N5027PHughes Helicopters HUGHES 369D · 1981

On July 9, 2023, at 1027 eastern daylight time, a Hughes 369D helicopter, N5027P, was substantially damaged when it was involved in an accident near Waldorf, Maryland. The commercial pilot and two observer crewmembers sustained minor injuries. The helicopter was operated by Haverfield Aviation, Inc., as a Title 14 Code of Federal Regulations Part 91 powerline inspection flight.

The flight crew reported that they planned to complete one flight before forecast weather moved into the area. The pre-mission risk assessment, preflight inspection of the helicopter, and inflight power-assurance check were all satisfactory and the helicopter performed “without incident” for two hours before the crewmember in the back of the helicopter reported hearing an “abnormal noise.”

The crewmembers concurred that they heard something “suspicious,” but agreed that it was so subtle they couldn’t decide whether the noise was “abnormal;” however, the pilot reported that he “felt a vibration in the collective” control, and the crew agreed to return to Maryland Airport (2W5), Indian Head, Maryland, where the flight had originated.

According to the flight crew, about 4 nautical miles from the departure airport, there was a “loud bang or pop noise and a hard left yaw.” The pilot entered an autorotation and selected a forced landing site adjacent to a pond in a residential area.

Company GPS track data depicted the helicopter in cruise flight about 900 ft mean sea level and 100 knots groundspeed for about 8 minutes on a southwesterly heading toward 2W5. The data indicated that the helicopter entered a steep, descending left turn through about 180° before the track data ended at 0 knots about ground level in the vicinity of the accident site.

Home security video captured the final seconds of the flight. The sounds of the helicopter’s engine and the turning main rotor could be heard before the helicopter appeared in the frame, descending steeply in a level attitude. The helicopter dropped from sight behind a house and reappeared just as the helicopter’s nose flared upward and the tail rotor impacted the surface of the pond about the same time. The tailboom was separated by the turning main rotor at or about water contact. The helicopter’s skids impacted the fence that bordered the pond, and the helicopter subsequently came to rest upright. The engine, which continued to run briefly, was shut down by the pilot and the damaged main rotor blades stopped turning. The crewmembers egressed the helicopter without assistance.

The helicopter was manufactured in 1981 and was powered by a Rolls-Royce 250-C20B turboshaft, 420-horsepower engine. The helicopter’s most recent 100-hour inspection was completed on June 7, 2023, at 24,799.4 total aircraft hours, and included an inspection of the overrunning clutch for oil leakage, correct operation of the clutch, and regreasing of the clutch splines. These overrunning clutch inspections were signed off with no discrepancies noted.

The helicopter came to rest upright straddling the damaged fence. The cockpit and cabin area appeared intact. The transmission, main rotor mast, and main rotor head all appeared intact and secure in their mounts. Two of the five main rotor blades displayed significant impact damage, and all five blades were secure in their hubs. The tailboom was severed just aft of its mount; the damage was consistent with main rotor contact and impact with water and terrain. A fractured tail rotor gearbox with the tail rotor attached was found separated from the tailboom. One tail rotor blade was severed near its grip.

A detailed examination of the helicopter revealed that the overrunning clutch outer race’s splined shaft had fractured.

Figure 1. A cutaway view showing the internal components of the overrunning clutch assembly. The input drive (right side) is driven by the engine. The output drive (left side) provides drive to the engine-to-transmission (Kaflex) driveshaft. (Image courtesy of MD Helicopters)

An overrunning clutch permits the output, or driven member, of the clutch to freewheel whenever the input, or driving member, is stopped or is rotating at a slower speed. In a helicopter transmission, overrunning clutches are used to disengage the engines from the rotor, allowing the rotors to turn without engine drive. In multiple-engine aircraft, the overrunning clutch permits individual engines to be started without rotating the remaining engines. Safe landings may be executed by autorotation without the use of engines because the overrunning clutch automatically disconnects the engines from the rotor head when the engines are stopped.

The overrunning clutch subassembly on the accident helicopter comprised the outer race, inner race, sprag assembly, forward and aft sprag bearings and the output bearing. Adding the clutch housing and retaining ring to the clutch subassembly creates the overrunning clutch assembly.

The accident helicopter make and model, when equipped with a hook for external load operations, was subject to FAA Airworthiness Directive (AD) 90-19-02. The AD required disassembly of the overrunning clutch assembly to inspect the condition of the clutch inner race, clutch outer race, and sprag assembly at 300-hour intervals, with replacement of the sprag assembly at 1,800 hours time since new (TSN).

The accident overrunning clutch assembly was removed from another helicopter in February 2023 in order to complete the AD inspection. The inspection was completed in April 2023 and the overrunning clutch assembly was installed on the accident helicopter in June 2023, with 1,199.4 hours TSN on the sprag assembly. At the time of the accident, the overrunning clutch had accumulated 22 hours time in service since the AD inspection.

The overrunning clutch assembly was examined by the NTSB Materials Laboratory in Washington, DC. Examination of the fracture surface showed signatures consistent with fatigue. Other notable findings included evidence of a gap between the forward-most bearing’s inner race and the clutch inner race shoulder, off-axis sprag contact marks around the circumference of the outer race, and asymmetric contact marks on the wave washer. Additionally, secondary microcracks were observed on the shaft portion of the clutch outer race. Closer examination of the microcracks revealed that the black oxide layer on the surface of the clutch outer race that had been applied during the manufacturing process appeared to continue onto the microcrack surfaces in the first grain. For additional details of the clutch assembly examination, see NTSB Materials Laboratory Report No. 24-030 in the docket for this investigation.