Unleaded Fuels Part 1

Airworthiness By Design

  • Industry Insights
  • Michael J. Kraft, Senior Vice President and General Manager Lycoming Engines

This article is the first of three that explains in more detail Lycoming Engines's position on automotive fuels. The bottom line is that it's possible to make automotive gasoline "fitfor-purpose" for aviation. However, in doing so, you deviate from "pump gas" and end up creating a low octane unleaded aviation fuel from automotive gasoline blend stock - "mogas." Why do we need to have fuels "fit-for-purpose" for aviation? You achieve Airworthiness by Design, not by luck. Using "pump gas" from the corner gas station is achieving airworthiness by luck.

At EAA AirVenture 2008 Lycoming Engines presented one of its most information-packed press conferences of the last five years. New products, new services, new initiatives - including one that stated Lycoming would seek unleaded automotive gasoline approval for its standard-compression parallel valve O-360 and IO-360 engines.

Automotive gasoline. The ultimate solution for inexpensive, readily available fuel for aviation. The easy answer to replacing leaded avgas for 70 percent of the existing fleet. Since 2008, we've received a lot of questions about when the approval was going to be ready. One would think we would have shouted it from the rooftops when available. Last summer we published an update to our "Approved Fuels" Service Instruction 1070. Yes, Lycoming approved automotive gasoline - "mogas" - on several 360 engine models - and we did not splash the news out in a press release.

There are several reasons Lycoming refrained from the "end zone dance":

1. We firmly believe that the much bigger issue is making certain we identify and bring to market unleaded aviation grade replacements for 100LL - a solution that would serve the entire fleet

2. Our "mogas" approval was for the engine only. In order to be legally used in certificated aircraft, the aircraft must also be approved via TC or STC for utilization of automotive gasoline.

3. The automotive gasoline Lycoming approved controls specifically four parameters to values required to maintain airworthiness of the engine without mechanical modifications or operational limitations. These parameters are also controlled for ground transportation vehicles but in different form.

4. We did not approve "pump gas". People cannot think that they can simply drive down to the local gas station and purchase automotive gasoline that is known to provide the same level of airworthiness as aviation gasoline.

5. Before we move further into the subject of automotive gasoline's potential place in aviation, let's repeat why Lycoming believes our existing fleet needs an unleaded replacement to 100LL avgas. First and foremost, the "workhorse" aircraft that drive our industry's economics are mostly designed around 100LL octane capability. If Page 2 of 2 the "workhorse" demand goes away we will all suffer higher costs and reduced FBO service levels. Second and no less important, octane capability is just one element of avgas. Not all aircraft and engines need 100 MON octane. However, it's the parameters beyond octane that enable us to achieve airworthiness for the specified operating envelopes of the aircraft we fly today. Third, the fleet that we fly today - both workhorse and recreational - is about fifty times larger than annual new aircraft production and is based on technology designed around avgas properties. We cannot abandon the existing fleet.

The words written above might be controversial to some, but let's leave this as Lycoming's opinion and return to the discussion about automotive gasoline - "mogas" and "pump gas." Broadly speaking, both of these are fuels conform to ASTM Specification D4814 or Euronorm Specification EN228. These fuels are designed for ground vehicles and controlled to ensure proper startability, driveability, seasonal emissions control and of course, performance. As ground transport fuels, they are highly influenced by environmental regulations (EPA). They are not subject to aviation regulations (FAA). The specifications change on an almost yearly basis in response to changing environmental regulations and political mandates such as ethanol inclusion. What is made available to the market for retail consumption is "pump gas" which is NOT "fit-for-purpose" for aviation precisely because it is controlled for ground transportation purposes. So how did Lycoming, self-admittedly one of the most conservative engine companies in the world, approve "mogas" and what exactly did we approve?

Lycoming approved "mogas" by controlling automotive gasoline properties differently than what is done for ground transport vehicle "pump gas." The specifics:

  • 93 AKI for detonation margin (hot day OAT and 500F cylinder heads).
  • Vapor pressure Class A-4 to prevent vapor lock.
  • No ethanol and maximum 1% oxygenates.
  • ASTM D4814 Revision 09b and EN228 Revision 2008:E.

That's pretty specific stuff and only one of those values is listed on the filling station pump. The first three parameters are what fuel wholesalers control when ordering pump gas." In order to get to aviation "fit-for-purpose," Lycoming controlled those same parameters - differently - and locked in the fuel specification revision to avoid a moving target.

Long story short: This isn't your corner market "pump gas." It is automotive specification fuel with carefully controlled characteristics that help repurpose it from a ground mission in automobile fuel systems - "pump gas" - to a flight mission in aircraft fuel systems - "mogas." It can be done - but you need to sweat the details. The other detail to sweat is that Lycoming's approval is for the engine only, not the airframe. TCs and STCs are still needed to be legal on certificated aircraft.

In the next articles, we will explain in more detail the controls we placed on "pump gas," why we decided to approve an automotive gasoline on some of our engines, and why Airworthiness by Design should be everyone's first consideration.

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