This EFATO, caused by the wrong fuel, made me thing whether any mixing is allowed
This has been asked before but nobody knew the answer.
I would think that just about anything would dissolve pretty well in avgas so say a 5% mix of avtur might be OK. But then it might affect the detonation margin massively…?
I don’t think 5 or even 20 % would do much except making increasing amount of soot and roughness. Lots of people have accidentally filled diesel in their gasoline cars (don’t ask me how they manage to do it). A medieval technology Lycoming would probably keep on running until the plugs gets all bogged down by soot. The rule of thumb is diesel in gasoline is OK up to a certain amount, bit gasoline in diesel is not OK at “any” amount. This is how I remember it, but I haven’t actually filled up a car in months running the much better option – electricity 
gasoline in diesel is not OK at “any” amount.
Quite common practice here when it gets really cold*, to keep the filters from clogging. I’ve done it several times and never had any trouble.
*which, with our very temperate maritime climate, means below -10 degrees C or so
In practical terms I guess the question is: what do you do with the information when you have it?
If someone else has misfueled your aircraft, obviously they can pay for a new tank.
If I misfueled mine, I think I would want to pay for a new tank of fuel (the question being, what do I do with the old tank?)
If I found out about it in mid-air, it would clearly have been too much.
Quite common practice here when it gets really cold*, to keep the filters from clogging. I’ve done it several times and never had any trouble.
*which, with our very temperate maritime climate, means below -10 degrees C or so
Sounds like a very dangerous practice for the diesel pump. Here in Norway, the oil companies changes the diesel according to season. Winter diesel and summer diesel typically. But ordinary diesel (summer diesel) won’t start to crystallize before below -20C. A common myth is the winter diesel is diesel mixed with kerosene (Jet A1 essentially), but this is not the case. It is a special product from the refinery designed to have the correct lubricity. My ex father in law used to fill with kerosene mixed with 1-2% 2 stroke oil. He did that for years with no problems.
Slightly off topic: Phil Croucher writes in one of his pilot textbooks that in emergencies of the military kind when you have no jet fuel but really have to take off to save yourself and the aircraft, a very close equivalent of Jet B can be made out of 1/3 mogas and 2/3 diesel fuel.
Turbines can use a wide range of fuel. The PT6 is even certified to run on Avgas for some time.
I wouldn’t mix diesel in any quantity with avgas, it can completely change the detonation characteristics even in small quantities.
Yes – 50 hours on 100LL, IIRC, for a Citation. Then a mandatory and expensive inspection 
Yes – 50 hours on 100LL, IIRC, for a Citation. Then a mandatory and expensive inspection
Depends on the Citation. The older ones with JT15 engines tolerate a certain amount of AVGAS (3500 gallons between overhauls IIRC). The newer ones with PW530 series engines don’t like AVGAS at all and will go straight to see Mr. Pratt and Mr. Whitney if accidentally operated with it. (Don’t know about Williams and Honeyell engined series though).
With respect to piston aircraft engines, even a small amount of JetA mixed with Avgas will significantly reduce the octane. The engine will run at full power for a minute or so before catastrophic failure. Jet A and Avgas mix and retain the Avgas color. The test that we used to detect proper Avgas fuel, was to pour a small sample of the fuel on a white piece of paper. After a minute or so, it will evaporate and not leave a residue. If their is any JetA in the mix, it will leave an obvious oily appearing residue.
