What was found
NITROGEN PRE-CHARGE LEAKED PAST ACCUMULATOR PISTON CAUSING PRESSURE BUILD-UP IN RETRACT SIDE OF LANDING GEAR SYSTEM AND COLLAPSE OF LEFT MAIN GEAR WHILE AIRCRAFT WAS BEING TOWED. SUBMITTER STATED LATER DASH NUMBER HYDRAULIC POWER PACK INCORPORATES A VENT VALVE TO PRECLUDE THIS PRESSURE BUILD-UP IN THE EVENT OF ACCUMULATOR LEAKAGE. SUBMITTER RECOMMENDS THIS POWER PACK BE RETRO FITTED TO EARLIER AIRCRAFT. MAIN LANDING GEAR WERE FITTED WITH SAFETY PINS FOR GROUND HANDLING OPERATIONS.