START BUTTON DEPRESSED ERRONEOUSLY ON OPERATING ENG ACFT EQUIP PROB MALFUNCTION RESULTED IN OVERHEAT AND SHUTDOWN.

Narrative

DURING TAXI FOR TKOF WITH L ENG RUNNING; I SELECTED THE R ENG FOR START BUT DEPRESSED THE L START BUTTON AGAIN. IT ILLUMINATED AND REMAINED SO. I PUSHED THE L ENG STOP BUTTON; THE ENG BEGAN TO STOP; APPARENTLY NORMALLY; AND THE CAPT STEERED THE ACFT CLR OF THE TXWY. ABOUT 10 SECONDS INTO THE L ENG SPOOL DOWN; ITS EGT ROSE RAPIDLY (900 DEGS SHOWING ON GAUGE; OVER 1000 DEGS ON THE TPE 331 ELECTRONIC MEMORY) AT WHICH POINT THE CAPT SECURED THE ENG WITH THE CONDITION LEVER (TO 'FUEL SHUTOFF'). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR FO TRIED THE SAME SCENARIOS IN A SIMULATOR; BUT COULD NOT DUPLICATE. EVIDENTLY; THE AUTOMATIC SEQUENCING WAS OUT OF SYNC BECAUSE DEPRESSING THE STOP BUTTON SHOULD SHUT OFF FUEL; IGNITERS AND PROP CTL OIL PRESSURE DUMP. IN THIS SHUTDOWN; THE FUEL CONTINUED SUPPLYING THE ENG AS WELL AS THE PROP OIL PRESSURE DID NOT DUMP. ALSO; DEPRESSING THE START BUTTON ON AN IDLING ENG SHOULD HAVE NO EFFECT SINCE IT IS LOCKED OUT ABOVE 60 PERCENT RPM. THE ENG WAS CHANGED AND SENT BACK TO COMPANY MAINT FOR INSPECTION AND REPAIR.