Avgas and jet fuel: fuelling the rotorcraft sector

Avgas and jet fuel: fuelling the rotorcraft sector

2-Dec-2022 Source: Air bp

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Avgas and jet fuel are specifically developed for fixed and rotary wing aircraft and have been designed to improve the safety and reliability of flight. How did these distinctive aviation fuels come about and how do they meet the fuelling requirements of helicopters? Dr Alistair Clark, Air bp aviation fuels research & development manager, reports.

Following the success of the Wright Flyer in 1903 and the first helicopter flight by Frenchman Paul Cornu in 1907, early aviators were reliant on a low boiling hydrocarbon gasoline as used in ground vehicles. The link between engine operation and fuel quality remained unexplored for several years. That is until the First World War, when the need for reliability and high-power output led to the introduction of aviation gasoline (Avgas). This high-quality fuel was, and still is, specifically designed for use in spark ignition piston engines.

Such engines were still in use when a key break-through in rotorcraft design came in 1939, with Igor Sikorsky using a tail rotor to counter-act the torque or the main rotor, as seen with the Vought-Sikorsky VS-300.

Today Avgas is recognised by the distinctive blue colour of the most common grade, 100LL (one hundred low lead) and is used in piston-engine helicopters worldwide such as the Robinson R22 and R44.

With the arrival of the turbine engine in the 1930s and 1940s, another change swept across the aviation industry. Avgas is a flammable liquid which can easily ignite in temperatures down to -40oC and below. While an aviation turbine engine could use such a fuel, significant safety and engineering advantages are available by adopting a specialist aviation kerosene grade for flight. This fuel, originally designated RDE/ F/ KER has been developed into the product which powers the majority of aviation operations around the world today. The most common Grades, Jet A and Jet A-1, are supplied by Air bp globally for use in turbine fixed and rotary wing aircraft, possibly also featuring a specialist anti-icing additive (Fuel System Icing Inhibitor ‘FSII’).

Why using a dedicated aviation fuel is important

Both Avgas and Jet have been developed by the aviation industry as dedicated aviation fuels with strict specification and quality control measures. This is to ensure they are fit for flight operations and that the required aviation fuel reaches the aircraft in good condition. For example, these measures include filtration standards for removing dirt and free water which could block filters, cause corrosion or impact engine performance.

In addition, the aviation industry has stringent procedures for approval of additives with a focus on aircraft engine and fuel system reliability. Only a limited number of chemical types are approved and carefully regulated. For example, unlike automotive diesel fuel, no blending of fatty acid methyl esters is permitted in jet fuel due to risks to low temperature properties, as well as the impact on aircraft range and fuel stability.

Another factor to consider specifically with Avgas is the fuel octane quality – if the fuel is of poor combustion quality it may start to explode uncontrollably, which could damage a piston engine. Avgas ‘Motor Octane Number’ (MON, ASTM D2700) quality is 91.0 and 99.6 minimum for Grades UL91 and 100LL respectively in excess of typical automotive gasoline which tends to quote the less severe ‘Research Octane Number’ (RON, ASTM D2699) test. There’s also fuel volatility and carburettor icing, which can affect engine performance and reliability.

Given aviators fly at altitude where temperatures decrease, low temperature operability is another consideration for aviation fuels. Jet fuel and Avgas are manufactured to keep flowing under these circumstances. Jet fuel can also act as a coolant for the engine oil and maintain clean injector nozzles, avoiding carbon deposits in the severe temperature regime of modern turbine engine design.

As aviation fuelling requirements evolve there is increasing focus on sustainable aviation fuel (SAF), a fully approved jet fuel produced from sustainable feedstocks which can drop straight into existing infrastructure and aircraft.  In 2021, Air bp collaborated with CHC Helicopter Service to provide the fuel for its offshore flights out of Sola Airport in Norway. It has since supplied SAF to other helicopter operators including Bristow and ADAC Luftrettung. Air bp also now has a role to play as a member of international aviation industry groups, such as ASTM, in securing approvals for new SAF pathways, or indeed challenging inappropriate technology.

Given helicopters are designed to perform complex operational manoeuvres often at low altitude and in confined airspace, it’s good to know the industry is watching over a key component for flight, the fuel. Air bp has an integral role to play here in seeking to meet the customer requirements of today, while looking to help to provide a sustainable future for aviation.

To find out more about the fuels we supply click here.

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