Hughes 369D N173AL accident near West Alton, Missouri

Probable cause

The pilot’s failure to maintain helicopter clearance from the powerline during the powerline maintenance operation for reasons that could not be determined.

NTSB analysis

The pilot and a lineman were conducting maintenance work on a powerline at the time of the accident. The operator’s chief pilot reported that the pilot had transported two linemen onto a powerline transmission tower to complete their work. The pilot then returned to pick up a third lineman. The pilot and third lineman were attempting to remove a marker ball from one of the transmission lines at the time of the accident.

A barge flotilla and tow vessel were located along the south bank of the river and were stationary at the time. The helicopter was positioned above the forward portion of the barge flotilla.

A witness located on the tow vessel reported that a lineman appeared to be standing on the left skid, wearing a safety harness. On the second attempt, as the pilot brought the helicopter closer to the powerline, the tail of the helicopter struck the powerline. The tailboom appeared to partially separate from the airframe with a portion of it “dangling.” The helicopter pivoted, and the cockpit hit the powerline, which created sparks similar to when an electrical transformer fails. The helicopter then fell onto the barge deck and a post-impact fire ensued. The witness did not recall seeing the helicopter on fire during flight, only the sparks when the fuselage struck the powerline. The crew of the tow vessel moved the tow vessel alongside the barge and used its pilothouse fire station to extinguish the fire.

According to the chief pilot, the accident pilot and lineman had discussed accomplishing the marker ball maintenance via the helicopter platform versus utilizing a rope. The pilot had wanted to use the rope since it provided more clearance and, hence, a greater safety margin. He noted that the static line upslope was more pronounced near the transmission tower and presented more of a challenge than working farther from the tower. However, the lineman recommended utilizing the platform. Ultimately, the decision was made by the pilot to utilize the platform.

The high-tension power lines spanned the river at the accident site, and the forward portion of the barge flotilla was positioned under the lines. One transmission line appeared to be frayed consistent with contact damage.

The helicopter main wreckage consisted of the fuselage, engine, main rotor transmission, main rotor hub, and forward portion of the tail boom (fragmented into two sections). The aft portion of the tail boom, including the tail rotor assembly, was not present with the wreckage on the barge and likely landed in the river during the accident sequence. Each of the main rotor blades separated from the hub; two main rotor blade segments and one main rotor blade end weight were located on the barge. The remaining portions of the blades were not present and likely landed in the river. A post-impact fire consumed significant portions of the airframe.

The engine remained within the engine compartment and was subjected to impact and post-impact fire damage. The compressor and turbine cases exhibited no evidence of release of high energy debris from within the engine. Visual inspection of the compressor inlet, 4th-stage power turbine wheel, and 1st-stage gas producer turbine exhibited no anomalous damage. Continuity of the compressor-to-turbine and power turbine (PT) shafts was confirmed. The interconnecting driveshaft from the engine to the main rotor transmission was fractured consistent with torsional overload. The main rotor transmission was continuous from the engine side driveshaft input outward through the tail rotor drive shaft connection.

Based on the examination conducted, no evidence of a preimpact failure or malfunction was observed. However, portions of the airframe and tail rotor were not recovered from the river and could not be functionally tested.

Although the autopsy revealed the presence of mild coronary artery disease, there was no forensic evidence of adverse effects or that the condition contributed to the accident. Toxicology testing did not identify any substances that might have contributed to the accident.

Text from the NTSB. Open the NTSB docket for CEN25FA307 for the full record.