What was found
MECHANICS WERE WORKING ON LANDING GEAR DOWNLOCK (J-HOOK) SOLENOID. THEY WERE CHECKING OPERATION OF THE SOLENOID BY ACTIVATING THE RIGHT SQUAT SWITCH. ACFT WAS NOT ON JACKS AT THAT POINT. GEAR HANDLE WAS VERIFIED IN DOWN POSITION PRIOR TO ACTIVATION OF THE SQUAT SWITCH. AS MECHANICS ACTIVATED SQUAT SWITCH, GEAR MOTOR STARTED TO RUN IN THE RETRACT DIRECTION. LANDING GEAR MOVED ENOUGH TO BREAK OVER CENTER POSITION ON BOTH MAIN GEARS AND ACFT COLLAPSED ONTO THE FLOOR. NLG ACTIVATED OVER CENTER, BUT NOT ENOUGH TO CAUSE GEAR TO COLLAPSE. MAJOR LANDING GEAR DAMAGE, ENGINE WHEELWELL NACELLE DAMAGE, FLAP DAMAGE, HORIZ STAB DAMAGE FROM COLLAPSING ONTO THE NOSE OF ANOTHER ACFT, BUT NO ENGINE DAMAGE.