Korean SAR S-76B Mountain Rescue Accident 2020 (HL9646)
On 1 May 2020 SAR Sikorsky S-76B HL9646 of Sejin Aviation crashed on Mount Jirisan, South Korea, while undertaking a mountain rescue tasking. One casualty being hoisted, a climber who had suffered a heart attack,, and another climber on the mountainside below were both killed. The five crew of the helicopter suffered minor injures.
Sikorsky S-76B HL9646 Accident Sequence (Credit: ARAIB)
The helicopter was under contract to Gyeongsangnam-do Province Fire Department, from September 2019 to February 2022 as a stopgap awaiting the delivery of new helicopters.
The Korean Aviation and Railway Accident Investigation Board (ARAIB) safety investigation report was published on 16 December 2024 (in Korean only).
The Accident Flight
The accident timeline is as follows:
- 11:28: HL9646 took off from the Gyeongsangnam-do Fire Aviation Rescue and Paramedic Unit.
- 11:42: The crew commenced a search of the mountains between Cheonwangbong and Beopgye Temple, at an altitude of approximately 5,900–6,200 feet, to locate the casualty. Mount Jirisan is the highest mountain in mainland South Korea. However, due numbers of hikers in the area, the casualty’s location could not be identified until the third orbit of the area.
- 11:50: HL9646 attempted it’s first approach to the casualty, but aborted due to strong crosswinds.
- 11:54: The crew arrived at the site for a second attempt from a different direction, and a two person rescue / paramedic team were successfully hosted to the casualty. Afterwards HL9646 moved away until requested to return. It moved into position, hovering approximately 5,935 feet above sea level and 30–35 feet above ground level. Outside Air Temperature was c15-16ºC (cISA+12). It appears one of the rescue / paramedic team was hoisted aboard first.
- 12:06: While hoisting the casualty, “the helicopter’s nose rotated to the right, and the aircraft shifted forward, descending unexpectedly”. The main rotor blades contacted the terrain below.
Sikorsky S-76B HL9646 Wreckage (Credit: ARAIB)
Aircraft debris was scattered over a radius of about 15 m. The climber on the ground who was killed was struck by a rotor blade.
Sikorsky S-76B HL9646 Rotor Blade Damage (Credit: ARAIB)
Fir trees around the hiking trail, with a diameter of c 35 cm, prevented HL9646 from rolling down the steep slope.
Sikorsky S-76B HL9646 Wreckage Against Tree Trunk (Credit: ARAIB)
ARAIB Safety Investigation
HL9646 did not have a flight data recorder (FDR), only a cockpit voice recorder (CVR).
The wreckage was recovered using a Korea Forest Service S-64 Sky Crane.
Sikorsky S-76B HL9646 Wreckage (Credit: ARAIB)
In interview, the Aircraft Commander (Pilot Flying) recalled that…
…while the stretcher carrying the patient was being raised, [the helicopter] was hit by a vortex and sank. To prevent this, I increased the power, but the altitude did not recover, so I turned the aircraft nose to the right and made an emergency landing.
The Co-pilot (Pilot Monitoring) recalled that:
We approached the rescue site at about 5,700 feet, and the wind at that time was not strong at 5-6 knots. We hovered at an altitude of about 30-35 feet above the rescue site, and when the cardiac arrest patient rose to the middle, the aircraft lost power and felt like it was being pushed forward, and then it immediately started to sink. It rose slightly near the ground and then crashed.
The hoist operator indicates that as the aircraft started to descend they tried to lower the patient to the ground.
ARAIB highlight a range of factors that affect hover performance:
In order to hover for mountain rescue or other missions, you must first check whether hovering is possible at the altitude and temperature of the terrain and then fly.
NTSB spectral analysis of the CVR for ARAIB showed that:
During the hover, the captain used the collective a lot, which caused changes in the main rotor rotation speed (RPM). This indicates that the pilot experienced a lot of power changes on the steep mountain slope during the hover.
ARAIB say that:
The Handling Qualities Model of the S76 aircraft predicted that the efficiency of the tail rotor would decrease when the aircraft main rotor speed fell below 88%.
…through video data and acoustic analysis, the phenomenon of the nose turning to the right occurred more sharply when the main rotor speed fell below 88%.
Sikorsky S-76B HL9646 NR (Credit: ARAIB)
Main rotor speed (NR) was initially 106-108%.
Three seconds before impact with the ground, the main rotor speed was reduced to 78%, and the nose of the aircraft at the time of impact turned 180 degrees to the right compared to the initial hovering nose in a 120-degree direction and crashed.
Sikorsky S-76B HL9646 NTSB CVR Spectral Analysis (Credit: ARAIB)
ARAIB say that:
The sudden change in the main rotor speed indicates an unstable state of the aircraft. This includes the environment such as aircraft power, gross weight, wind strength and direction changes in hovering, decreased tail rotor efficiency, and power changes due to electrical use due to hoist operation. While lifting the paramedic, the rotation speed of the main rotor was constant.
Sikorsky S-76B HL9646 CVR / NR Analysis (Credit: ARAIB)
However, there was a change in the rotation speed of the main rotor from the time the hoist was lowered to lift the emergency patient, and from the moment the emergency patient was lifted, the rotation speed of the main rotor decreased rapidly and the aircraft crashed.
ARAIB observe that during the final hoisting the helicopter had yawed to the right. This resulted in a tailwind state, needing more power at a time they calculated hover performance was already marginal.
ARAIB Probable Cause
HL9646 crashed after exceeding the hover weight limit (performance) during a mountain rescue mission.
The contributing factors were determined as follows:
- The flight crew took off without checking the hover performance chart as specified in the flight manual prior to the mission.
- Engine maintenance, such as engine washing, was not conducted in accordance with the aircraft manual, leading to reduced engine performance due to internal contamination and corrosion.
- The Engine Electronic Control (EEC) trim adjustment was not performed, resulting in discrepancies between the actual internal engine temperature (ITT) and torque compared to the cockpit instruments.
ARAIB do not quantity the two technical contributory factors.
Safety Resources
The European Safety Promotion Network Rotorcraft (ESPN-R) has a helicopter safety discussion group on LinkedIn. You may also find these Aerossurance articles of interest:
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