Compressor, power shafts fused by friction, halting engine

Unprecedented failure type prompts detailed US analysis

An AH-1S Cobra helicopter of the Army Aviation Operations Command fires its 20mm Vulcan cannon. [Herald Business DB]
An AH-1S Cobra helicopter of the Army Aviation Operations Command fires its 20mm Vulcan cannon. [Herald Business DB]

The cause of the crash of an Army Cobra (AH-1S) attack helicopter in Gapyeong-gun, Gyeonggi Province, in February, which killed two pilots, has been confirmed as an engine stoppage caused by a defect in the internal cylinder of the aircraft's exhaust diffuser.

A joint civilian-government-military crash investigation committee said at a briefing Thursday that friction heat fused the engine's compressor shaft and power shaft, halting their rotation and causing the crash.

The failure has been attributed to a so-called "shaft whirling" phenomenon, in which vibration increases sharply as the power shaft rotates — an unprecedented type of accident that prompted engineers from a US engine manufacturer to travel to South Korea for detailed analysis.

Byun Soon-cheol, co-chair of the joint investigation committee, described the flight conditions at the time of the crash at the briefing. The helicopter, assigned to the 5th Corps, took off one morning in February from a flight path at Gapyeong Airfield for its seventh takeoff-and-landing training session, designed to hone emergency procedures. Black smoke appeared as the aircraft climbed, and the engine then stopped. The helicopter simultaneously lost power, went into a steep dive and veered off the flight path, crashing into a stream near Sinha Bridge about 760 meters north of the airfield's runway.

The committee retrieved and analyzed the aircraft's onboard recording device (MP-4), the pilot's handheld location tracker (H-TAB), and CCTV footage from around the crash scene. Both devices were damaged in the ground collision, but the Army Criminal Investigation Command's forensic science center recovered some of the data, allowing investigators to confirm the aircraft's altitude, speed and main rotor rotation speed at the time.

Footage analysis showed the helicopter's speed, altitude and attitude were all normal during takeoff. However, 21 seconds after liftoff, smoke appeared and the engine stopped. Even with no evidence of pilot over-control or mishandling, readings on various instruments and the main rotor's rotation speed dropped sharply, and the aircraft hit the ground 12 seconds after the engine stopped.

The committee explained that during a climbing takeoff, the main rotor faces greater air resistance than in level flight, so an engine stoppage at that stage causes rotor speed to drop rapidly, making an emergency autorotation landing impossible.

An Army official said, "If it had come down somewhere like a wooded area, the crew might have survived. A witness — a local resident — said the helicopter appeared to have come down over the stream out of concern about an explosion." The official added, "We concluded that the pilot, worried about harming civilians, tried to steer away from residential areas until the very end and attempted an emergency landing."

The committee added that when it reenacted the crash scenario using a flight training simulator, all instrument readings dropped sharply in the same way as in the actual crash once the engine stopped, confirming that normal flight would have been impossible.

The T53 turboshaft engine installed in the helicopter is designed so that its compressor shaft and power shaft combine to generate power. The committee's analysis found no damage from external foreign objects; instead, friction between the power shaft and compressor shaft generated heat that fused the two together, stopping their rotation and halting the engine. The committee said an in-flight engine stoppage caused by the fusing of the two shafts was an unprecedented type of accident.

To determine the cause, a quality engineer from Ozark, the US engine manufacturer, traveled to South Korea to conduct a joint analysis with committee members and officials from Hanwha Aerospace. The damaged parts were also sent to Ozark and to Metl in the United States for further detailed analysis.

The analysis found that the central axis of the internal cylinder in the engine's rear exhaust diffuser was misaligned with the engine's central axis, causing vibration when the power shaft rotated. At low torque, this vibration increases sharply, potentially producing the shaft whirling phenomenon in which the shaft spins like a jump rope. Because the helicopter had reduced torque while conducting emergency-procedure training, engine vibration is believed to have been at its greatest at that point.

The committee concluded that as torque was increased again for takeoff, vibration in the power shaft grew, causing friction between the power shaft and compressor shaft. The resulting heat partially melted the power shaft, fusing it to the compressor shaft and stopping the rotation of both, which halted the engine.

In response, the Army said it would consult with the Joint Chiefs of Staff on the timing for retiring the Cobra helicopters, given concerns that the aging aircraft pose a safety risk. The Army also said it will continue coordinating with the Defense Acquisition Program Administration (DAPA) to adjust the existing deployment plan for the LAH-1 (Mireon) light armed helicopter, the Cobra's replacement, to ensure it can be fielded safely and stably.

The Army will also strengthen its aviation safety management system. By April next year, it plans to transfer safety management functions and organization currently under the Aviation Operations Command to the Army Headquarters' combat readiness and safety division, bolstering aviation safety oversight and management at the headquarters level. Airfields where helicopters and unmanned aerial vehicles operate together will shift from individual inspections to integrated inspections covering both manned and unmanned aircraft. The Army also plans to regularize aviation safety assessments conducted by outside experts, including the Ministry of Land, Infrastructure and Transport.


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