Sikorsky S-64F accident near Libby, Montana, September 1, 1994

The NTSB's narrative final · quoted from the NTSB record

THE HELICOPTER WAS ON A MAINTENANCE TEST FLIGHT TO TEST A RETARDANT TANK SYSTEM THAT WAS USED TO CARRY AND DISPENSE WATER. AFTER ARRIVING AT A LAKE, THE FLIGHT CREW HOVERED THE HELICOPTER WHILE LOWERING THE SNORKEL TO ONLOAD WATER. HOWEVER, THE QUANTITY INDICATOR WAS MALFUNCTIONING, AND THE CREW MEMBERS WERE UNSURE IF WATER WAS BEING ONLOADED. THE PILOT OPTED TO FLY OUT OF THE HOVER AND DUMP THE WATER FROM THE RETARDANT TANK. AS THE FLIGHT BEGAN TO DEPART THE AREA, THE PILOT NOTED THAT THE HELICOPTER WAS LACKING RESPONSE TO THE COLLECTIVE AND THE RATE OF CLIMB WAS SLOW. WHEN THE WATER WOULD NOT DUMP FROM THE TANK, THE FLIGHT CREW TRIED TO JETTISON THE TANK. NEITHER SYSTEM WOULD WORK. THE ROTOR RPM BEGAN TO DECREASE AND THE HELICOPTER SETTLED INTO THE WATER AND SANK. DURING AN INVESTIGATION, NO MECHANICAL FAILURE OR MALFUNCTION OF EITHER ENGINE WAS FOUND. WHEN CHECKED, THE RETARDANT (WATER) QUANTITY INDICATOR SYSTEM WAS INOPERATIVE AND WOULD ONLY SENSE THAT THERE WAS NO WATER IN THE TANK. TO RELEASE WATER, THE SYSTEM NEEDED TO SENSE THAT THERE WAS ENOUGH WATER FOR THE SELECTED SETTING; THEREFORE, THE TANK DOORS WOULD NOT OPEN TO DUMP WATER WITH THIS MALFUNCTION. NO EVIDENCE WAS FOUND TO DETERMINE WHY THE EMERGENCY TANK DROP SYSTEM DID NOT FUNCTION. USING ESTIMATES, PERFORMANCE DATA INDICATED THE HELICOPTER WAS ABOVE THE MAXIMUM GROSS WEIGHT AND WAS POWER LIMITED FOR THE ENVIRONMENTAL CONDITIONS. ELEVATION OF THE LAKE WAS ABOUT 6500 FEET; AIR TEMPERATURE WAS ABOUT 63 DEGREES.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.