In practice, I'd have thought it would be tiresome trying to ensure that the composition was correct.
Scenario 1) - you add the additive to your aircraft's fuel tanks. You are billed for 10 gallons of fuel, but realise that you failed to check how much was put into each tank - was it 10 into the left tank, 10 into the right tank, or 5 into each. How much will it matter if you have too much or too little of the additive? Will you have to stir the fuel in the tanks of a high-wing monoplane after it's been delivered, in order to mix the additive properly.
Scenario 2) - You own a fuel bowser and you add the additive to it whenever fuel is delivered. But if you are buying a substantial amount at once, why not simply get it delivered ready-mixed and save the hassle?
My understanding is that all the additives invented so far are pretty toxic, so at the end of the day I'm not sure I'd want to handle them neat in any case.
You could invent a device to optionally inject the additive into the fuel-stream as it came out of the nozzle into the tank, so that the same tanker could supply unleaded or leaded fuel. And if you designed it carefully nobody should need to come into contact with the additive.
One way of boosting octane would be to add aromatic hydrocarbons. For example, technical xylene (natural mix of ortho-, meta- and para-isomers plus ethylbenzene) has a RON of about 118. It is somewhat toxic, but not nearly so toxic as tetraethyllead. You have to add a fair amount of it to the fuel, but it is inexpensive - in some countries it may be cheaper than avgas, so in principle you could run your engine on neat xylene. During WWII, benzene and toluene were routinely used as fuel for highly supercharged piston engines; you can use them as well, but they are more volatile, more toxic and have a lower octane rating (though still above 100).
Inspected a couple of cylinders from an air cooled test engine recently 1 Unleaded with Castrol booster 2 Unleaded with Tetra Lead
Both were clean, no corrosion, no fouling. Plugs were clean. Valve seats and valve faces normal.
Many years ago the BGA conducted some tests on 4 star fuel. They used a Beagle Airdale at the time and the tests were overseen by their Tech man Dick Stratton. Tugging is as good a test for fuel suitability as anything and does not involve a passenger,plus the tug is usually within close proximity to an airfield,so perhaps the BGA can be prevailed upon to help out again.
Adding stuff to fuel is not nearly as problematic as one might think, because many pilots flying at high altitude add stuff already. I add ~ 0.5% IPA when doing certain flights, as a precaution even though there is no known case of fuel icing on a TB20.
But the amount would have to be small in % terms. Even the 0.5% is a hassle to carry if you had to do it on every leg of say a 20hr trip. That's why the pros use EDME because that is only 0.15%.
Turboprop pilots add a fuel icing inhibitor called Prist. Very toxic, stinking and tedious to apply. Some fuel stations serve kerosene with Prist already mixed in, others don't. In 2009, a PC-12 crashed in Montana because the pilot did not apply the mandatory icing inhibitor.
So it is possible to manually apply mandatory fuel "enhancers". Lycoming/Continental could just issue a SB allowing the use of 91UL + octane enhancer. They have done that in the past, e.g. when 100LL replaced the higher leaded 100.
I'm sure this octane enhancer would be a nice substance, very useful to put in your neighbor's fish pond when you're having a little quarrel.
But avtur takes very cold temps to freeze the water in it - the BA 777 report is standard reading material there...
Actually not that much. The PC-12 POH makes it mandatory to use Prist for cold weather operations. Bigger airplanes heat the fuel which they return to the tanks, small turboprops don't and -40°C is not that rare at cruise level. The Jetprop has the same problem as the PC-12.
achimha, according to MSDS on Prist's site, Prist Hi-Flash is diethylene glycol monomethyl ether - neither stinky nor very toxic, though it can damage paint (many years ago, I participated in the restoration of ancient frescoes in an Orthodox monastery in the north of Russia, and we used it in various mixtures to remove layers of oil paint on top of egg tempera). The stinky and toxic liquid you are talking about is probably tetrahydrofuran, also used for this purpose.