N745BW

On July 20, 2020, about 0938 Pacific daylight time, a MD Helicopter 600N, N745BW, was destroyed when it was involved in an accident near San Andreas, California. The pilot and two passengers were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 aerial observation flight.

The pilot reported that he was performing low-altitude operations to visually assess power lines. While on the second circuit of the morning and at an altitude of about 250 ft above ground level, he and the right seat passenger detected the smell of smoke in the cockpit/cabin area and saw smoke rising from under the forward pilot and copilot seat area. As a result, the pilot made a rapid descent to an open area. During the descent, the helicopter’s airframe began to vibrate, and the pilot reported that the vibration got “progressively worse” as the descent continued. As the helicopter descended to about 30 ft above ground level, the pilot heard a loud “pop” and thought that the engine lost power. The helicopter then yawed to the left about 90° despite the pilot’s application of full right pedal. The helicopter landed hard, causing the right skid to fail. The pilot and the passengers evacuated the helicopter. A postcrash fire erupted, which consumed the helicopter.

Data recovered from the engine control unit revealed primary exceedance messages for a main rotor droop, which was followed almost immediately by an engine surge and then a flameout. Immediately after the main rotor droop, torque and fuel flow dropped to zero, consistent with a complete loss of load. Additionally, the engine control unit data showed that the rotational speed of the engine rose above the rotational speed of the main rotor,

Postaccident examination of the wreckage revealed that most of the fuselage and tailboom, including the cabin area, instruments, flight controls, and antitorque system, was consumed by fire. Most of the main rotor and antitorque flight control systems was fragmented and exhibited varying degrees of thermal damage. As a result, flight control continuity could not be confirmed.

Examination of the drive system, which transmits the torque that the engine produces to the main rotor system and the antitorque system, revealed the overrunning clutch (located between the engine and the main transmission driveshaft) had extensive thermal damage. (The overrunning clutch acts as a freewheeling unit if the engine fails to deliver power or the main rotor systems turns faster than the engine output speed.) The outer housing was consumed by fire, leaving the subassembly exposed but still attached to the power takeoff gear of the engine accessory gearbox. The subassembly separated from the main transmission driveshaft at the inner race output shaft. The output bearing and its retainer separated from the overrunning clutch. An exemplar overrunning clutch is shown in figure 1.

Figure 1. Exemplar overrunning clutch (Source: MD Helicopters).

Note: The output bearing, and retainer, subassembly, and inner race output shaft are internal and thus not visible in this figure. The red line indicates the location of the fracture in the accident overrunning clutch.

The main transmission driveshaft remained attached at the opposite end to the input to the transmission. A photograph of an exemplar clutch, main transmission driveshaft, and transmission is shown in figure 2.

Figure 2. Exemplar overrunning clutch, main transmission driveshaft, and transmission (Source MD Helicopters.)

Note: The engine (not shown) is located behind the overrunning clutch.

Examination of the engine revealed significant thermal damage from the postaccident fire. Despite the thermal damage, no evidence indicated a mechanical malfunction or failure that would have precluded normal operation.

The overrunning clutch subassembly, output bearing, and retainer were sent to the National Transportation Safety Board Materials Laboratory. Examination revealed that the overrunning clutch inner race was fractured near its output shaft, as shown in figure 3. Circumferential scoring and metal deposits were observed on the external surface of the overrunning clutch inner race. The area of scoring and metal deposits corresponded to the installed location of the clutch output bearing on the clutch inner race.

Figure 3. Separation of the inner race output shaft of the overrunning clutch subassembly.

Examination of the output bearing revealed that the inner race was substantially deformed radially outward, wrapping around the balls such that the inner diameter had a U-shaped profile. The output end of the retainer was also deformed radially outward, as shown in figure 4. Stainless steel flakes were observed in between the balls, cage, and outer race. A sample of the flakes was analyzed and found to be consistent with the stainless steel used on the grease seal.

Figure 4. Output bearing and retainer damage to the inner race.

ADDITIONAL INFORMATION

According to the maintenance logbooks, the overrunning clutch was overhauled in October 22, 2017 when the helicopter had about 8,613 time since new. The clutch was installed in the helicopter on October 3, 2018. According to the manufacturer’s Maintenance Manual, 300-hour inspection checklist, the overrunning clutch bearing should be removed, inspected, and repacked with grease every 300 hours. The last 300-hour inspection occurred on February 10, 2020, about 5 1/2 months before the accident.