GAS Why Wing It?
The Importance of Aviation Gas Handling Standards
For many aviators, flying is the ultimate expression of freedom, and most would agree that the price of freedom has never been cheap. With crude oil driving fuel costs skyward, pilots are feeling the pinch even more than their earthbound counterparts. For environmental reasons there has been growing interest in finding an alternative to leaded aviation gasoline (avgas) for general aviation, but pilots are now motivated for financial reasons as well, and are increasingly eyeing automotive unleaded as a cheaper substitute. However, there are important differences between the two fuels, and aviators should beware that a switch could be risky.
By the 1950s the use of tetra-ethyl lead (TEL) as an anti-knock additive had become standard for both the aviation and automotive industries. Since then grades and standards for both have morphed, but an important difference has remained the same. Avgas is a very pure petroleum derivative, with TEL being the only additive. More importantly, due to the potentially disastrous consequences of aircraft engine failure, the production and distribution of avgas is regulated closely to insure the characteristics of the avgas at the pump are uniform and the fuel is fit for its purpose as an aviation fuel.
As an important example, the flash point of avgas is optimized for aircraft operating at high altitudes and must be held in tight tolerance to avoid vapor lock in fuel lines. Unlike most cars, which use submerged pumps to push fuel from the tank toward the injectors, many small planes rely on external pumps that pull fuel up from wing tanks. Under some conditions, inappropriate fuels can vaporize in the lines, robbing the pump of traction and starving the engine of fuel. Even less predictable are the effects of various additives found in automotive fuels. The same performance additive packages that differentiate one brand of gasoline from another (in on/off road vehicles) can harm aviation systems. It would be a mistake to assume the additives in unleaded gasoline would preclude frequent tune ups in aircraft, as they do in automobiles, when in fact some of those additives can have a detrimental effect.
Newer components like ethanol in unleaded gas (E10) can also cause a decline in aircraft engine performance, resulting in an unexpected decrease in range, corrosion to aviation fuel supply systems and other unintended consequences. Further, buyers may not know if ethanol is in their gas unless they ask their dealer. Some states don’t require gas pumps to be labeled, leading to the risk that pilots won’t know if the fuel they intend to use contains ethanol.
While a variety of aircraft have been certified to fly on unleaded gasoline, receiving a Supplemental Type Certificate (STC) from the FAA, the certification is solely based on the engine’s technical ability to fly on the fuel, it does not take into account the differences in the aviation and ground fuel supply chains, the variability of additives packages and the advent of ethanol in the unleaded gasoline.
For many years, industry and specification bodies have worked together to devise a safe system through which aviation fuels are supplied; ensuring that as they move through the system, they continue to be fit for the purpose of flying. Ground fuels have their own excellent set of tried and true handling practices which are tied directly to their use in cars, trucks and other ground equipment, but they were never intended to cover risks that are solely in the realm of aircraft.
While there is a movement afoot to bring more eco friendly, and possibly less expensive unleaded gasoline to general aviation, perhaps within the next decade, the infrastructure in production, pipeline, and pumps does not yet exist. For the moment, aviators would be wise to avoid using fuels not specifically formulated for their type of craft. Thanks to strict standards and careful oversight at each phase of production and distribution, avgas is still the safest fuel on which to fly.
By: Linda Gallaher, Manager-Product Quality, Chevron Global Aviation