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seriousnotshirley t1_jdvlmym wrote

Reply to comment by pavlik_enemy in Does living in an airplane flight path, near an airport, pose a health risk? What happens to the lead from the jets fuel? by [deleted]

Student pilot here, the industry is in the process of moving in that direction. There's a lot of barriers that are slow to get past.

The first thing to understand is that the aviation industry moves slowly and purposely. If a car dies on the highway you coast to the side of the road and usually don't cause an accident. When a plane's engine has a failure that plane is going to need to land somewhere. There's a much higher risk of that causing death and property destruction. Sometimes you're over farmland and get lucky, sometimes you're not. Just google "plane crashes into house" to get a sense of it. There's many many other things that can happen when a plane's engine dies. Because of this the FAA takes slow deliberate steps in changing regulations. One of these is how they certify which engine in which airframes using which gas are permitted. Changes to this need to be certified by someone and for a long time no one had the incentive to do this. I believe getting unleaded fuel certified for most general aviation planes and engines took a change in the way they regulated this, possibly from political pressure.

Next, someone needed to develop an unleaded fuel that can run in existing airplane engines, and there needs to be refinery and distribution capacity. Most planes need 100 octane fuel. While many engines could run 94 octane many need 100 octane. Development on 100 UL didn't start until 2010. FAA certification didn't happen until July 2021 for a single plane (Cessna 172) using a single engine manufacturer. Last September they finally certified it for all planes and engines, though the owner or operator of the plane needs to get a supplemental type certificate for their plane (don't ask me why). Every owner/operator of a piston engine aircraft needs to pony up about $600. Great, except it's not available yet. The developer of the fuel needs to ramp up production capacity and distribution. This is where we are currently at. It's expected to be available in California next year and around the country in 2026.

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The next thing is that airports need to start stocking unleaded fuel. Until there are absolutely no planes that require leaded fuel airports will need to stock leaded fuel. Airports serve a public purpose to the flying community by making fuel available. You don't want to leave a big gap in the us where a pilot flying from point A to point B can't get the fuel they need. Now they need to keep another type of fuel. This requires new tanks and either new pumps or trucks. This will be the next roadblock.

If you want to learn more about general aviation you can do what pilots all do and listen to AVWeb’s Paul Bertorelli.

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pavlik_enemy t1_jdw2fza wrote

Thanks, I've watched his two videos on leaded gas and difficulties of developing new aviation engines. My thought was that since modern engines aren't made for extreme performance you could detune them and use lower octane fuels. But since 100LL was probably widely available and there were no concerns about lead there was no reason not to use it otherwise a manufacturer just crippled their engine design.

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seriousnotshirley t1_jdwdo3q wrote

Something to remember is that general aviation planes don't typically use modern complicated engine designs. Complicated engine designs are one more way for something to go wrong and planes often fly in very different conditions than cars. Moreover, 40 year old planes are flying 40 year old engines, or at least the designs are that old if the engine has been replaced. It's not legal to just swap a new engine type into an old plane.

GA planes also have tight weight tolerances. They just aren't designed to carry a lot of extra weight and so an engine with lots of extra stuff could easily use up a lot of useful capacity, especially on a four seat plane which is really "two seats and luggage" or "four seats and no luggage".

One of the common questions people have is why planes don't use automobile engines since they are commonly mass produced. The issue becomes that you need a transmission and the extra weight of the transmission makes it difficult to convince anyone to even try getting it certified. I only know of one attempt at it.

Using the Cessna 172 as an example since it's so common, the Continental O-300 engine used for many years is a 6 cyl 300 CI 145 HP engine producing about 230 ft-lbs of torque at 2200 RPM. Note: plane engines can't rotate fast because the tips of the propeller would break the sound barrier and the shockwaves would cause issues (and by issues I mean death and destruction). How many 5 liter/300 CI 6 cyl engines do you know of in cars and how many produce much more torque than HP?

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