It's a few days since I have been able to log on to PPRuNe, so quite a few replies on this since I last visited !
I just want to clarify a couple of points.
Gasax says, quite correctly that
The simple fact is that low compression engines were not designed to run on 100LL and all this stuff about how dangerous Mogas is in comparison conveniently ignores the fact that these engines were not certified to use 100LL.
Well true, the early engines were not certified on Avgas 100LL as it did not exist at the time. Neither were these engines certified on Mogas. Avgas 100LL is a closer in its properties to the fuels that the early engines were originally certified on in terms of boiling point range and hydrocarbon content. Believe me, I work in this field.
Don't be fooled into thinking that because Mogas is an unleaded fuel of mid 80's MON octane number then this is a close analogue of leaded Avgas 80/87, or even the pre war unleaded Avgas 80; the base fuel composition and properties are quite different.
All of these engines are now certified to run on Avgas 100LL, but only some aircraft carry limited use STC's to use Mogas. The use of Mogas is not approved at all by the engine manufacturers (Rotax aside). This should tell you something.
As I have said in my earlier posting, Notice 98 does put a limitation on altitude and tank temperature for Mogas use, and notice that it is a tank temperature NOT an air temperature.
Think about this for a minute in this scenario.
You fly on a nice summer's day to visit another airfield and leave your aircraft parked outside in the sun. Air temp can be low 20's, but the sun beating down on the tank can easily raise it's temperature significantly above the air temp. You have no way of measuring the tank temperature, so you have to guess you could be OK, but don't know.
What will you do if you suspect that you are near the limit, drain the fuel and replace it? Postone your flight until the next day? Or will you do what most people do, not even realise that it could be a problem and set off for home. The AAIB will investigate the crash and conclude that you were probably outside the guideance of Notice 98 and so the CAA is in the clear. The fault is yours. Notice 98 does not get changed, as there is no need to -you did not observe the restrictions. People will also continue to read notice 98 and assume that, if their aircraft has an STC then they have carte blanche to use Mogas, but they do not.
As has been said before, Avgas is for aircraft, Mogas is for cars. If you use Mogas for aircraft do not think for one minute that it is as safe and controlled as using Avgas.
This brings me on to another point.
Philip Whiteman makes another point.
However, avgas has too much lead for engines like the A-65, and aerodrome fuel storage/throughput can, I suspect, be poor enough to allow more water into the fuel than happens at busy--and well-run--garage forecourts.
Well fair point on the Lead levels for some engines that run notoriously low combustion temps like A series Continentals, but fiitting a hotter plug and doing an engine run up at the end of a flight is normally enough to let the Lead scavenger in Avgas 100LL to do it's job and clear plugs. The run up involves getting to the parking spot, stabalising the CHT and running at 1800 rpm for 20 - 30 seconds before going back to idle and immediately bringing the mixture lever back for a lean cut.
More info on this can be found in Lycoming's excellent "Flyer" publication - advice that transers to the TCM engines just as well.
As for the water issue - I would have to differ with Philip.
There are rules covering the storage and handling of Avgas in a CAA publication called CAP 434. This stipulates not only equipment standards for storage that are beyond those required for storing Mogas, but also that Avgas tank and filter drains must be checked and cleared of water and debris every day, after a bulk fuel receipt and after rain or snow. Not so for Mogas.
There are also filtration requirements. Speaking for Shell supplied Avgas sites, the final filtration before aircraft delivery is a water absorbant filter. This contains media which chemically reacts and traps any water in the fuel and so Shell can, hand on heart, say that the fuel supplied is filtered to 5 microns for particles (human hair is about 20 microns in diameter) and contains less than 10 parts per million of water.
There are no tank draining or sump inspection criteria for Mogas tanks, and neither is there any filtration requirements for the outflow. If you are lucky your local Mogas forcourt may have a plain mesh filter of about 200 microns. If you are lucky.
There are plenty of people who have run for years on Mogas without a problem, but my point is that it is incurring an additional risk that one day might catch up with you. Remember that no public cat aircraft are allowed to use Mogas. Ask yourself why this might be? It is because inflicting this additional risk on the fare paying public has been deemed unacceptable.
My view is that it is unacceptable for me and my loved ones also.
Good luck.