Wednesday, July 18, 2018
Transport Canada - Aviation Safety Letter Missed Engine Cover Still Installed Brings Down Helo

Missed Engine Cover Still Installed Brings Down Helo

On August 6, 2008, an MD 369D helicopter was operating near Alice Arm, B.C. The helicopter took off at about 07:09 Pacific Daylight Time (PDT) for the first flight of the day, with one pilot and three passengers; it was headed to a drill site up the Kitsault River valley. As the helicopter departed in a shallow left climbing turn, it emitted an unusual sound, reached about 150 ft above ground level (AGL), then suddenly banked 90° to the right and fell to the ground. It broke up on impact and all four occupants suffered fatal injuries. The investigation by the Transportation Safety Board of Canada (TSB) is complete, and the following is based on TSB Final Report A08P0244.

Other factual information
The pilot had 38 years of flying experience and had accumulated over 11 000 hr of flight time. The work schedule was relatively light. Prior to this tour of duty, the pilot had been off for 10 days. This was well within the limits of duty time, and he had a quiet, restful evening before the accident flight.

There was an apprentice aircraft maintenance engineer (AME) 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 down the main rotor. 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 did install the cover.

The cover did 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 clues of the cover’s installation. Before this occurrence, it was expected that the engine would not start with the cover installed, but we now know that as soon as the rotor started turning, the cover opened enough to let in sufficient air for the start.


Photo 1: Doghouse cover

On the morning of the accident, the pilot was 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 that day. When the third passenger arrived at the helipad, the pilot was loading the other passengers’ equipment. They loaded some more equipment, embarked the helicopter and departed immediately after. Examination of the helicopter wreckage at the accident site revealed the cover tightly wrapped around the main rotor control system, around the swashplate (see Photo 2). Nearly all the control linkages (pitch change rods) were broken and the damage was not consistent with that normally found from crash impact forces. Rotor blade damage was consistent with low rpm at impact. Tree damage and scars at the accident site were consistent with a vertical descent, and no rotor rpm.


Photo 2: Doghouse cover wrapped between controls

It is clear that the removal of the cover was missed during flight preparation and that the cover damaged the main rotor controls while the rotor was turning. Also, rotor rpm was likely lost due to the binding effect that the cover had as it squeezed between the stationary and rotating components of the swashplate assembly. This rendered the helicopter uncontrollable after the takeoff, and it collided with terrain.

To determine how this item could be missed, the following analysis focuses on human factors.

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.


Photo 3: Modified doghouse cover

The TSB final report lists the following three findings as to causes and contributing factors:

  1. The doghouse cover was not removed before flight and it wrapped around the flight controls linkage, damaged the linkage, and rendered the helicopter uncontrollable.
  2. The helicopter fell in an uncontrolled state, with reduced rotor rpm, until it collided with terrain.
  3. Anomalies in the pilot’s routine and the lack of physical barriers likely caused him to miss removing the cover before the flight.

Safety action taken
As a results of this occurrence, the operator 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 Photo 3).

Closing comment from the lead investigator to the Aviation Safety Letter (ASL): “I would hope that the ASL readers don’t think this was just a stupid error. This cover was missed by a very well-respected and careful pilot. If it happened to him, it can happen to any of us. It is also important for all operators to get that message. Our analysis looked at human factors to determine how a fastidious, experienced pilot could miss such a basic and critical item.”

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