Failure to regard the human-machine interface leads to damage and injury
By Myron J. Boyajian, Contributing Editor
Getting an early start, a refuse truck crew rolled through a subdivision in the dim morning light of a summer day. While passing signs indicating a construction area, the driver moved cautiously as he peered into the darkness to find the address of a newly added customer. Without warning, the truck’s front wheels dropped into a depression in the road. The shock threw the driver upward to the roof of the truck cab. He suffered fractured neck bones and momentarily lost control of his truck. At the last second, he recovered enough to jam on the brakes and stop. His co-worker was thrown from the riding platform at the truck’s back end, but sustained negligible injuries.
The driver required surgery and extensive rehabilitation. His attorney retained me to determine how the accident occurred and if the road condition was the proximate cause. By the time I was retained in this matter, the road repairs were finished leaving me with only a police report and photos as evidence. The photos showed a depression in the street that was made for a sewer line in the expanding subdivision, but it had been filled with sand. No measurements were made of what appeared to be depressions in the sand made by the refuse truck. They appeared to be two or three inches deep, seemingly not deep enough to throw the driver up from his seat.
My initial reaction was that the driver was going faster than he said, and that he was not wearing a seat belt as he claimed in his statements. I expressed these thoughts to my client, but he said the driver recalled having difficulty with the seat height adjustment, and that other drivers had similar complaints on this refuse hauler. With the possibility of equipment malfunction, I obtained product spec sheets, brochures, and the purchase order and factory build sheets for the hauler in question. I learned that this vintage truck was equipped with a seat that was raised and lowered by an air cylinder controlled by a pneumatic valve with a push-pull knob located at the lower left front corner of the seat cushion. Pulling the knob lowered the seat. Pushing the knob raised it. I also obtained prints of the seat structure and pneumatic circuits from the seat supplier.
The subject refuse hauler had since been altered with a new seat, and was in service and not available for an inspection. I inspected a similar hauler at a local refuse company. It was built one year after the subject truck, and its seat height control used a low-profile toggle button surrounded by a raised guard, and was located on the side of the seat. My “Whoa, what’s this?” moment came as I compared this control to the one on the subject truck.
I could visualize the hauler hitting the road depression and the impact causing the driver’s leg to strike the knob, thus causing the seat to rise unexpectedly. I made an accurate scale drawing of the seated driver, who was quite lanky, that showed the calf of his left leg in close proximity to the seat height control. I now felt confident that the driver’s leg did strike the height control knob. When informed of this finding, the truck and seat manufacturer argued that the height control had a restrictor that limited the seat’s upward speed.
To put this argument to rest, I purchased a used seat with the knob control. I loaded the seat to equal the driver’s weight and charged the air system to the truck’s spec. Pushing in the knob raised the seat in only one second, surely not in any controlled way. It seemed likely that the fast rising seat and the upward thrust caused by the truck’s wheels hitting the excavation could cause the driver’s head to hit the roof. My opinion report and deposition testimony cited poor use of human factors and ergonomics, and the use of a control that invited inadvertent operation. My attorney-client reported that the case settled shortly afterward with what he called “a just and fair settlement.”
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