Citing Valve Damage, UND Drops Unleaded Fuel And Returns To 100LL (Updated)


After an extensive trial, the University of North Dakota’s flight school has dropped Swift UL94 fuel and resumed use of 100LL. The school said ongoing maintenance monitoring of aircraft using UL94—almost exclusively Lycoming-powered Piper Archers and Seminoles—resulted in measurable exhaust valve recession. The school made the switch back to 100LL on Oct. 27.

Partly because students were interested in a less-polluting fuel, the school switched to Swift Fuel’s ASTM-spec UL94 in late June. In four months of flying totaling 46,000 hours, the school found evidence of significant valve recession in some of the Archers. The school’s director of maintenance, Dan Kasowski, told AVweb Wednesday (Nov. 8) that the total number of aircraft impacted isn’t known yet because the data is still being collected. The data is being forwarded to Lycoming for analysis. “The fuel is on-spec, so that’s not a problem. I don’t want to speculate on what it could be. We’re waiting for Lycoming to tell us,” Kasowski said.

Valve seat recession was a known problem when unleaded fuels were introduced in the automotive market starting in the 1970s. As explained in this AVweb video, the precise mechanism is debatable, but hardened valve seats and cylinder heads tamped down recession damage. Lycoming addressed this issue in its cylinders during the 1990s and was believed to have valve seats and guides suited for unleaded fuels. Lycoming said it was “proactively evaluating” the data received from UND and would provide appropriate guidance based on its analysis. As of today’s deadline, Swift’s Chris D’Acosta confirmed that Lycoming is looking at materials, pilot operating methods, flight telemetry and additional data to understand the findings at UND. “The comment attributed to equivalent spark plug fouling and replacement vs. 100LL is generally not consistent with our own experience with UL94. This may be an indication of different operating methods at UND than elsewhere,” D’Acosta said.

UND’s Jeremy Roesler said when the school made the switch in June, it did so only after setting up a maintenance monitoring program to track potential issues with UL94. “We are overcautious,” he said. “When we started finding these issues, we needed to ask ourselves how bad does it have to get before we switch back to 100LL?” he said. With intense daily flight operations, aircraft down for maintenance were a worry for UND. “We’re big on how it affects our students. It is causing some delays. So it’s not in our best interest to accelerate back to UL94,” he added.

To monitor for cylinder wear, the school conducted regular compression checks on its fleet of Archers and also did the so-called “dry tappet” check. This involves removing and cleaning the tappets and reinstalling them with the pushrods in place. The clearance between the rocker arm and the valve stem is then checked. If the valve seat is recessing, this clearance will progressively diminish as the valve recedes farther into the cylinder head. Kasowski said the Lycoming specified minimum clearance is 0.028 inch and some of the cylinders exceeded this limit. If the recession is deep enough, the valve won’t close against the seat and power loss or burned valves can result. Recession was measured only on exhaust valves, not intake valves.

In addition to data sent to Lycoming, UND is also sending both cylinders and engines for further analysis. Kasowski said after 46,000 flight hours, the school had about the same replacement rate for spark plugs as on 100LL, nor was spark plug fouling noticeably reduced.

Another operator we’ve been monitoring for more than two years is Rabbit Aviation Services in San Carlos, California. The company oversees maintenance and fueling of a pair of flying clubs totaling nearly 10,000 hours of flight per year. Rabbit CEO Dan DeMeo said experience with UL94 has been universally positive with less spark plug fouling and cleaner oil. Rabbit has been offering UL94 for two and a half years.

We’ll update the story as more information becomes available. 

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      • I disagree, and probably you do too, if you are using gasoline in your incredibly reliable Honda or Toyota. Lycoming’s metallurgy is just not keeping up. Most of the advances in aircraft power plant design and fuel efficiency, have come from better metallurgy and tighter tolerances and component design. I would also put a side bet on the Avstar issue mentioned by I Silver. Leaning is definitely different with this carb.

        • Actually most of the automotive efficiency improvements are in part throttle operation and mostly due to direct fuel injection and variable valve timeing which allow for stratified charge at part throttle and the use of the Atkinson cycle for the latter . Atkinson cycle allows a
          Larger expansion ratio vs the compression ratio, boosting thermodynamic efficiency.
          These are both driven by digital engine control

          Metallurgy supports these technologies but is not the controlling technology.

      • Yes, the problem is ancient engine technology. Air cooled engines with no electronic fuel injection and ignition with with knock sensing that could automatically adjust the timing. Also add variable valve timing. You can run most automotive engines on any unleaded pump fuel even if is turbocharged although you will get more power with higher octane if you have a higher performance engine. Too bad GM does not make modern aircraft engines.

        • Variable valve timing probably wouldn’t help much in aircraft engines. They operate in such a narrow RPM range that the power benefits would be minimal at best. Cars are another story when operating from 1500 rpm at low power cruise to 6000+ at high power demands.

          Aircraft have the option of variable ignition timing with a safe failure mode. Engine knock monitoring along with it would be a great addition.

          The bad news is poor metallurgy and/or tolerances, if that’s what happening on the Lycomings, isn’t going to be solved with any of those ideas. GM is well-known for failed lifters on their Displacement-on-Demand engines and they have all the technology you mention. The ultimate cause was usually out-of-spec engine blocks.

          • Variable valve timing allows for variable effective compression ratio. If you are running lower octane fuel, opening the intake later or leaving it open during the beginning of the compression stroke will change the effective compression ratio or leaving some of the exhaust in the cylinder will lower the burn temperature.

        • To expand on my previous post: I just did a compression check on my 300 hour Rotax 915 iS, an electronically fuel injected, variable timed, turbocharged engine that runs on avgas or mogas. My results were 80, 80, 79, 80.

  1. I ran a collegiate flight program that was the launch customer for the Archer TX in 2013. We had extensive valve problems with those engines on 100LL The problems then crossed over into our O-320 equipped aircraft as engines were replaced with factory remans. We guessed that it could be due to a change in spec for the Avstar carbs installed as OEM equipment by Lycoming. The bench flow spec resulted in the engines running leaner at high power as compared to the older model MA-4 carbs. So I’m not sure the apparent premature wear can be linked to 100LL. We defiantly saw premature valve recession, burnt valves and valve guide wear suddenly appear with the new carb specs, unrelated to a fuel change. We weren’t as big as UND, but we were flying 60,000 hours a year so it was obvious when the wear pattern changed.

    • IF the fuel is the issue, wouldn’t we see valve seat recession on all the cylinders in engines running UL94? If just a seat or two here and there scattered around the fleet it would suggest something else as the cause, Like the Avstar carb fuel distribution or lack thereof combined with operational practices. I Silver, what did you guys see for a pattern of valve problems?

      • Valve recession, discoloration and burnt valves. We started installing older overhauled carbs and the problems resolved. It was pretty cut and dry in our view. If your carb is running lean, there’s less excess fuel to assist with heat scavenging from internal surfaces.

  2. I am surprised they didn’t change to the new
    GAMI’s 100 UL fuel. That might help solve the mysteries that are associated with unleaded fuels. It might answer the question is it the octane or the lead that might cause the problem they are seeing.

  3. If “students were interested in a less-polluting fuel” then I have to wonder if this flight school (as well as every other in the US) has run the numbers on investing in a Pipistrel Velis Electro?

    I appreciate that because it’s electric any trainee pilot would have to do some time on top with a fossil fuelled aircraft but, like learning to drive a car, eliminating much of the mundane and fiddly bits at first (mixture, carb heat, dipping tanks, yada, yada…) would allow students to spend more time concentrating on actually flying with a proportionate increase in rate of learning.

  4. Whatever the root cause, certainly gives pause to the idea that universal acceptance without universal testing could still be a concern.

    Some years ago I was offered the opportunity to be a lab rat for one of the alternative fuels, but declined when the sponsor wasn’t willing to pony up for any potential engine damage. Realize they were probably concerned about any maint issues I created, but since my mason jars of spare cash had already been emptied, I didn’t see a reasonable upside without their backing.

    All this circles back to the biggest concern…it’s technically doable, but is it economically (engine test/adaption, infrastructure, pump price) doable without parking the majority of the rec fleet in the weeds.

    • “Whatever the root cause, certainly gives pause to the idea that universal acceptance without universal testing could still be a concern.”

      Great comment, Rich. This is why we chose to stay with the PAFI program, because of all the testing hurdles it imposes on us. The STC route is no easier, but it doesn’t provide the rigor or transparency of PAFI. Nor does it require an ASTM standard for the fuel, an essential safety and legal requirement for the FAA and OEMs.

      The UND experience will certainly give us pause and reason to think about VSRs in our unleaded fuel even though we still don’t understand the cause of the UL94 failure. Rest assured that we have taken notice and will ensure we don’t have similar issues.

      • Isn’t the ASTM Standard for fuel based on 1950’s technology and thus has a large, acceptable standard deviation in consistency? The technology to refine fuels today is much more precise. It seems that an updated standard would promote a better product. We’ve seen this in the auto industry. I’ve heard there exists a wide variation in 100UL at the pump.

        I don’t hold much hope for the UL94 because it is not a direct replacement for all that is considered the GA Fleet. Swift and others are working on a 100UL. GAMI has their G100 which I’d like to see broader use and testing of. It has been STC approved and advertises some significant benefits which include the POTENTIAL for longer TBO’s.

        I did speak with an FBO in Nor Cal who referenced some notable wear to valves in some engines using UL94 but with too small of a sample it is impossible to know for sure.

        Finally, tetraethyl lead was never intended to be a lubricant. It was added to eliminate knock and stabilize octane at altitude. It’s questionable why this article makes the assumption that UND thinks valve recession is due to lacking lubricant in fuel. UND doesn’t actually say this do they?

  5. Given the number of hours being flown, one would suggest that the valve “recession” was simply wear, and the monitoring had never been done before; likely nothing to do with the fuel change; just more monitoring.
    Other experience with unleaded fuel in Lycomings has been longer cylinder life; as measured by oil consumption and differential compression checks.

    • To add a bit to Brian’s post: was there a control group in this test? Such as a group of planes that continued to use 100LL while undergoing the same level of scrutiny?

      Or did they apply these same tests to the fleet for some period of time to establish a baseline before switching to 94UL?

      They may well have, but it’s hard to tell from this article. But if not, it could be as Brian suggests – a ‘problem’ that was there all along, just never noticed without the additional testing.

    • The article also says they “conducted regular compression tests”. Since the compression test is well known to not be a reliable indicator of engine health, either they were hopefully doing other evaluations, or their monitoring program was faulty. Were they also doing oil analysis during every oil change? Borescoping the engines? And as suggested, they would have needed a control group too.

    • Checking valve clearances was done every 100hr on every reciprocating engine when I worked there. I doubt it has changed, since it is a licensed repair station with standard procedures and checklists. Decades of records should serve as a sufficient control group.

  6. Back in the 80’s I took apart automobile engines
    The ones running on unleaded fuel had pitted and galled exhaust valves. None of the valves were reusable.

    The engines that ran on leaded fuel were caked with crud, but after cleaning, were perfectly good. I attested that the umbrella seals at the time were the culprit of the crud.

    I never took apart an engine with valve seat recession that ran on leaded fuel.

  7. “Whatever the root cause, certainly gives pause to the idea that universal acceptance without universal testing could still be a concern.”
    There’s too many variables to this puzzle. Yes, slow transitioning is the way to go. After the “testing” by all involved and data collections established we would be in a better position to evaluate what is really going on. Other than that it is speculation over assumptions.

    • There still seems to be something missing here. There has been a mogas STC for Archers to run on UL94 for years, so UND wasn’t really doing anything different other than using Swift’s UL94. But it was formulated to an ASTM spec, so it shouldn’t have been any different.

      • The difference may be in the recommendations. Swift has always maintained that lead has absolutely nothing to do with valves. We (Petersen Aviation) acknowledged that it does relate to valve seat recession, especially when breaking in a newly overhauled engine, which is when the need for lead is most critical. However, it is also important to know, when running exclusively on unleaded, that you must operate slightly rich of peak, not lean of peak. Lean of peak operations on unleaded can lead to engine damage. Our recommendations for using autofuel or UL91/94 include that information specifically.

  8. In the classic car world this is not unheard of. Years ago, I restored a classic (1958) British Sports Car. Now I’m life-long pilot, and a not-too-experienced mechanic, but doing my homework before launching into the engine overhaul, I found that some restorers recommended hardened valve seats to avoid the possibility of seat recession or loosening. I took the entire engine head to a specialty shop and they told me it wasn’t necessary. I asked them to do it anyway. I’ve been driving the car since 2015, and being a really cool sports car, I usually drive it pretty hard. Haven’t had a single engine problem. BTW: I also add ZDDP (zinc) to the oil for extra lubrication of the guides, etc., although I’m skeptical of the conventional wisdom that lead lubricates the guides. I’m not sure if any of this is applicable here, but I hate cleaning “clinkers” out of my (Lycoming engine) plugs every year, even though I always lean aggressively. (The only time I use full-rich mixture is during a cold start or above 75% power.) Lead causes lots of problems in our little air-cooled engines and I’m hoping a good alternative comes along soon.

  9. Click on the attached picture in the article then zoom in on the seat area in question. There appears to be a precise radius of recessed seat towards the valve stem with a trough of missing material. The key is the precise radius of the trough, there is a clearly delineated failure of a SPECIFIC portion of the valve seat related to radius, call it 60 degrees. This would be the result of the valve not sitting square on the seat NOT a function of fuel octane or lead content. Maybe there was just a run of marginal valve seats or cylinder heads and UND’s scrutiny has revealed them. To me this seams to be a clear problem with geometry or metallurgy not the presence or absence of lead. No mention of detonation so octane is not involved. BUT, Lycoming can come to the conclusion that some or all of it’s cylinder heads are defective / marginal OR they can blame Swift 94UL, any bets…… Also, it should be pointed out, UND depends on Lycoming parts to keep their fleet operational, Publicly and perhaps prematurely (lot’s of unaccounted variables) throwing lycoming under the bus is not in their long term interest.

  10. If there is going to be a true comparison of valve seat wear possibly due to 94 UL, how much of this type of monitoring occured with UND with 100 LL prior to them switching to 94 UL? If there is no study before then the whole process of blaming this on 94 UL is moot. A study accurately done should utiilze planes running on 100 LL with the same engines and planes running on 94 UL, then let’s see after time what the differences are.

  11. Bertorelli has been “retired” for only a few days–and look at the discussion he’s caused! (smile). I would hope that this would be a recurring role for Bertorelli–throwing a “stink bomb” into the room and causing a discussion on “who the culprit is.” (laugh)

    This has been one of the most entertaining threads I’ve seen in a long time–KEEP UP THE GOOD WORK!

    I’d like to ask the question: “Since some attribute the poor results to improper leaning or fuel distribution–would simple fuel injection (non Electronic) have made a difference? Would using an IO-360 (Piper Arrow) have made a difference?

    The carbureted O-360 on a Piper Arrow was originally designed for 91/96 octane. I’d like to see the results of a study on engines designed for 80 octane–the 150 hp engines used on Cherokee 140s and early Warriors, as well as the Continental O-200s.

    I’d also like to ask–“Were these engines using EXCLUSIVELY 94UL–or did they get the occasional dose of 100LL while fueling away from home?

    We sell 94UL at my FBO–and have since it was first legalized–and have never had a problem. Yes, our airplanes do get an occasional shot of 100LL when fueled away from home.

    Even more telling–for years, we flew pipeline patrols in Cessna 172s, 150 hp Cardinals, 182s, and Citabrias–all engines designed for 80 octane, and they rarely got a dose of 100 octane. The engines all went FAR BEYOND TBO (3000 nearly 5000 hours)–but then, they weren’t being used in a training environment–only cruise (and it was at 65-75% power)–and the aircraft were flown 100 hours a month or more. Even though it was at low altitude, the operator DID teach his pilots to lean aggressively. Obviously, the airplanes weren’t getting anywhere near the number of takeoffs and landings of a flight school program like UND–or the “slow flight and stalls” of a training program–but they WERE being flown often–more like normal use than a flight training program.

    This “News ABOUT the News”–putting press releases in context–and “slaying aeronautical dragons” (debunking myths) would be the perfect “retirement” for Bertorelli–no deadlines–just report the findings–and Bertorelli’s acerbic wit would provide entertainment and education for us all!

  12. I agree that the damaged area in the 7 o-clock location looks weird.
    I had an exhaust valve recede into the seat and it was uniform around the seat.
    I have topped the engine [ GO-300 ],twice and this 600 hr cylinder was the only one to have valve problems. I suspect that the seat was not properly heardened.
    I run 100LL and lean aggressively on the ground and at all power settings lower than 75%.

    • Note that the Rotax 9xx are water cooled so the heads run 100’s of degrees cooler than the 350+ on air cooled engines, so the valves run much cooler too.

      Maybe we need water cooled heads on our Lycoming and Continental engines.
      [ they are already about 30% oil cooled ..!!]

  13. I see a possible research design problem here. If UND had had a similar valve recession monitoring effort in place when running 100LL, or if they ran 94UL on half the fleet and monitored both, we could be more certain that this amount of recession doesn’t happen regardless of fuel.

  14. A Bertorelli “stink bomb.” … that’s GREAT! 🙂

    He’s probably just cleaning out his ‘in’ box but — still — THIS is an example of where he could keep his toes in the foray and keep us all on the edge of our seats … waiting. 🙂

    I think the new Rotax 916iS is the answer to all of this. Water cooled heads designed for mogas. Problem solved. They’re costly but so are new Lycosaurus’.

  15. Ironically, there’s a good discussion by Ben Visser on the fuels topic dated yesterday in GA News. I think he’s ‘on’ to a good idea in his article:

    Here in WI, I know numerous people who burn non-ethanol premium autogas (gas stations advertise that here) in their 80 octane engines AND switch back and forth to 100LL with no issues. They don’t leave the mogas stuff in their airplanes when winter comes for reasons of stability — they fill with 100LL — but other than that … I’ve never heard any of them having any issues. Back in the day when 80/87 was available alongside 100 or — later, 100LL — we’d see no problems with the 80 octane fuel but as soon as 100LL started becoming the only fuel available, lots of issues started popping up in the military aero club I worked at. The ‘old’ green 100 gas had twice the lead of the blue 100LL so be glad of that for most of the engines flying today.

    This whole thing blaming the minute amount of lead in aviation fuel as being some horrible monster killing the population is borderline lunacy IMHO. The ‘SMUF’ crowd — “Students Marching for Unleaded Fuels” (I just made that up!) needs to grow up and take a chemistry class or two.

  16. I’m not as much of a mechanic as most the posters here, but I do have experience selling planes into schools.

    Schools are stupid resistant to change and if they bought the cars for the students as well as they do the planes, the students would all be running around campus in Checker Marathons. Every new vehicle purchased would be different from the Marathon and after sufficient time those differences would be catalogued and used to point out why it was inferior. Whatever benefits the new car offered were quickly forgotten and dismissed.

    So in the end, I’m not surprised at all with their findings. They maybe right or wrong, but I’m not surprised. Nor am I surprised by a rush to defend Swift 94 because if only we could get rid of all the high performance planes, GA would be better. Right?

  17. The elephant in the room is the glaring absence of a control group in this fuel experiment. While the fleet may switch back to 100LL after burning UL94 there doesn’t seem to be a part of the fleet which continued to run 100LL in parallel with aircraft burning UL94.

    The lack of scientific rigor in this experiment, especially when it was carried out by a university, is, to put it mildly, underwhelming.

  18. These general aviation piston engines are unique situation with regards to exhaust valve/seat wear. They are air-cooled, low compression ratio, 2-valves per cylinder – hence a single exhaust valve per cylinder. This combination of parameters makes for a rather extreme situation with regards to exhaust valve temperatures. It may also be the case they are run excessively lean which can also exacerbate exhaust valve temperatures.

    I don’t know what materials Lycoming uses at this interface (exh valve seat face / exh seat insert) but this is potentially the area where an improvement may exist. Any proposed change to this interface would need to be fully proven out with extensive testing.

    • That testing is likely very expensive. Who is going to pay? This is how the FAA has been killing GA. Every improvement is a financial gamble with an ever dwindling return potential. We’re going to go down from 10,000 paper cuts.

  19. In addition to my comment above (regarding exhaust valve/seat interface) –

    Using a toxic fuel additive to solve a wear issue at a mechanical interface in an engine is a very old, outdated method to solve a problem.

    The vast majority of automotive engines, for many years, do not have wear problems at valve/seat interfaces. The main difference is these are liquid-cooled engines, most of them 4-valves/cylinder with two exhaust valves per cylinder, and the valve/seat materials were chosen and proven out for the non-leaded fuels in automotive use.

    Perhaps general aviation would be better served with more-modern liquid-cooled engines having better efficiency, can be run on the most available aviation fuel – jet fuel, and may also be quieter, easier to control and have less toxic emissions.

  20. As a previous poster suggested, why not include a few fuel injected engines in their fleet test issues to see if this is a fuel issue or a carburetor (fuel distribution) issue? Lycoming fuel injection systems do a pretty good job of uniform fuel distribution, but to be sure, add a set of GAMIjectors for more accurate results.

  21. Kind if ironic that the 100LL issue could be resolved for most of us here if not for the same knuckleheads who banished lead only to replace it with it with another problem (MTBE), and then prevent its use as a 100LL replacement in most states because you can’t reliably get ethanol-free mogas…and ethanol brings its own macro-environmental baggage. Sorry for the high compression guys if it comes down to it, but I’m not excited to pay $10/gal to subsidize their fleet if I could get a $4/gal solution available today after some state regulatory action.

    All this unintended consequence management is a reminder that just “doing something” is not always the best answer.

    • MTBE and ETBE are allowed in all 50 states for engines and aircraft certificated for MOGAS. FAA regulates AVGAS, not the States. EPA upgraded its UST standards in 2015 and >90% of AVGAS tanks are above ground, so the risk of groundwater contamination has been mitigated. If we make it through PAFI full-scale testing, you won’t have to worry about paying $10/gal for AVGAS.

      • No argument over who regulates avgas, but if no ethanol-free mogas is available in an area, then mogas as a cheap replacement for the low compression segment probably isn’t.

        As far as promise that $10/gal isn’t a threat for 100LL replacement…aviation doesn’t have a strong reputation for delivering solutions as cheaply as touted, I would welcome an outlier, but ref the MO state motto.

  22. What was proven and reported here is absolutely no surprise to myself or those who have studied the effects of unleaded fuel in air cooled engines over the last 20+ years. There have been several research papers published but none of them made the press and all of them were swept under the rug. Now that unleaded fuel is about to become an irreversible reality, the truth of its effects on air cooled piston engines can no longer be hidden from view.

    Just ask yourself – where did all the air-cooled automotive engines go when leaded automotive fuels went away? They were taken off the market or converted to water cooling. Sure some lame excuses were made about air-cooled engines polluting more but that was a ruse for the real reason. Exhaust valve seat erosion was shortening the life of air-cooled engines running in unleaded fuel to unacceptable short times that manufacturers could not support from a warranty perspective. Before anyone starts ranting about the continued use of air cooled VW engines in some countries running on unleaded fuel – I will remind you that (a) they are no longer manufactured for production cars and the ones still in use are mostly used in very low demand, low power situations – rarely producing more than 30% of rated power even when cruising on the highways.

    The exhaust valve seat erosion issue is just basic metallurgy – irrefutable laws of physics and chemistry. Air-cooled aircraft piston engines run at very high head temperatures that result from running at high (>65%) power levels for extended periods of time. We know that the typical the air-cooled cylinder head temperature is around 400F at typical engine power settings. That head temperature is but a crude average given position of the sensor and the thermal gradient from the combustion chamber surfaces to the sensor. With exhaust gas temperatures typically around 1400-1500F the exhaust seat is also running at around 900-1000F. That steel seat is being hammered by the steel exhaust valve also running at a higher temperature which then slams shut on the seat under considerable pressure where it then transfers its surface heat to the seat while it is closed.

    What none of the armchair physicists and mechanical engineers commenting here realize is that at 900-1000F the hardened exhaust valve seats are much softer. The identical metals of the exhaust valve hammering the exhaust seat causes micro welding and sublimation of the exhaust seat material which results in seat erosion. Why the seat and not the valve. Well you can argue all you want about that.

    Why doesn’t seat erosion happen when running leaded fuels?

    Besides the anti-knock properties that TetraEthyl lead provides to leaded fuels like 100LL, very few understand that the TetraEthyl lead additive has the beneficial side effect of depositing a microscopic layer of lead on the face of the exhaust seat and valve as a tiny portion of lead condenses out of the exhaust stream onto the slightly cooler exhaust valve and seat on every combustion cycle.

    The deposited lead is a dissimilar metal that insulates the two identical metals of the valve and seat when the valve slams closed against the seat. You cannot weld dissimilar metals and the lead deposited on the surfaces acts a buffer to absorb the impact of the closing valve against the seat.

    Take TetraEthyl lead out of the fuel and it is only a matter of time (about 500 hours) before the exhaust seat erodes to the point where the valve lash is used up and the exhaust valve fails to close properly and bingo – burned exhaust valve in a matter of minutes at high engine power. That is the good scenario. The worse scenario is that the exhaust valve face happens to microweld to the seat and after opening, the tiny weld slams into a different position as the valve rotates causing a leak that again burns the exhaust valve face in a matter of minutes.

    The results of the actual fleet wide testing performed by the University of North Dakota’s flight school makes clear that no matter what anyone wishes for – physics and chemistry laws cannot be bent. Engine life will be reduced to about 500 hours or less for the typical air-cooled cylinder before they will require the exhaust valve and seat to be replaced.

    While this article didn’t report on the amount of time that was accumulated on the various engines, the University of North Dakota’s flight school had the foresight to monitor the exhaust valve lash as part of their regular maintenance. Wonder why? Because they were obviously aware of the research data that has been published over the last 20+ years and they wanted to catch any problem before it resulted in an engine failure in flight. Good for them!

    I’m sure there will be plenty of commenters that we counter my explanation with – hey I know aircraft engines that have been running on Mogas for years! Right – and for everyone one of those – I can show you that they also burned 100LL when they had to fuel up at an airport with MoGas so the engine was getting regular doses of leaded field deposits on the exhaust valves and seats. The University of North Dakota’s flight school was able to run their engines strictly on unleaded fuel which then depleted and lead deposits on the exhaust valves and seats and allowed the seat erosion to take place.

    What is the solution? How can air-cooled piston aircraft burin unleaded fuels?

    Either convert the engines to water cooling or plan on 500 hour top overhaul times for every air cooled engine that is flying.

    Why do water cooled engines not suffer the same problem? Because the exhaust seat temperatures are around 300F not 900-1000F allowing their hardened metallurgy to resist the impacts of the much hotter (but still way cooler at around 400-500F) exhaust valve when it slams shut against the seat.

    Bob Atkins
    Liquid Cooled Air Power

    • The have sat on delivering 80-91 unleaded to GA for around 40 years now!
      We don’t NEED to wait another 40 years for “one fuel” when we can have 2 or 3 at the airport like we used to have in the 50’s and 60’s. Sorry but AOPA and FAA just don’t care about supporting small planes.

  23. The “news” is short on data.

    How was VSR tracked, before and during the study period?

    Simply finding valve damage, by color and pattern change via boroscope, for instance, is only a part of the needed data. Dry tappet clearance, before and after, would need correlation to start to call it VSR.

    Knock sensors have not worked on the huge, noisy, air cooled aircraft cylinders.

    I would not expect a training environment to protect 94 octane-fueled legacy engines, even at lower, say 7.5:1 compression ratios.

    Without the data, this “news” reads like a supply-side economics issue and business decision. I don’t see the data “droids” we are looking for.