For as little as I know about the subject, extinguishers are only of use if aimed at the base of the fire. This would normally be at the business side of the firewall, and thus not accessible from the cockpit i.e. by the crew.
Also I remember from my own crash - 2-stroke engine quit on take-off, instructor got the ship down but ran out of airspeed some 10-20 feet up, plane dumped vertically, broke the nose wheel and tumbled her tail over - both occupants were hanging upside-down from their belts, and must have been stunned for a good while before looking in each other's glazy faces. Cannot imagine when an extinguisher would have been any good, we were either stunned, or just hanging there, or working our way out (which took some doing from the inverted position, well aware of the fire risk), or RUNNING.
Can anyone describe a practical case where a carried extinguisher was of any use? I can't imagine any. The disadvantages are obvious, though: ballistics, as already pointed out, and the sheer weight. Not that much of an issue in a C152 or anything heavier, perhaps; but a real concern in a 450 kg gross ultralight.
Last edited by Jan Olieslagers; 12th Aug 2010 at 19:42.