BE18 PILOT REPORTS LANDING GEAR COLLAPSE DURING TKOF ROLL.

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NORMAL PREFLT; ENG START; AND TAXI. NORMAL RUN-UP. TKOF PWR AND AT APPROX 60 KTS; R MAIN GEAR AND L MAIN GEAR COLLAPSED. ACFT SLID TO A STOP ON RWY CTRLINE. NOTIFIED TWR OF PROB; SHUT DOWN ALL SWITCHES; AND TURNED FUEL OFF. LNDG GEAR SWITCH REMAINED IN DOWN POS WITH GREEN LIGHT AND NO WARNING HORN THROUGHOUT ENTIRE EVENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: UNDER NORMAL CIRCUMSTANCES THE CIRCUIT WHICH RUNS THE LANDING GEAR MOTOR PASSES THROUGH THE SQUAT SWITCH AND THE LANDING GEAR SWITCH; WHICH PREVENTS THE GEAR FROM BEING RAISED INADVERTENTLY WITH THE ACFT ON THE GROUND. THE DEPARTURE RWY HAS UNDULATIONS WHICH CAUSE THE ACFT TO COME OFF THE SQUAT SWITCH AS FLYING SPEED IS REACHED. IT WAS AT THIS POINT THAT THE GEAR RETRACTED EVEN THOUGH THE GEAR SWITCH WAS STILL IN THE DOWN POSITION. A CRANE WAS BROUGHT IN TO RAISE THE ACFT SO THE LANDING GEAR COULD BE EXTENDED AND PINNED FOR THE TOW BACK TO THE HANGAR. WHEN THE MASTER SWITCH WAS TURNED ON THE LANDING GEAR EXTENDED EVEN THOUGH THE LANDING GEAR SWITCH HAD NEVER BEEN TOUCHED AND REMAINED IN THE DOWN POSITION. NO ELECTRICAL FAULTS HAVE BEEN DISCOVERED AND NO REASON FOR THE INCIDENT HAS BEEN FOUND.