N506PH

On February 21, 2019, about 0659 Hawaii-Aleutian standard time, an MD Helicopters, 369E helicopter, N506PH, was substantially damaged when it was involved in an accident near Kukuihaele, Hawaii. The pilot was seriously injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 positioning flight.

The pilot reported that the helicopter had undergone a 300-hour inspection the night before the flight, which included an engine wash, and he had performed post-maintenance flight tasks before departing on the flight. He stated he was maneuvering to land at an off-airport site to pick up passengers when he observed, out of the corner of his eye, a yellow flash, followed by a red flash. The pilot then looked inside the helicopter and saw the engine out light illuminate and heard an alarm activate. He noticed the turbine outlet temperature (TOT) at about 909° C and the engine torque gauge indicated 0%. He established an autorotation descent and found a suitable landing area. During touchdown to uneven terrain, the tail rotor assembly and left skid assembly broke away and the helicopter rolled to the left. The helicopter came to rest on its side with the engine still running. The pilot shutdown the engine with the fuel shutoff valve and exited through the front windscreen area with the help of responders.

All major components of the helicopter were found in a debris area around the accident site. The main rotor blades remained attached to the hub and displayed chordwise bending and wrinkling along their span. The left landing skid separated at the front and rear struts. The empennage separated from the tailboom. The tail rotor assembly remained attached to the end of the tailboom. About 42 gallons of jet fuel were drained from the fuel tank. The helicopter remained at the accident site for three days and was then recovered to the operator’s T-hangar, which was not equipped with climate control.

A post-accident examination, performed on March 5 and 6, 2019, revealed no mechanical malfunctions or failures with the airframe that would have precluded normal operation. A preliminary engine examination revealed the compressor bleed valve unit (bleed valve) was in the incorrect, closed position, had an accumulation of corrosion on the back face of the bleed valve, and the bleed valve would not move with hand force (figure 1.). According to a Rolls-Royce engine training manual, when the engine is not in operation, the bleed valve is positioned fully opened by a spring. The bleed valve is open during starting and idle operations and modulates from open to close once predetermined pressure ratios are obtained. The engine was shipped to an authorized service facility on March 21, 2019.

The engine was received at the test facility and examination revealed that the bleed valve was still in the closed position. The engine was placed in a test cell and prepared for a “run as received” test run. The bleed valve was manually placed in the open position prior to starting the engine. The engine was subsequently started and brought to idle.

Figure 1. Compressor bleed valve on the accident engine.

The engine ran normally up to takeoff power and was shut down. The bleed valve was observed to close “reluctantly,” or at a rate that was slower than normal during the engine run.

The engine was then fitted with an exemplar bleed valve unit modified to open and close manually. The engine started normally and was accelerated to takeoff power. The bleed valve was manually held in the closed position while engine power was then reduced. About 78% N1 speed (rotation of the engine compressor section), the engine power began to surge, and TOT increased. Any attempts to increase power at that time increased the severity of the surging.

The accident bleed valve unit was further examined. The valve moved with moderate force and required the use of a mallet to extricate it from the bushing in which the valve stem resided. An accumulation of corrosion was present on multiple surfaces of the bleed valve assembly including the valve stem, spring, bellows, and the bushing, as shown in figure 2. Measurements of the bleed valve stem and bushing were taken. The valve stem measured .3079 inch outside diameter, including the corrosion, and the bushing measured .307 inch inside diameter, including the corrosion. Regarding compressor bleed valve assembly, the Rolls-Royce maintenance manual indicates that the compressor bleed valve stem outside diameter is 0.307 to 0.308 inch and states in part, “D. Check the inside diameter of the bushing (120) is .310 in. (7.87mm).”

Figure 2. Components of the accident bleed valve assembly, showing corrosion.

A review of maintenance records revealed the bleed valve assembly was overhauled on March 27, 2018, and installed on the accident engine on January 23, 2019, at an engine total time of 29,702 hours. According to the party member for the operator, the bleed valve had operated on the engine for about 94 hours without maintenance writeups.

The valve and bushing were relocated to the NTSB Materials laboratory. Examination of the corrosion, using a Fourier Transform Infrared (FTIR) spectrometer, revealed it contained the compounds contained in Ardrox 6367, an engine cleaner used by the operator.

According to the operator’s director of maintenance, an engine wash was performed the day before the accident as part of the 300-hour inspection. The company maintenance program used the Rolls-Royce engine wash procedures and diluted Ardrox engine wash solution with water (10 parts water to one part engine wash). The company procedures also included using a V-shaped block to manually close the bleed valve during engine washes and performing a rinse procedure after the engine wash and a 10-minute engine run after the rinse.