N103SU

HISTORY OF FLIGHTOn December 29, 2020, about 0835 central standard time, a Hughes 369D helicopter, N103SU, was destroyed when it was involved in an accident near Tuckers Crossing, Mississippi. The pilot was fatally injured. The helicopter was operated as a Title 14 Code of Federal Regulations (CFR) Part 133 external load flight.

According to the operator, the pilot departed for the first flight of the day from a private landing zone to begin aerial tree trimming operations along a transmission line, which was about 2.7 miles from the departure location. The helicopter contained about 400 pounds of fuel and was equipped with an external saw assembly that was about 120 ft in length. The mission’s ground support specialist (GSS), who was in communication with the pilot via radio, stated that the helicopter approached the trimming zone from the south and flew northbound to the east of the transmission line. He estimated that the external saw was about 40 to 50 ft above the trees. The GSS and the pilot discussed hazards located in the trimming zone. Shortly thereafter, the pilot announced over the radio that he would be making an emergency landing. The GSS reported he did not see any smoke or hear any “odd noises” coming from the helicopter and that “the helicopter made a quick forward descent until the saw disappeared into the east tree line.” The helicopter subsequently impacted trees and terrain, and a postimpact fire ensued.

The accident site was located in wooded terrain to the east of the transmission line (see figure 1). The external saw assembly was found connected to the helicopter’s trapeze assembly and entangled in the trees.

Figure 1. Main wreckage (Source: Federal Aviation Administration). PERSONNEL INFORMATIONOn August 11, 2017, the pilot successfully completed the “MD520N Pilot Transition Training” course provided by MD Helicopters.

The operator provided the pilot’s two resumes, which were not dated. On one resume, the pilot reported 785 hours of flight experience with external load/long-line operations; on the other resume, the pilot reported 1,100 hours. The operator hired the pilot in April 2019, and he completed the 14 CFR 133.37 knowledge and skills tests on May 11, 2019.

Between June 2019 and March 2020, the pilot completed 126 flights and accumulated 119.4 total flight hours, of which 117.1 hours were for external load operations. The available information for this accident did not show the pilot’s total flight experience with the operator. AIRCRAFT INFORMATIONA review of the airframe maintenance records revealed that, on December 20, 2020, a 100hour helicopter inspection was completed, which included the removal of the overrunning clutch and the installation of a serviceable over-running clutch. On December 21, 2020, a 300-hour airframe inspection was also completed with a total airframe time of 12,807.2 hours. The engine records revealed that 150/300/600-hour inspections were completed on December 17, 2020, with a total engine time of 3,332.3 hours, and 3,468 cycles. On December 7, 2020, the fuel control unit was replaced “due to erratic engine operation during start up sequence.”

According to company text communications regarding helicopter maintenance and flight operations information, on December 13, 2020, a pilot reported that “had the ‘warning’ tone annunciate 3 [times] in the last flight. Had no secondary indications. Early in the flight so I don’t think it was the low fuel light blinking.” A company mechanic responded, “Trans oil pressure may be the cause on [N103SU], a quick flicker will cause tone.” A company employee replied that another helicopter in the company’s fleet “does it all the time on windy days with the warning audio going off and no secondary indication. If you’re not low on fuel then it’s likely the trans pressure [gauge].” This company employee also stated that he had “caught it faintly illuminating the panel light with a good gust pushing things way out of trim for a second.” The pilot then responded that he “was figuring one or the 2” and that he “tried inducing a trans press light but couldn’t get it to do it.” The pilot further stated that he would “keep an eye on it.” No additional related communications were provided through December 23, 2020, when the text conversation ended.

According to the rotorcraft flight manual emergency and malfunction procedures, if the main rotor transmission oil pressure indicator (red warning light) illuminates, the procedure is to land as soon as possible. The main transmission oil pressure warning does not produce an audible tone; audible and visual warnings are only provided for engine-out and low-rotor conditions.

A review of the accident helicopter’s instrument panel showed a warning tone switch below the annunciator panel lights in the center of the upper instrument panel (see figure 2.). The maintenance records did not indicate an installed separate warning tone system for the helicopter.

Figure 2. Instrument panel with warning tone switch (Source: MD Helicopters).

According to the operator, previous flight experience showed that the main rotor transmission oil pressure light can illuminate when ferrying the external saw at higher speeds or turbulent conditions due to the aerodynamic drag placed on the external saw in forward flight and the nose-low attitude required to pull the external saw during flight. The operator also stated that slowing the airspeed or adjusting the helicopter into trim would extinguish the light and allow for normal operation. The operator added that “the nose-low attitude is not seen during normal flight profiles without the external load’s drag.” AIRPORT INFORMATIONA review of the airframe maintenance records revealed that, on December 20, 2020, a 100hour helicopter inspection was completed, which included the removal of the overrunning clutch and the installation of a serviceable over-running clutch. On December 21, 2020, a 300-hour airframe inspection was also completed with a total airframe time of 12,807.2 hours. The engine records revealed that 150/300/600-hour inspections were completed on December 17, 2020, with a total engine time of 3,332.3 hours, and 3,468 cycles. On December 7, 2020, the fuel control unit was replaced “due to erratic engine operation during start up sequence.”

According to company text communications regarding helicopter maintenance and flight operations information, on December 13, 2020, a pilot reported that “had the ‘warning’ tone annunciate 3 [times] in the last flight. Had no secondary indications. Early in the flight so I don’t think it was the low fuel light blinking.” A company mechanic responded, “Trans oil pressure may be the cause on [N103SU], a quick flicker will cause tone.” A company employee replied that another helicopter in the company’s fleet “does it all the time on windy days with the warning audio going off and no secondary indication. If you’re not low on fuel then it’s likely the trans pressure [gauge].” This company employee also stated that he had “caught it faintly illuminating the panel light with a good gust pushing things way out of trim for a second.” The pilot then responded that he “was figuring one or the 2” and that he “tried inducing a trans press light but couldn’t get it to do it.” The pilot further stated that he would “keep an eye on it.” No additional related communications were provided through December 23, 2020, when the text conversation ended.

According to the rotorcraft flight manual emergency and malfunction procedures, if the main rotor transmission oil pressure indicator (red warning light) illuminates, the procedure is to land as soon as possible. The main transmission oil pressure warning does not produce an audible tone; audible and visual warnings are only provided for engine-out and low-rotor conditions.

A review of the accident helicopter’s instrument panel showed a warning tone switch below the annunciator panel lights in the center of the upper instrument panel (see figure 2.). The maintenance records did not indicate an installed separate warning tone system for the helicopter.

Figure 2. Instrument panel with warning tone switch (Source: MD Helicopters).

According to the operator, previous flight experience showed that the main rotor transmission oil pressure light can illuminate when ferrying the external saw at higher speeds or turbulent conditions due to the aerodynamic drag placed on the external saw in forward flight and the nose-low attitude required to pull the external saw during flight. The operator also stated that slowing the airspeed or adjusting the helicopter into trim would extinguish the light and allow for normal operation. The operator added that “the nose-low attitude is not seen during normal flight profiles without the external load’s drag.” WRECKAGE AND IMPACT INFORMATIONThe main wreckage, which consisted of the fuselage, engine, main transmission, and rotor assembly, was consumed by the postimpact fire. The tail rotor assembly had separated from the tailboom near the tail rotor gearbox and was found adjacent to the main wreckage.

Postaccident examination of the helicopter found that the airframe fuselage, cockpit structure, and helicopter instruments and avionics exhibited extensive thermal damage. The tailboom had separated from the aft fuselage boom fairing tail cone. Damage consistent with three main rotor blade strikes was observed on the tailboom. The cargo hook system, which consisted of a primary (electrical) and secondary (hydraulic) release system, was intact, and no anomalies were noted with the system. The cargo hook remained attached to the external saw.

Due to thermal damage, flight control continuity could not be established to the cyclic, collective, and anti-torque controls. A damaged section of the anti-torque control tube located in the aft tailboom section moved appropriately when the tail rotor blades were manipulated by hand.

The overrunning clutch and main...