What options are there for those with older planes that require leaded fuel? Is there a relatively easy fix so that leaded fuel isn't necessary?
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$\begingroup$ Can you give an example of an aircraft that requires leaded fuel? $\endgroup$– Mike SowsunCommented Feb 25, 2022 at 21:08
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1$\begingroup$ My understanding was that aircraft engines manufactured before 1980 would require hardened valve seats and newer valve guides. I hope that isn't true, because of the bad effects lead has on the engine, not to mention the lead, which I'm pretty sure isn't a good thing to be breathing! $\endgroup$– Indelible SnarfCommented Feb 25, 2022 at 22:16
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1$\begingroup$ In car engines it was the cast iron heads that were damaged by unleaded fuel valve recession. I expect any high alloy seat material would resist the wear. $\endgroup$– blacksmith37Commented Feb 26, 2022 at 17:11
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$\begingroup$ The problem concerns older exhaust valve seats being softer than hardened seats currently used. Lead provided a protective coating preventing the hot valve flange from welding to the seat. Unleaded fuel removes this protection resulting in valve-to-seat welding. Upon opening the valve causes damage by plucking the weld from the seat and not subsequently closing tightly. On subsequent ignition and exhaust, the valve and seat will burn from hot exhaust blow-by. Hardened valve seats prevent this. But originally, it was a lead additive. Details are in links given in my answer, below. $\endgroup$– Thomas PerryCommented Feb 27, 2022 at 16:55
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4 Answers
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They don't require leaded fuel to run. The real problem is, 100LL has too much lead, for those engines intended to run on 80 octane fuel, to handle. Today, 80 octane engines are much happier running on alcohol free automotive premium unleaded.
Leaded fuel provides a minor benefit by lubricating valve seats (there is always some sliding contact as the valve centers in the seat while closing, depending on how loose the valve guides are), but aircraft cylinders went to harder valve seats some years ago the way cars did in the 80s, and seat wear is a negligible issue.
Otherwise, the lead in the fuel is nothing but grief for the engine itself and if it wasn't necessary for the octane rating, the engine would be happier without it.
Anyone with an old 80 octane engine should he using mogas if the higher vapour pressure isn't a problem (lower boiling point, and higher tendency to vapour lock).
If your concern is the introduction a new unleaded avgas to replace 100LL, it'll be happy days for all those 80 octane engines. It's nothing but a plus to have a fuel with avgas's stability and low vapour pressure, but without the tetraethyl lead regardless of the octane rating.
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1$\begingroup$ You can run all kinds of engines you'd think wouldn't like mogas, even supercharged engines like the R985, and certainly non-turbo engines rated for 100LL octane also run fine on 94 octane mogas. My old glider club had thousands of hours of running "high compression" O-360s, a 100LL engine, on premium mogas. I run my homebuilt's O-290 on mogas and it did away with having to dig the lead out of my lower spark plugs at 20 hrs after they started to misfire. Current mogas can be a problem if the fuel system under the cowl has too much heat exposure. I put a heat shield on my engine fuel pump. $\endgroup$– John KCommented Feb 26, 2022 at 2:45
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Well, yes, there are workarounds regarding the use of leaded aviation fuel. This link provides some answers about the use of unleaded aviation fuel in aircraft piston engines. See this answer for a general perspective on the use of unleaded aviation fuel. The following from that answer should be noted -
In July of 2021 the FAA approved Supplemental Type Certificates (STCs) authorizing use of G100UL high-octane unleaded avgas in aircraft piston engines, without modification. At least this is something to consider, in the foreseeable future piston-engine aircraft are not likely to be rendered unserviceable due to the unavailability of leaded avgas, or the need to modify engines to use unleaded G100UL fuel.
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To first answer the question. For many, almost all older aircraft you may avoid leaded fuel in one of two different ways:
- Fly on mogas. As mogas is simply a mix of various stuff you take your chances that one day the mogas will be "off" enough to create a problem. (First check that all rubber components and inside tank coatings and stuff like that can handle alcohol).
- Fly on one of the unleaded Avgas formulations allowed by you engine manufacturer. Type and availability varies between locations. As aviation engines has a limited life span, often on the order of 2000 hours, you could probably convert you plane at next engine change without any extreme extra costs.
Now a few words why it has not happened. There are two different problems around the move to unleaded aircraft fuel.
The first problem is related to the supply side with 100LL beeing a niche product, used in diminishing volume over time. There really is very little economic reason for the fuel providers to introduce and sell and support one more type of aircraft fuel at airfields. It is possible to find unleaded aircraft fuel in some markets and they tend to work very well without any specific problems for the airplanes that can use them.
It might be tempting to assume that you could switch to mogas straight away. And it will often work, but without guarantees. Gasoline is a mix of various stuff, where the producing plants selects the lowest price components at any time. As such there is no guarantess that mogas will have the same composition from time to time, except that it will work in most auto engines. The production might involve changing the composition so that it is "thicker" in summer (as the thicker components might be cheaper) which generally is no problem for autos. But aeroplanes fly high up where it is cold even in summer, and the thicker components may freeze. Avgas 100LL (and other aviation fuels) have specific limits on what temperature they should be able to sustain as well as how high up at lower air pressure they should not boil off. The limits for mogas are often not really defined. In addition mogas often contains alcohols, metanol or ethanol, which is no problem for a modern car but can dissolve certain rubber or plastic components used in older aeroplanes -- remember they were never expected to handle alcohols. Regardless, lots of smaller enginges and smaller operators run on mogas without experiencing problems.
The second problem is the structure of consuming side of Avgas. The main part is used by the larger two motor planes used by aviation companies for short hops or special flights. These tend to run under commercial FAA rules, meaning that they have to follow all regulations to the letter. And one of the regulations state that the fuel used should follow the engine manufacturers minimum requirements. And they seldom allow the use of anything else than 100LL for their larger turbocharged engines used in these planes that use most of the 100LL. As the engine manufacturers will be responsible, possible sued for damages, they have a difficult time allowing mogas. They could start programs to certify unleaded aviation fuel but it may need degrading performance on the large turbocharged engines -- after all the 100LL was "invented" to allow high performance. For smaller aircraft engines this has often already been done and they allow unleaded fuel. But as the larger engines have not been certified, even if the fuel company would offer unleaded fuel at airports, the largest buyers would not be allowed to use it, which sort of completes the cycle.
I believe that the change has happened already or is very imminent when it comes to smaller, private type of flying -- they can and often do fly on unleaded fuel. For the larger planes flying commercially and beeing the large buyers of 100LL my guess is that they will over time probably convert to Jet fuel instead, most probably by completing the change over to turboprops or perhaps diesel type engines.
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$\begingroup$ You use premium unleaded in part for the octane safety margin, and in part because it is normally alcohol free. You still test each batch anyway just to be sure using the simple water volume test. In Canada at least there is no distinction between commercial and private use for an airplane with the mogas STC. Many bush operators switched to mogas back in the 90s when 80 octane production in Canada stopped. The air service I worked for way back started using it in their DH Beaver with no problems. $\endgroup$– John KCommented Feb 26, 2022 at 15:05
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I'm not 100% sure if this is applicable to aero engines, but there are fuel additives that you can buy for older engines that provide the same anti-knock properties as leaded fuel without actually using lead (which we all know isn't great for human or environmental health!). We use such an additive for our vintage cars, about half a cap-full per full tank, and it has definitely helped the engine out, particularly with all of the different additives present in modern fuel to help with emissions, etc.
