Propellers always be treated as ‘live’.
A report in the AAIB Bulletin concerns a Beagle Pup whose engine burst into life while the pilot was turning the propeller as part of the external pre-flight checks as described in his copy of the aircraft’s Flight Manual. Because the throttle was fully open and the mixture fully rich, again in accordance with the written procedure, the aircraft broke free of its tie-downs, clipped the wing of a parked aeroplane, and crashed into an earth embankment.
It appears that the ignition switch was actually selected to ‘left’ despite there being no key in the switch. However, the investigation was particularly concerned with the Flight Manual procedure, and amendments have been made to all Pup Flight Manuals to require an idle throttle setting and mixture to CUTOFF during the pre-flight check.
Nonetheless, the accident should remind us yet again that a piston engine has the capability of springing into life at the slightest provocation. All possible safety measures should be taken, magneto switches must be positively (double) checked, and propellers always be treated as ‘live’.