NTSB Factual Report

On May 6, 2016, about 1700 Pacific daylight time, a Bell UH-1B helicopter, N486SA, was substantially damaged during a forced landing following 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.

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, however, the attempt was unsuccessful and he initiated a landing within an almond orchard. The helicopter landed hard and came to rest upright.

The pilot further reported that 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 of the helicopter by a Federal Aviation Administration (FAA) inspector revealed that the fuselage and tailboom were substantially damaged.

Following a visual examination of the engine, it was 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 of the engine 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 the engine operation or monitored parameters. The engine was shut down and a pressure gauge was added to the engine air inlet anti-ice overboard/vent port to confirm that pressurized air was escaping from this port. The engine was restarted and accelerated to the same performance levels as before. The pressure gauge confirmed that pressurized air was escaping from the anti-ice overboard/vent even though the valve was being commanded closed. Further examination of the engine revealed that when cycling on and off the hot air valve solenoid, a clicking noise could be heard, and when felt by hand during the operation, a slight vibration was detected. The hot air valve and solenoid were removed and found in a partially open position. The engine's bleed air port, where the hot air valve was removed, was blocked using a blanking plate and the engine was run. During the run, it was noted that the engine obtained full rated power prior to reaching the EGT limitations. A representative from the Honeywell manufacture stated that initially the engine was producing approximately 80% of its rated power prior to removal of the hot air valve.

Disassembly of the hot air valve revealed that the main body of the valve appeared to be undamaged. The forward surface of the valve appeared to have deposits/corrosion present, and the solenoid body appeared to be discolored with most of the protective plating missing from the unit. Debris, similar to adhesive/sealant, was found within the valve after disassembly of two body halves. The top surface of the piston, as well as the retaining nut displayed evidence of a buildup of foreign material, consistent with corrosion. The edges of the piston also displayed evidence of corrosion.

The valve's pistons shaft displayed evidence of corrosion and wear. The valve's spring and piston cavity displayed evidence of corrosion. The inside piston to cavity surface displayed evidence of corrosion and linear scoring. A buildup of debris and corrosion were displayed 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 of the valve in the areas where sliding contact is made.

According to a Honeywell representative, during manufacture of the hot air valve, a dry film lubricant coating is applied to the flow surfaces of the valve. Once applied, the thickness of the coating is between 0.00015 to 0.0005 inch. Molybdenum-disulfide is a constituent of the dry film lubricant which reduces friction between the sliding elements allowing for smooth, unimpeded motion of the valve. The representative further stated that the lack of lubricant coating in these areas would increase the sliding friction such that the closing spring would be unable to overcome the friction forces thus 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.

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.

NTSB Probable Cause

A reduction in available engine power during takeoff due to a stuck hot air valve, which resulted in low rotor rpm and a forced landing. Contributing to the accident was the lack of dry film lubricant and the presence of corrosion on the hot air valve assembly, which resulted in the sticking of the valve.