UAV, near Imsil-gun South Korea, 2009

UAV, near Imsil-gun South Korea, 2009


On 3 August 2009, control of a rotary UAV being operated by an agricultural cooperative for routine crop spraying in the south western part of South Korea was lost and the remote pilot was fatally injured when it then collided with him. The Investigation found that an inappropriately set pitch trim switch went unnoticed and the consequentially unexpected trajectory was not recognised and corrected. The context was assessed as inadequacies in the operator’s safety management arrangements and the content of the applicable UAV Operations Manual as well as lack of recurrent training for the operators’ qualified UAV remote pilots.

Event Details
Event Type
Flight Conditions
Flight Details
Type of Flight
Aerial Work
Take-off Commenced
Flight Airborne
Flight Completed
Phase of Flight
near Imsil-gun
Inadequate Aircraft Operator Procedures, Ineffective Regulatory Oversight, Unmanned Aircraft Involved
Inappropriate crew response - skills deficiency, Procedural non compliance
Flight Management Error, Aircraft Flight Path Control Error, Incorrect Aircraft Configuration
Damage or injury
Aircraft damage
Non-aircraft damage
Non-occupant Casualties
Number of Non-occupant Fatalities
Off Airport Landing
Causal Factor Group(s)
Aircraft Operation
Safety Recommendation(s)
Aircraft Operation
Aircraft Airworthiness
Investigation Type


On 3 August 2009, control of a Yamaha Motor Company RMAX L17 rotary UAV (S7012) being operated by the Osu Agricultural Cooperative at Imsil-gun in day VMC was lost and it subsequently collided with the remote pilot who sustained fatal injuries and then fell to the ground. The only external damage to the UAV was the breaking off of parts of the main rotor blades, damage to the skid due to uncontrolled ground impact and the severing of the two pesticide canisters from the airframe. There were no other immediate consequences of the Accident. 

UAV Imsil-gun 2009 UAV

The UAV involved prior to the Accident. [Reproduced from the Official Report]


An Accident Investigation was carried out by the Korean Aviation and Railway Accident Investigation Board (ARAIB). The FDR was removed from the remains of the UAV and its data (including the complete accident flight once the engine rpm was above 3000 rpm and ceasing when it dropped below 3000 rpm) were successfully downloaded.

The deceased 46 year-old pilot operating the UAV had been employed by the Osu Agricultural Cooperative for 6½ years and worked at their Rice Processing Centre (RPC). When the Cooperative decided to start using an unmanned rotorcraft for aerial spray to support their members’ farming operations in 2008, he and one other person were selected to act as pilots. He completed an initial training course provided by the importer of the UAV in February 2008 and in July 2008 obtained a ‘skill certificate’ from the Korea Agricultural Unmanned Helicopter Association.

The pilot continued working at the RPC but thereafter operated UAV aerial spray equipment during the summer spraying season as required. His initial UAV pilot training which led to the award of his skill certificate was provided on the same type as was involved in the Accident. However, the Investigation was unable to establish how much experience he had of flying it since he was found not to have maintained the log book in which he was supposed to record each flight.

The Accident UAV was found to have been imported to South Korea on 20 July 2008. It had a 94kg MTOW and was 3.63 metres long with a height of 1.08 metres. It was capable of carrying approximately 18 litres of pesticide which was enough to spray an area of 2 hectares. Just over a month after it arrived in South Korea, it was involved in a crash after colliding with an overhead electric wire during its inaugural aerial spray flight whilst being controlled by the same pilot who later was killed in the accident now under investigation. The damage to the UAV was minor and it was repaired the same day and returned to service. Its most recent safety certification by the Korean Transportation Safety Authority was 2½ months prior to the accident and its flight check log did not record any pre or post flight defects since that time.

By the time of the accident, there were around 75 RMAX L17 UAVs in use in South Korea with most, excluding those dedicated to pilot training, being operated by agricultural cooperatives or farming corporations for the aerial spraying of pesticide.

What Happened

On the day of the accident, the surface winds were light and the air temperature was 24°C. The accident flight was the second of the day in the same area for the team involved after a 90 minute flight in the morning had been followed by a lunch break. Because the only other pilot qualified to operate the UAV was on leave that day, the Cooperative’s 51 year-old ‘Executive Director for Guidance and Economy’ (referred to below as the ‘Guidance Executive’) volunteered to act as the co-pilot. 

According to witness statements, having arrived at the spray site, the necessary preparations were made with the spraying planned to be conducted by flying to the left and right from southwest to northeast as shown in the illustration below.

UAV Imsil-gun 2009 spray site

The spray site showing the intended tracking and the position of the spray team members. [Reproduced from the Official Report]

After starting the engine, the pilot and the other aerial spray team members “moved away about 15 metres from the UAV”. The pilot then increased the engine RPM sufficiently to cause it to take off and once it had, the Spray Team Leader and the Guidance Executive began to move to the locations from which they would signal to the pilot the boundary of the aerial spray to ensure pesticide was correctly applied to the crop. Whilst moving to the assistant signal position, the Guidance Executive reported having twice turned round to watch the pilot and the airborne UAV. He stated that he saw the UAV approaching at speed towards the pilot with its tail pointing towards him and it then hit the pilot who, after a few backward steps, fell to the ground. The Aerial Spray Team Leader also reported having turned around to watch the pilot whilst he was moving to his position but had not seen the collision, only the pilot on the ground afterwards. The UAV had been airborne about 20 seconds and had travelled approximately 18 metres in the final 6 seconds. The pilot was found to have died immediately after collision, the cause of death being subsequently identified as “hypovolemic shock caused by left femoral open fracture”.   

Why It Happened

No airworthiness defects relevant to the accident were found and it was found that the unexpected behaviour of the UAV once airborne had been attributable to the pitch trim switch being in an abnormal position (3 units pitch up) when the engine was started after this had not been noticed by the pilot. FDR data showed that the pilot had not turned on the UAV's GPS system when a corresponding signal quickly became available once it was airborne. It then began to fly backwards towards the pilot who tried to stop it but his signal input was not enough to achieve this and after it had flown 18 metres from its airborne position in 6 seconds at a speed of around 6 knots, it hit him before he and the UAV fell to the ground.  

The Investigation noted that whilst guidance on UAV operation for the purposes of insect pest control was provided by the ‘Unmanned Helicopter Association’, not all such guidance was being followed, in particular an ‘operational limitation’ to only spray before noon and a maximum flight time of one hour, were not being followed. It was also found that although regulations covered the airworthiness of UAVs there were no regulations in respect of qualifying as a pilot of one and thereafter remaining as one.  

The Cause of the Accident was formally documented as “an inadequate setting of pitch trim switch at 3 units pitch up was not recognised and corrected and the rearward movement of the rotorcraft was not properly controlled”.  

Four Contributory Factors were also identified as:

  1. The safety regulations of the Osu Agricultural Cooperative regarding the aerial spray work were insufficient and the supervision of the safety management did not work properly.
  2. There was no requirement to check if the trim switches were set at the 'zero' position in the UAV’s Operator's Manual.
  3. A safety distance of 15 metres was considered to be insufficient to provide the pilots with enough protection against abnormal movement of the unmanned rotorcraft.
  4. There was no method or requirements for pilot training to enable a pilot to maintain their proficiency after receiving initial training.

Eight Safety Recommendations were made as a result of the Findings of the Investigation as follows:

  • that the Osu Agricultural Cooperative establish and implement the following safety procedures for aerial spraying operations:
    • the personnel requirement for the aerial spray team 
    • management and supervision requirements for aerial spraying operations 
    • working conditions for pilots and limitations on their working hours. [UAR0903-1]
  • that the Osu Agricultural Cooperative add the trim switches position check item to the checklist in the Pilot Operating Manual. [UAR0903-2]
  • that the Osu Agricultural Cooperative consider adopting the 'challenge and response' method for completion of checklists where one pilot calls the check items out loud and another pilot takes action accordingly, [UAR0903-3]
  • that the Osu Agricultural Cooperative improve their current training programme in order to reduce the number of breaches of safety involving human error to include but not be limited to: 
    • a human error prevention programme    
    • training provisions which will enable pilots to maintain their proficiency. [UAR0903-4]    
  • that the Yamaha Motor Company consider improving the trim switches and panel of the radio control box and/or modification of relevant software, including the disabling of engine start if the trim switches are incorrectly set to prevent the risk of unintentional switch setting by inappropriate handling. [UAR0903-5]
  • that the Yamaha Motor Company provide unmanned rotorcraft operators with the detailed information which is necessary to establish safety procedures. [UAR0903-6]
  • that the Yamaha Motor Company add a procedure for checking the position of trim switches of the radio control box in the Operator's Manual. [UAR0903-7]
  • that the Yamaha Motor Company reconsider the appropriateness of the current 15 metre safety distance between an unmanned rotorcraft and its pilot. [UAR0903-8]

The Final Report was published on 31 December 2010.

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