livestock mustering, often at low altitudes with high power settings during high ambient temperatures. And unlike fixed-wing piston aircraft, which use the aircraft’s propeller and cowling intakes for cooling, piston Robinsons utilize a rear cooling fan connected to the engine’s driveshaft to move air over the cylinders and the oil cooler and direct exhaust downward.
Temperature Warnings
In February, Robinson amended the Pilot’s Operating Handbook (POH), reinforcing the need to observe proper engine cooldown procedures and providing for an extended cooldown period when ambient temperatures top 100 degrees F.
Recognizing the problem, earlier this year Australia’s Civil Aviation Safety Authority (CASA) issued Airworthiness Bulletins AWB-85-024 and AWB-85-025 and issued warnings about the unreliability of cylinder head temperature (CHT) as an indicator of valve temperatures, counseling, “Even a cylinder displaying a moderate CHT can be suffering accelerated wear. Be mindful that a single probe CHT will not necessarily be indicative of all cylinders, nor represent even and consistent cooling of the entire cylinder assembly.” CASA also reminded pilots that, “The risk to the integrity of avgas in relation to being fit for purpose can occur at any point in the supply chain from the point of manufacture to final delivery to the aircraft, thus creating the potential to adversely affect aircraft systems and components. Fuel suppliers need to verify that all product supplied to the market complies with Defence Standard 91-090 avgas and has been manufactured, stored and handled in compliance with Energy Institute 1530 aviation fuel quality system compliant supply chains to control contamination risks.”
In its study, the AHIA recommends that: authorities access historical data to determine composition characteristics of fuel delivered to the terminal servicing the region (Vopak); further study the performance of avgas containing moderate levels of aromatic compounds; consider potential airworthiness implications for piston aircraft operating in high ambient temperatures with fuel with increased levels of aromatics; and prepare and circulate relevant advisory materials to operators flying in high ambient temperatures using fuels with moderate levels of aromatics. Helicopter operators and maintainers are counseled to remain vigilant with regard to the issue.