David Pears wrote:

I though gas turbines (as used in aircraft engines) would run on just
about any old liquid, as long as it has a hint of hydrocarbon in it?

The problem is that turbine fuel ( effectively an aviation kerosine ) has to
also satisfy several requirements, both with regard to engine performance and
safety. The fundamental ones are flash point ( temperature at which the fuel
will give off sufficient vapours to ignite if a flame is applied ) and freezing
point ( engines tend to stop when fuel turns to sludge ).

Avgas has suitably low freezing point, but the flash point is far lower than
the fuel systems on a commercial Jet A1 fuelled airliner are designed to cope
with. Military airliners often use a wide cut kerosine with a flash point well
below airfield ambients.

If a plane is carrying passengers, it's usually running on Jet A1, a narrow cut
fuel with a flash point above most airfield ambient temperatures ( 38C ).
Flash point is also the easiest method of detecting whether a kerosine has been
contaminated by a gasoline. The 1997 explosion of the "empty" centre tank in
TWA Flight 800, has reignited interest in fuel flammability issues.

Kerosines tend to have more lubricity than gasolines ( higher sulphur and
viscosity ) but high pressure hydrotreating and hydrocracking have made fuels
"harder" ( less lubricity ), and NZ aviation authorities have convinced other
countries to include a lubricity specification in DefStan 91-91 but, AFAIK,
it's not yet in the more common ASTM D1655.

Note that refractory elements ( silicon, calcium, potassium, vanadium etc )
will erode-deposit on turbine blades, and they are present in higher boiling
fractions such as diesel ( which wouldn't pass the freezing point requirement -
but is used on ship and power-station gas turbines ). Fuels for gas turbines
tend to ensure such elements aren't present, even though falling out of the sky
isn't an option, as overhauls are very expensive.

There are a whole heap of other criteria for Jet fuels associated with
cleanliness, corrosivity, combustion properties, stability and approved
additives that mean than commercial aircraft tend to use a well defined narrow
cut kerosine fraction.

Bruce Hamilton