Kukuihaele, HI, USA
N506PH
MD HELICOPTER 369
The pilot was performing an approach to land when the engine power surged, and the engine turbine outlet temperature (TOT) rose after the pilot lowered the collective. The pilot performed an autorotation to uneven terrain where the helicopter landed and rolled over onto its left side. The engine continued to run after impact, and the pilot shut it down. Examination of the wreckage found no anomalies with the airframe. The engine bleed valve was found in the incorrect, closed position. The engine was relocated to a service center where it ran normally using the accident bleed valve and after the bleed valve was opened manually before the test run. The valve was observed to operate at a slower rate than normal during the engine run. The engine was then equipped with an exemplar bleed valve modified to operate manually. When the engine was brought to full power, the bleed valve held closed, and the engine power reduced, the engine surged, and the TOT increased like it did on the accident flight. A teardown examination of the bleed valve revealed that the bushing in which the bleed valve stem resides was manufactured undersized. The bushing measured .307 inch inside diameter, and official documents state the bushing should measure .310 inch inside diameter. This reduced the clearance between the valve and the bushing; however, the bleed valve assembly had operated on the engine for about 94 hours prior to the accident and was not identified in any maintenance writeups prior to the accident. The examination of the bleed valve also revealed an accumulation of corrosion on multiple internal sub-component surfaces, which included the valve stem and the undersized bushing. The corrosion tested positive for the engine wash compound used by the operator, and the helicopter had undergone an engine wash the day before the accident. The operator indicated the engine manufacturer’s guidance for the wash was followed, which included blocking the bleed valve closed to prevent wash intrusion, washing the engine with a diluted wash solution, rinsing the engine, and drying the engine by conducting a 10-minute engine run. The engine manufacturer stated that engine wash intrusion into the internal components of the bleed valve would be very small if those steps were followed. Due to the extended storage time in non-climate-controlled environments, the investigation could not determine the exact amount of corrosion on the bleed valve at the time of the accident; however, the presence of the wash compound in the corrosion found on the bleed valve indicates that some wash solution penetrated into the bleed valve during a wash and remained following the rinse and drying of the engine. The investigation could not determine if the wash procedures were strictly followed or why the wash solution entered the bleed valve and remained. Although the bushing installed on the bleed valve stem was manufactured incorrectly, the bleed valve likely operated properly until corrosion developed inside the bushing following the engine wash. However, the bleed valve was likely more susceptible to this type of failure due to the incorrect bushing dimensions. Due to these factors, the bleed valve failed to function properly during the accident flight, which resulted in a partial loss of engine power.
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.
The partial loss of engine power due to corrosion in the engine bleed valve due to wash solution intrusion. Contributing to the accident was the installation of a bushing manufactured to incorrect dimensions in the bleed valve, and the uneven terrain at the landing site.
Source: NTSB Aviation Accident Database
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