Unleaded fuels and exhaust valve recession — General Aviation News

By Ben Visser · May 30, 2024 · 33 Comments

A few years ago, I was giving a talk during a state aviation maintenance symposium when someone in the audience asked if there where any potential problems associated with the new proposed unleaded 100 octane avgas.

I explained that there could be a problem with an adequate rich knock rating for the fuel, which could lead to knock complaints in large engines and possible exhaust valve recession on some certified aircraft engines.

At that point someone else raised his hand and stated that he read on the internet that exhaust valve recession was a complete myth and there was absolutely no risk of this occurring.

I then pointed out that in the 1980s an oil company in California looked over the ASTM D-910 specification for 80/87 avgas and noted that the spec only listed a maximum lead level of 0.5 grams per gallon and no minimum level. It then started selling an unleaded 80/87 avgas. There were no problems for a while, but then there was a rash of engine failures because of exhaust valve recession.

Many of these problems occurred in engines such as the Continental O-200 engine powering Cessna 150s used in flight training operations.

A well-known engine rebuilder from the Twin Cities then got up and shared several slides of engines that he had worked on that failed after using unleaded auto gas and had experienced exhaust valve recession.

Following this information, the person in the audience who said exhaust valve recession was a myth said maybe the engine rebuilder was right, but he could not believe that because the article he read was on the internet, so it had to be true.

Since that time, I have written several articles about the possibilities of exhaust valve recession once all of the lead is deleted from aviation fuel. These articles had a fair number of comments, including some from people who did not believe that it could happen. I also talked to engine manufacturers who felt that it was not a concern.

And then the University of North Dakota, which has a large well-maintained fleet of aircraft, started running on unleaded Swift UL94 fuel and guess what? They started seeing exhaust valve recession on several aircraft.

So why is this happening at flight schools and not in private aircraft operated on Swift fuel or unleaded auto gas?

The big difference is the single source of fuel.

Many private aircraft were broken in on 100LL, plus many are used on cross-country flights, where they are refueled with 100LL occasionally. Most flight schools do all of their refueling at only one location, so most aircraft in a school’s fleet never see any lead in normal operation.

The reaction to the UND valve recession problem was interesting. Lycoming thought it could be due to the aromatic content of the Swift fuel.

But guess what? Most 100LL blends have contained aromatics since the early 1970s.

The only difference between 100LL and the Swift fuel is — wait for it — lead.

What are the main factors that prevent exhaust valve recession?

In the 1970s auto manufacturers found that hardened exhaust valve seats in liquid-cooled engines equipped with knock sensors helped prevent the problem.

The problem was also associated with load and the RPM that the engines experienced.

The problem for general aviation is that aircraft operate at high load, high RPM, with air cooling, and high exhaust valve and seat temperature — all factors that are in the danger zone.

Some of the discussion about the problem at UND included the thought that the university’s planes were operated on the lean side of stoichiometric, which would aggravate the problem.

But I believe they have multi-point temperature probes on all cylinders and they lean based on the monitor.

There is also a very real possibility for knocking on these engines using the 94 octane Swift fuel. Almost any knocking will raise the temperature of the exhaust valve and seat significantly and can lead to exhaust valve recession.

A multi-point EGT or CHT system may be mandatory for all aircraft before they are operated on an unleaded fuel.

In addition, knock sensors would help, but are not too practical in the aviation world because of individual cylinder assemblies.

What else needs to be done before we switch over to all unleaded fuel?

There are possible engine modifications, such as liquid cooling, but that would be a very high cost for every general aviation airplane out there.

I think the best answer would be to put an additive in all unleaded fuels to reduce or eliminate the problem. Something like MMT — Methylcyclopentadienyl manganese tricarbonyl, a fuel octane enhancer produced by Afton Chemical Corporation — might work.

The problem with MMT is that at normal treat levels the additive produces dark red whisker deposits on the spark plugs and other surfaces. These deposits can cause fouling and other problems.

But at very low levels it may not cause any problems and may prevent the recession problem.

What needs to happen to move forward towards an unleaded future for general aviation?

A repeatable valve recession test to determine if MMT or other additives — such as TCP, an FAA-approved aftermarket product that helps prevents lead fouling on valves and spark plugs — will eliminate possible exhaust valve recession.

But like lead and other fuel additives, TCP has its own health problems, including irritation of the eyes, skin and respiratory tract, drowsiness or dizziness, and depression of the central nervous system.