When Habits Kill - Canadian Hughes 500 Helicopter Accident
Establishing habits can be effective ways to achieve safe outcomes, by sub-consciously completing routine actions. Examples include automatically fastening harnesses and seat belts or routinely scanning for other traffic. However, sometimes habits can be dangerous, for example when circumstances or equipment change. Just such a case occurred in Canada on 6 August 2008 to an operator supporting the mining industry.
The Helicopter Accident
The MD Helicopter 369D [aka MD500] (serial number 500715D, registration C-GZIO), operated by Prism Helicopters, was involved in support operations for mining exploration near Alice Arm, British Columbia. The helicopter took off at about 0709 Pacific daylight time for the first flight of the day to a drill site up the Kitsault River valley with one pilot and three passengers on board. As the helicopter departed in a shallow left-climbing turn, it emitted an unusual sound. It reached about 150 feet above ground level, then suddenly banked 90º to the right, and fell to the ground. It broke up on impact with a fallen tree and the main section, with the occupants, fell into the Kitsault River and remained mostly submerged just below the surface. All the occupants suffered fatal injuries on impact. The emergency locator transmitter operated until it was turned off by first responders. There was a small post-crash fire on the river bank that extinguished on its own.
Tree damage was consistent with a vertical descent and minimal rotor rpm. The TSB found heavy fabric cover wrapped tightly around the main rotor swashplate and broken pitch change rods). Rotor blade damage was consistent with low rpm at impact. The TSB go on that:
The pilot had 38 years of flying experience and had accumulated over 11 000 hours of flight time, with a substantial amount in similar operations. There was an apprentice aircraft maintenance engineer on-site to help with elementary tasks on the helicopter. He would normally look over the helicopter at the end of the day and secure it for the night. This included installing a synthetic heavy material cover (doghouse cover) over the engine intake and around the main rotor control system and tying the main rotor down. He would normally get up in the morning to remove the cover and untie the rotor, among other things. A couple of days before the accident, the pilot suggested that the apprentice need not get up early and that he would prepare the helicopter for the day’s flying. On the last two nights, the apprentice did not tie the rotor down but he installed the cover.
TSB explain that:
The cover does not have any straps or physical barriers that fall at or below human eye level. Once the pilot gets into the helicopter, there are no visual cues of the cover’s installation. On the morning of the accident, the pilot got up at the usual time, but stayed at the lodge a little longer and arrived at the helipad later than normal. Two of his passengers arrived before him. The pilot was loading their equipment when the third passenger arrived at the helipad. They loaded some more equipment, embarked the helicopter, and departed immediately after. One piece of equipment was left behind.
The TSB concluded that the cover was left in place during pre-flight preparations. Consequently the cover damaged the main rotor controls and ultimately main rotor rpm probably decayed due to “the binding effect that the cover had as it squeezed between the stationary and rotating components of the swashplate assembly” resulting in a Loss of Control – inflight (LOC-I).
Because humans are easily distracted and our memories are less than perfect, we use training, routine, checklists, visual cues, and physical defence barriers to help us carry out required tasks. There were no physical barriers to prevent the pilot from operating the helicopter with the cover installed. When the cover was installed without the main rotor being tied down, two things happened. The physical barrier and visual cues were removed. Since the pilot arrived at the helipad after some of his passengers, it is possible he was distracted from his normal routine by the need to assist them loading their equipment. Also, mental imprinting of the task to remove the cover could have been lessened by the practice of having someone else install it. In summary, anomalies in the pilot’s routine and the lack of physical barriers likely caused him to miss removing the cover before the flight.
Aerossurance has previously written about Professor James Reason’s 12 Principles of Error Management. Reason highlights that “errors are consequences not causes”, “people cannot easily avoid those actions they did not intend to commit” and “you cannot change the human condition, but you can change the conditions in which humans work”. In this case the TSB report that procedural and equipment changes have been made after the accident to prevent a reoccurrence:
Prism Helicopters implemented procedures requiring that blade tie-downs be installed whenever the doghouse cover is installed. Also, the covers have been modified with tape/straps that hang down and are to be placed in the front doors. The manufacturer of the doghouse (Aerospace Filtration Systems Inc.) has taken safety action in modifying the cover…[see below]
However, when designing or introducing such equipment it is vital to consider potential failure modes, such as failure to remove pre-flight. What Reason’s 12th principle (“Effective error management aims as continuous reform not local fixes“) reinforces is that we should not just look at the accident as an example of how to reduce the risk with one type of cover on one helicopter type, but wider as a trigger to consider what other safety critical practices or habits could breakdown in the future, what other interfaces between different departments and specialists may be assumed but not assured and what other equipment could be enhanced to help prevent errors. Its these issues that an effective Safety Management System would seek to find.
- Time Pressures and Take-Off Trim Trouble
- EC120 Forgotten Walkaround
- Flawed Post-Flight and Pre-Flight Inspections Miss Propeller Damage
- Too Rushed to Check: Misrigged Flying Controls
- UPDATE 10 June 2019: Troublesome Tiedowns: The Sequel A Robinson R22 suddenly fails to respond to control inputs and descends and impacts the sea. Investigators find a tie-down wrapped around the main rotor head.
Also see our review of The Field Guide to Understanding Human Error by Sidney Dekker presented to the Royal Aeronautical Society (RAeS): The Field Guide to Understanding Human Error – A Review
- Identify: What was I doing?
- Ask: Where was I distracted?
- Decide/act: What decision or action shall I take to get ‘back on track’?
Aerossurance is pleased to sponsor this Royal Aeronautical Society (RAeS) Human Factors Group: Engineering conference on 12 May 2015 at Cranfield University: Human Factors in Engineering – the Next Generation