The Metro 23 (Garrett TPE331-12U turbo prop engines) uses a water/methyl alcohol mixture for high density altitude take-offs.
It's a mixture of 40% methyl alcohol & 60% distilled or demineralised water.
When the performance charts require it's use it's turned on just after you start rolling. It gives approximately a 30% increase in torque and is turned off at 500'.
The Metro has a 16 gallon tank for water/meth and you'll get approximately two take-off's out of one tank.
It's a bit of an eye opener for the passengers when you get to 500 feet, turn the water/meth off and suddenly get a 30% torque decrease!