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Caution Urged in the Use of Unleaded Mogas in Aircraft Engines

By Jack Neima, EAA 413636, EAA Canadian Council, Nova Scotia

December 2021 – In early 1991, Transport Canada accepted U.S. STCs authorizing the use of auto fuel in Canadian registered aircraft. Transport Canada document TP10737, which you can read here, provides guidance to aircraft owners who are strongly encouraged to be completely familiar with operational considerations and to be aware of the various limitations. Ethanol-free auto fuel has now been in wide use for over 25 years and many Canadian EAA members report very satisfactory experience with mogas, notwithstanding some of these limitations. One of the major considerations with this approved fuel is the clear requirement that only ethanol-free auto fuel be used. This has been relatively easy to obtain in Canada as most retailers have been selling ethanol-free premium grades. Recent developments suggest this may be changing and if you’re using auto fuel in your aircraft you are cautioned to pay close attention to ongoing developments.


The recent announcement by Maritime Fuels about their plans to transition to ethanol-blended gasoline is reason for concern. Modern auto engines are designed to run on alcohol-blended fuels but the problem is that many engines still in service in older cars, boats, aircraft, maintenance equipment, etc. are not designed for these fuels and it has been seen that using ethanol-blended fuels in these older engines has many unwanted consequences including corrosive damage as well as the breakdown of hoses and seals that will not tolerate alcohol, not to mention loss of performance and a reduction of power output. This is a BIG problem for our older, low-compression, air-cooled aircraft engines that were originally designed to run on leaded aviation fuels, specifically 80/87, a type no longer available in Canada. Avgas 100LL is still widely available but it contains four times the amount of lead that was in 80/87. Among other things, this lead is an octane enhancer that is needed in higher compression aircraft engines. While most of our lower compression aircraft engines will run on 100LL we are now faced with other problems, such as fouled plugs and sticky valves caused by the excessive lead. Many lead scavenging solutions have been put forward to try and deal with these issues including a variety of additives like TCP, AvBlend and others (approved), and Marvel Mystery Oil and others (unapproved). Some additives are approved for amateur-built/experimental only and not for certified aircraft. Usually aircraft engine manufacturers, like Lycoming and Continental, do not endorse the use of auto fuels but we understand Rotax approves ethanol up to 10 percent.

The use of ethanol-free mogas is approved in Canada in certified aircraft if the aircraft document set is updated to reflect the installation of an approved STC. There are several STCs available, but the best-known ones are owned by and available for purchase from Petersen Aviation Inc. and EAA. No physical changes to the aircraft or engine are required other than the installation of a tag on the oil filler neck and the display of special placards on the instrument panel and at each tank. Anyone who wants to avoid airworthiness and insurance headaches will go the STC route, but we know that some people have been known to cut corners and take the chance to burn mogas without bothering with the paperwork or the associated cost. I understand the thinking but it is a mistake in my mind and for my aircraft I have always had the STC to give me some peace of mind. For me, even though mogas is cheaper than 100LL avgas, it's never been an issue of cost — it's always been about what's best for the engine. My own experience over many years has led to over 1,000 trouble-free hours on unleaded auto fuel in a variety of aircraft. Simply put, these older engines like mogas better than 100LL.

Until now we've easily been able to find alcohol-free mogas almost everywhere, particularly in high test, but since many pumps have "May contain up to X% alcohol" it's been a best practice to do a simple test at the pump to make sure the fuel is pure gasoline before purchasing. In my case I usually buy high-test unleaded from my local Irving station, although from time to time I've bought from others, like Petrocan. Since the Irving high test has always tested clean, I usually test at the beginning of the float flying season and perhaps occasionally during the summer. It is a simple process to test for alcohol in fuel samples.

Upon reading the linked Maritime Fuels piece above, I contacted a friend who works at Irving's fuel distribution facility in Dartmouth to find out their intentions. He reported back that the Irving refinery in Saint John will start blending ethanol in their regular and mid grades sometime early next year, but they say they will not be adding it to the premium grade. I'm relieved to hear this, but still a bit skeptical and I plan to modify my process a bit to ensure I get the pure stuff. Given the recent announcements I'll be checking much more closely next season. The Irving refinery is Canada’s largest and many retailers sell branded fuels that are refined at Irving’s Saint John refinery.

We’re unsure if these developments are unique to Atlantic Canada but we suspect they are more widespread and likely part of a broader trend. With this in mind, we would suggest members pay close attention to the issue and share with us and other Canadian EAA members any local issues you are experiencing.

If you're interested in finding out which gas stations sell ethanol-free fuel, check out Testing for ethanol in fuel is straightforward. Just put a mark on your fuel tester about 10 percent from the bottom. Fill water to this line. Add your gasoline, shake and verify that the water didn't expand above your mark by absorbing the ethanol. The gasoline won't dissolve in water but ethanol/alcohol does. - Ed.

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