KETCHIKAN, AK, USA
N549SW
HUGHES 500D
THE HELICOPTER PILOT WAS IN CRUISE FLIGHT ABOUT 250 FEET ABOVE THE GROUND, TRANSPORTING LOGGING PERSONNEL TO A REMOTE AREA. THE ENGINE SUDDENLY LOST POWER, AND THE PILOT BEGAN AN AUTOROTATION TO A GRAVEL PIT. DURING TOUCHDOWN, THE TAILBOOM WAS SEVERED, AND THE HELICOPTER ROLLED ONTO ITS RIGHT SIDE. DURING RECOVERY OF THE HELICOPTER, THE LIFTING CABLE BROKE, AND THE HELICOPTER DESCENDED TO THE GROUND FROM ABOUT 400 FEET. POSTACCIDENT EXAMINATION OF THE ENGINE FUEL CONTROL AND POWER TURBINE GOVERNOR DID NOT REVEAL ANY MALFUNCTION. EXAMINATION OF THE PNEUMATIC PC TUBE (AIR LINE) FROM THE PC FILTER TO THE GOVERNOR REVEALED AN AIR LEAK AT THE 'B' NUT ATTACHMENT TO THE GOVERNOR 'T' FITTING. LABORATORY TESTING REVEALED THAT A TORQUE OF 300 INCH POUNDS WOULD HAVE BEEN REQUIRED TO STOP THE AIR LEAK. THE MANUFACTURER'S SPECIFIED TORQUE VALUE WAS 100 INCH POUNDS. RECORDS SHOWED THAT ON 6/7/95, MAINTENANCE HAD BEEN PERFORMED TO REPLACE THE GOVERNOR AND TO CLEAN AND REINSTALL THE P2 AIR LINE; AND ON 6/11/95 (DAY BEFORE THE ACCIDENT), THE ENGINE SPEED HAD BEEN ADJUSTED. THE MAINTENANCE MANUAL FOR THE ENGINE WARNED THAT AIR LEAKS IN THE PNEUMATIC SENSING SYSTEM COULD RESULT IN FLAME-OUT, POWER LOSS, OR OVERSPEED.
On June 12, 1995, about 0515 Alaska daylight time, a Hughes 500D helicopter, N549SW, crashed during a forced landing, about 25 miles north of Ketchikan, Alaska. The helicopter was being operated as a visual flight rules (VFR) local area flight to transport logging personnel to a job site, under Title 14 CFR Part 91 when the accident occurred. The helicopter, registered to and operated by Silver Bay Logging Inc, Juneau, Alaska, received substantial damage. The certificated airline transport pilot and 2 passengers received minor injuries. Two other passengers were not injured. Visual meteorological conditions prevailed. Company VFR flight following procedures were in effect. The flight originated from a remote helipad in the area of Fire Cove on Revillagigedo Island, Tongass National Forest. The operator reported that the helicopter departed the helipad to place logging personnel on a nearby hill. While in cruise flight about 250 feet above the ground, the pilot noticed a rapid decay in the main rotor RPM. The pilot performed an autorotation to a gravel pit. During the descent, the pilot maneuvered around several trees and could not confirm if the engine had quit completely or if it was still running at flight idle. He did not report hearing an engine out warning horn or seeing any annunciator lights. During touchdown, the tail boom was severed from the main cabin fuselage and the helicopter rolled over on to its right side. The operator retrieved the accident helicopter by slinging it out with another helicopter. During the sling lift, the lifting cable broke, dropping the accident helicopter to the ground from about 400 feet. The second impact destroyed the helicopter. The engine completely separated from the airframe. After recovery, an examination of the engine revealed an air leak in the pneumatic tubing from the Pc filter to the power turbine governor. The engine manufacturer's operation and maintenance manuals contain numerous warnings concerning maintenance practices. These include: "Air leaks in the fuel system pneumatic sensing system can cause flameouts, power loss or overspeed." An additional warning states: "Proper tightening of engine tubing connections is critical to flight safety. Correct torque values must be used at all times. Excessive torque on pneumatic sensing system connections results in cracking of the flare or adjacent tube area in contact with the ferrule. This produces an air leak which can cause flameout, power loss, or overspeed." Another warning states: "Failure to properly install, align and torque fuel, oil, and air fittings and tubes could result in an engine failure." Examination of the maintenance records for the two days preceding the accident discloded that among other items on June 7, 1995, the governor was replaced because it would not hold the manufacturer's specified beep range. The P2 air filter was cleaned and reinstalled, and the fuel inlet strainer was cleaned and reinstalled. On June 11, 1995, the records indicated that engine idle speed was adjusted, the fuel filter was replaced. The engine fuel control module and the power turbine governor were inspected at the Allied Signal Aerospace, Engine Systems and Accessories facility in South Bend, Indiana, on August 9, 1995. The examination was overseen by FAA inspectors from the South Bend Flight Standards District Office. The fuel control exhibited impact damage and was not functionally tested. Disassembly of the module did not reveal any malfunctions. The power turbine governor was functionally tested and no malfunctions were reported. The governor "T" fitting, to which the Pc tubing from the filter to the governor attaches, exhibited an initial air leak. After retightening, the leak stopped. Both ends of the engine's flex driveshaft from the accessory case to the main rotor transmission were found separated from the shaft portion of the driveshaft. The driveshaft, the pneumatic tube from the Pc filter to the engine scroll, the tube from the Pc filter to the power turbine governor "T" fitting, and the governor "T" fitting were submitted to the Safety Board's Materials Laboratory for examination. The examination revealed that the driveshaft exhibited 45 degree shear plane separations, no evidence of flat fracture areas or crack arrest positions, and no evidence of torsional deformation. The Pc filter to engine scroll Pc tube did not conform to the manufacturer's specified shape of the tube; however, the end fittings aligned to the approximate positions of a standard shaped tube. One end of the tube exhibited bending deformation adjacent to the "B" nut ferrule. Pressure testing of the tube did not reveal any leak. Examination of the pneumatic tube from the Pc filter to the power turbine governor "T" fitting revealed minor bending deformation of the tube. The "B" nut that attaches the tubing to the governor "T" fitting was hand threaded onto the "T" fitting and rotated until patches of red torque paint on the fitting and the "B" nut aligned with each other. During tightening of the "B" nut, the flared end of the tube, and the nut ferrule are squeezed between the nut and the fitting to form a tight seal. A pressure test of this configuration revealed the presence of an air leak. The "B" nut was tightened to the manufacturer's specified torque value of 100 inch pounds. A small amount air leakage remained. Laboratory personnel conducted a test to determine the amount of torque required to properly seat the "B" nut, the nut ferrule, and the flared end of the tube against the "T" fitting. The application of 300 inch pounds of torque was required to tightly secure the tube ferrule within the "B" nut against the "T" fitting. All parts retained for testing were released to the operator on November 8, 1995.
AN AIR LEAK AT A 'B' NUT FITTING ON A PNEUMATIC PC TUBE TO THE POWER TURBINE GOVERNOR. COMPANY MAINTENANCE PERSONNEL DID NOT DETECT A LEAK AFTER PERFORMING MAINTENANCE ON THE GOVERNER AND P2 AIR LINE.
Source: NTSB Aviation Accident Database
Aviation Accidents App
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