“It is the conclusion of the investigation team that the selected tuning of the drive combined with the natural harmonics of the lift system, along with rapid speed changes, caused the rope instability resulting in (chair) 58 contacting tower five,” the report said. That contact knocked three people from the chair. One, Texas mom Kelly Huber, died after falling 25 feet to the snow. Her two daughters also fell and were injured, one seriously. Huber’s death was the first related to a lift malfunction in the U.S. since 1993, and the first death from a chairlift fall in Colorado since 2002.

“Electronic drives are not manufactured specific to ropeways,” the report notes. In addition, each lift is “unique in its design, terrain, vertical profile, length, tower spacing, tower sheaves, carrier spacing, and type of carrier.” As a result, the report explained, “there are no ‘standard’ settings for the drive. The purpose of ‘tuning,’ or ‘fine-tuning,’ a drive during installation is to find the right combination of drive parameter settings to optimize the lift performance under operational conditions. It appears the new drive was not comprehensively tuned to this particular lift during installation.”

The report continues, “The selected tuning of the drive resulted in oscillations of the drive bullwheel torque. The rapid periodic changing of the applied torque caused rope instability (surging) that was amplified by the natural lift dynamics. This rope surging is greatest near the bottom terminal where the tension is lowest. In this case, the maximum instability (rapid speed change) appears to have occurred as the lightly loaded Carrier 58 approached Tower 5.”

“No one on the investigative team has ever witnessed or heard of a similar event,” the report notes. “Likewise, literature does not describe such an event.”

The incident followed a modification to the electronic lift controls in early December, including an upgrade to the digital DC drive and fine-tuning to set parameters in the new drive that controls how the drive interacts with the electric motor that drives the lift. However, the state’s investigative team found no deficiencies, unusual dynamics, or anomalies in the December 12, 2016 Acceptance Test Report.

Witnesses stated that the lift made several sudden accelerations and decelerations in the moments before the incident. They reported that the chair Huber and her daughters were riding on, chair 58, began to violently sway as it approached lift tower five. When it got to the tower, the two collided.

“Several witness statements allege multiple stops immediately prior to the incident,” the report states. “However, the computer data logger does not show those stops. It was later understood with testing that those perceived ‘stops’ were actual changes in speed of the cable and gave the passengers the impression that the chairlift had momentarily stopped, indicating additional dynamic movement in the chairlift.”

Investigators said the speed changes resulted from changes in the drive control system that possibly created pulses of energy along the rope line and “could explain the rope instability.” They identified seven “potential contributing factors” to the fatal fall, including other equipment replacements and damage to a motor system.

Investigators made 10 recommendations in light of their findings, urging that chairlifts undergo more detailed testing to include different simulated load parameters. Investigators also suggested installation of a “black box” on all chairlifts to record stops, starts and speed changes, and longer time intervals between speed level changes, such as the interval between a lift stoppage and restart. The Quick Draw Express was programmed for 6.5-second intervals; the report suggested intervals for this lift as long as 30 seconds, to allow energy to dissipate and “naturally decay.”