A nearly decade-long search by the US government to find a new avgas formula that can replace leaded fuel in hundreds of thousands of piston-powered aircraft has entered a final, two-year evaluation with two remaining contenders: global heavyweight Shell and US-based start-up Swift Fuels.

Since 2014, both companies have emerged as the final contenders among 17 original bidders in a US Federal Aviation Administration-funded programme to develop a drop-in replacement for 100-octane low-leaded aviation fuel, or Avgas 100LL.

On 29 March, the FAA selected Swift Fuels and Shell to submit their fuels for the second phase of testing under the Piston Aviation Fuels Initiative (PAFI). Although it’s not guaranteed that either will qualify as a drop-in replacement for Avgas 100LL, the next round of testing will help the companies obtain an approved fuel specification, clearing the way for FAA authorisation for use on the US piston fleet.

“We’re on track to have unleaded aviation gasoline fully evaluated and ready to be authorized for use by the general aviation fleet in 2018,” says FAA Administrator Michael Huerta.

Lead additives have largely disappeared from road vehicles in the USA since the mid-1990s, leaving the general aviation community as the largest source of emissions of the carcinogen into the atmosphere.

Since the appearance of Avgas 100LL in the mid-1970s ­– allowing no more than 2.12g of lead per gallon – piston-powered aircraft have remained dependent on leaded fuels. The lead is used to boost the octane level, especially for supercharged piston engines. That reduces the chance that the fuel might detonate in the cylinder, potentially ripping the engine apart in flight.

Fuel suppliers face the daunting challenge of formulating an unleaded mixture that provides equivalent performance, requiring no engine modifications across a US piston-powered fleet of about 160,000 aircraft.

“This is critical to the future of general aviation,” says Pete Bunce, president and chief executive of the General Aviation Manufacturers Association (GAMA). “A successful transition from leaded to unleaded avgas will mean the continued safety and utility of the fleet, a reduced environmental impact, and lower economic transition costs for our industry.”

The Aircraft Owners and Pilots Association (AOPA) also welcomed the downselect, saying both candidate fuels look “promising”.

Source: FlightGlobal.com