Accident Summary
On May 6, 2016, about 1700 Pacific daylight time, a Bell UH-1B helicopter (registration N486SA) was substantially damaged during a forced landing after a loss of main rotor RPM near Maxwell, California. The commercial pilot was not injured. The helicopter was privately owned and operated by Jones Flying Service, Biggs, California, as a Title 14 Code of Federal Regulations Part 137 agricultural flight. Visual meteorological conditions prevailed, and no flight plan was filed. The flight was originating at the time of the accident.
Pilot Report
The pilot reported that after loading 300 gallons of chemical, he conducted a pre-takeoff checklist, noting that all instruments displayed normal indications. A takeoff was initiated from the loading truck platform, and as it climbed over the tree canopy, the rotor and engine RPM began to rapidly decay. The pilot initiated a right turn to gain airspeed and recover the rotor RPM, but the attempt was unsuccessful, and he initiated a landing within an almond orchard. The helicopter landed hard and came to rest upright. At the time of the accident, the helicopter weighed 7,854 pounds, which was 1,646 pounds under its maximum gross weight of 9,500 pounds.
Postaccident Examination
Postaccident examination by a Federal Aviation Administration (FAA) inspector revealed that the fuselage and tailboom were substantially damaged. The engine was visually examined, mounted on a test stand, and run for approximately 30 minutes. During the engine run, the engine was advanced to 75% of normal rated power followed by an acceleration to the maximum exhaust gas temperature (EGT) limit of 1,140° F. The engine was unable to reach full rated power before the temperature limit was reached. Inspection revealed that hot air was being discharged from the engine air inlet anti-ice overboard/vent port. Energizing and de-energizing of the hot air valve solenoid, which supplies compressor discharge air to the engine inlet for de-icing, had no effect on engine operation or monitored parameters. The engine was shut down, and a pressure gauge was added to the vent port to confirm that pressurized air was escaping. The engine was restarted and accelerated to the same performance levels. The pressure gauge confirmed that pressurized air was escaping even though the valve was being commanded closed. Further examination showed that when cycling the hot air valve solenoid, a clicking noise was heard and a slight vibration was detected by hand. The hot air valve and solenoid were removed and found in a partially open position. The engine's bleed air port was blocked using a blanking plate, and the engine was run again. During this run, the engine obtained full rated power prior to reaching the EGT limitations. A representative from Honeywell stated that initially the engine was producing approximately 80% of its rated power prior to removal of the hot air valve.
Engine and Valve Analysis
Disassembly of the hot air valve revealed that the main body of the valve appeared undamaged. The forward surface had deposits/corrosion, and the solenoid body was discolored with most of the protective plating missing. Debris, similar to adhesive/sealant, was found within the valve after disassembly of two body halves. The top surface of the piston and the retaining nut displayed evidence of a buildup of foreign material consistent with corrosion. The edges of the piston also displayed corrosion. The piston shaft displayed evidence of corrosion and wear. The valve's spring and piston cavity displayed corrosion. The inside piston-to-cavity surface showed corrosion and linear scoring. A buildup of debris and corrosion was present on the surface of the piston shaft guide bore. A microscopic examination of the hot air valve revealed that little to no molybdenum-disulfide coating was present on the flow surfaces in areas where sliding contact occurs. According to a Honeywell representative, during manufacture a dry film lubricant coating (0.00015 to 0.0005 inch thick) is applied to the flow surfaces. Molybdenum-disulfide is a constituent that reduces friction between sliding elements. The lack of lubricant coating in these areas would increase sliding friction such that the closing spring would be unable to overcome the friction forces, causing the valve to remain open even when commanded closed. It could not be determined when the hot air valve had become stuck in the open position.
Maintenance History
Per the T5311 Overhaul Manual (75-10-1), an inspection of the hot air valve should occur with an overhaul of the engine. Review of the engine logbooks revealed that the engine was overhauled on December 29, 2006. No logbook entries pertaining to the inspection of the hot air valve were observed. At the time of the accident, the engine had accumulated a total of 598.2 hours since overhaul.