This has been an interesting question for a number of years, but most people haven't been paying attention.
The heat was recently cranked up a notch by comments made by the new President of Teledyne Continental Motors, Rhett Ross.
In an interview with AvWeb Ross stated that he thought it would only be a matter of time before aviation was "forced out" of using 100LL. His company is proceeding full speed in the development of a 300 hp diesel engine that will be the first of a series of jet-fuel burning diesels from 100-300 hp. You can tell by the fact that they hope to have this new engine certified by late 2009 or early 2010 that Continental considers this a priority issue.
But is there a threat of 100LL disappearing in that sort of short term window?
The answer is an emphatic "yes" and GA needs to be ready for it.
We used to have four grades of Avgas - 80/87, 91/96, 100/130 and 115/145. The refining industry just stopped making each one of those over time and it caused some real heartache, especially for owners of aircraft that needed 115/145, because they couldn't go to a higher grade. A lot of engines stopped flying and some aircraft left service or got re-engined to turbines.
A few years ago there was a specification developed for 82UL (for unleaded), but no refiner has shown any interest in making any of it. Too small a market to bother.
What might cause 100LL to go away?
There are currently two different types of threats to the one remaining grade of avgas. The first is economic and the second is environmental.
From a refiner's economic perspective 100LL is hard to justify making at all. The market for it is tiny and spread out far and wide. The specifications for it are very tight which makes it expensive to produce. It also needs special handling at the refinery and during transport, because it contains tetra-ethyl lead (TEL) - it can't come into contact with other fuels. It requires dedicated everything as a result.
Overall making 100LL is a pain for the refiners and they wish they didn't have to do it. There are only two refineries in North America that make 100LL.
There is also only one company in the whole world that makes the TEL that is an essential ingredient and that is Innospec of the UK. If they decide to stop making this very toxic substance 100LL cannot be made anymore. On any given day the world has about a 30 day supply of 100LL on hand. Gone are the days of vast quantities of gas being stored - it is all "just in time" delivery these days.
These economic factors make 100LL's existence very precarious - today, not just years from now.
Everyone knows that leaded auto fuel was phased out in the early 1980s. Why? Because lead in gasoline is a toxic substance. No matter what the combustion process or temperature the tailpipe products of lead combustion are toxic to humans and most likely cancer-causing as well.
In the USA, the Environmental Protection Agency is once again looking at 100LL and why it is the last leaded fuel in use. Many environmental groups want it removed as well and not without good reason.
As Continentals' Ross pointed out this week, this will happen at some point, either from the economic or environmental perspective.
The question for pilots and aircraft owners is "what then?"
Auto fuel will still be available for some time, so if your aircraft is designed to run on auto fuel or can use it through an STC, you will be using that. I would bet that those empty 100LL tanks at the airport will start to be filled with premium autofuel, rather than sit empty.
What if you can't run on autofuel? Lots of aircraft can't, like 2007's best-selling new aircraft, the Cirrus SR22, for just one example. Cessna's new 350 and 400 can't either.
Companies like Continental will be very happy to sell you a jet-fuelled retrofit diesel engine a few years down the road. Hopefully you won't need one before they have them available. Many aircraft types will eventually have diesels available under STC. The folks at SMA diesels already have some STCs for their one product, the 230hp SR305 engine. It has been available for the Cessna 182 for a few years. There have been few takers, because of the cost.
If the cost of the SMA conversion is any indication then be prepared to pay around $100K for the conversion to jet fuel. The added bonus will be lower fuel consumption and longer range as a result. Of course for many older aircraft this cost will be several times more than the aircraft is worth. Also the removed gasoline engine will not fetch much when there is no fuel for it - perhaps scrap value.
I don't think used avgas-only powered aircraft are going to fetch high prices at that point.
So what can the lowly aircraft owner do in light of Ross's comments? Probably make like Boy Scouts and "be prepared". You now know what is going to happen when the 100LL taps run dry, so make a plan. Will you run on autofuel? Will you retrofit a jet-fuel burning diesel. Will you transition to rubber band power?
Just don't be surprised on the day when Ross's prediction happens and general aviation is "forced out" of 100LL.