Colorado Passenger Tramway Safety Board Issues Report On December 2016 Quick Draw Express Lift Incident
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Colorado Passenger Tramway Safety Board Report On December 2016’s Quick Draw Express Lift incident sates that the accident was the result of a mechanical malfunction
On the morning of December 29, 2016, three skiers boarded the Quick Draw Express Lift (SC-008) (“QDE”) at the Granby Ranch Ski Area in Granby, Colorado. As their carrier (Carrier 58) approached Tower 5, the carrier swung excessively and struck Tower 5. The three skiers were ejected from Carrier
58 and fell to the snow surface. Granby Police Department confirmed that one passenger was fatally injured and the two other passengers were seriously injured. No other injuries were reported. The lift was evacuated with the electric prime mover.
The QDE is a detachable quad lift built in 1999 by Leitner Lifts (See attachment B). The lift has a slope length of 4,360 feet and a vertical rise of 837 feet with 14 towers and a line gauge of seventeen feet five inches (17’-5”). The lift’s ultimate design capacity is 2,400 passengers per hour at the design speed of 1,100 feet per minute (ft./min.) in a clockwise rotation, but the current tested capacity is
1,800 passengers per hour.
The top fixed drive chairlift uses an Electric Prime Mover consisting of 500 horsepower (“hp”) electric motor through a Sew Eurodrive gearbox. The Diesel Prime Mover is a Cummins N14-P475 engine utilizing 475 hp through the same gearbox. The Evacuation Engine is a Cummins B5 9P-152 engine utilizing 152 hp through a torque converter and a belted connection to the same gearbox.
The original electronic drive controls were a Control Techniques drive with a Leitner low voltage system utilizing a Pilz “Programmable Logic Controller” (“PLC”). Until the new drive was installed in December 2016, there had been no major electrical changes since the lift’s construction.
The tension system is a single hydraulic cylinder used in a compression application, or “push” style design on the return bullwheel at the bottom terminal. The hydraulic controls are also located on the carriage at the bottom terminal.
The Investigation of this matter commenced on December 29, 2016, the date of the incident, and preliminarily5 concluded March 30, 2017. The investigation included the assembly of an investigation team, a review of the QDE lift history, licensure and inspections, extensive on-site testing of the QDE, witness interviews/review of statements, and review of the information from the data logger.
Initial Board Assessment and Assembly of Investigation Team
Immediately following the incident, Area representatives evacuated the lift and closed the lift for public operation. The Granby Ski Patrol was the first to respond to the scene at 10:55 a.m., and contacted the Granby Police Department, who arrived at approximately 12:05 p.m. The Area then notified the Board Supervisory Tramway Engineer (“STE”). Both the Granby Ski Patrol and Granby Police Department assisted in taking witness statements as the lift was evacuated with the primary drive (See attachment K). The weather was partly cloudy and the temperature was 30 degrees with wind less than five miles per hour (“mph”). Area maintenance personnel preserved the lift’s recorded drive performance data from the time of the incident.
The STE arrived from Denver at 1:30 p.m. on the day of the incident to coordinate the investigation. The STE assembled an investigation team of four licensed professional engineers with extensive tramway engineering experience.
The investigative team included:
1. Lawrence Smith, P.E. – Board STE (40*)
Responsible for overseeing the investigation
2. Thomas LaVenture, P.E. – Board inspecting engineer (30*) Responsible for technical support and documentation
3. Bruce Allen, P.E. – Board inspecting engineer (20*)
Contributing technical support via telephone
4. Charles Peterson, P.E. – Independent consultant (41*)
Responsible for directing investigation and report development
Others present during the investigation included:
1. Jamie Bunch, P.E. – Tramway Engineer (40*)
2. Jim Fletcher, P.E. – Tramway Engineer (40*)
3. Josef Gmuender, P.E. – Consultant (30*)
4. Rod Stocking – Senior lift technician for Leitner/Poma (40*)
5. Ski Granby Ranch managers and lift maintenance personnel
6. Granby Police Department
*Years of experience in the aerial ropeway industry
The QDE lift at Ski Granby Ranch (the “Area”) has been licensed to operate since December 9,
1999. During that time, the lift was inspected annually for both its licensing and unannounced operational inspections. During its 18 years of operation, there have been no reported incidents attributed to unstable lift dynamics.
The summer operational inspection of the QDE was conducted on August 7, 2016, by Board Inspector, Bruce Allen, P.E. The report (See attachment C) noted a deficiency related to the incorrect labeling of a tower test switch on the control panel. In their August 13, 2016, response to the deficiency (See attachment M), the Area reported that the deficiency was corrected.
In early December 2016, the electrical controls of the QDE were modified per the “Notice of Installation” received by the Board office in October of 2016 (See attachment D). The modification did not change the existing manufacturer supplied low voltage control system. The Engineer of Record for the modification was Joe Gmuender, P.E. The installation and drive setup was performed by Ed Thompson of Electramic Associates.
The modifications to the QDE lift included:
1. Replacement of the original 1999 Control Techniques digital DC drive with an ABB DCS-800 digital DC drive;
2. Addition of an interface controller between existing low voltage controls and the new DC
drive. The interface controller adds safety functions for both electric and auxiliary operation;
3. Fine-tuning to set parameters in the new drive that controls how the drive interacts with the electric motor that drives the lift; and,
4. Routine major maintenance of the electrical motor by the Area (off site - See attachment F).
The acceptance testing for the drive modification was conducted on December 5, 2016, by Board Inspector, Bruce Allen, P.E. The test included the verification that the new drive was capable of accelerating a fully loaded lift to design speed. Per ANSI Standard B77.1-2011, the test procedure (See attachment G) requires the lift be loaded with 110% passenger weight on the up-going line. Containers of water were utilized to simulate the passenger weight. The lift was driven backwards a minimum of three times during testing to keep the loaded carriers on the heavy side and non-run loaded carriers through the terminals. During the multiple acceleration tests, neither Inspector Allen nor any of the personnel (See attachment E - “Personnel attending”) who witnessed the test reported any unusual rope dynamics.
The annual licensing inspection for the QDE lift was completed by Inspector Allen, on December 5 and
8, 2016, concurrent with the new drive acceptance test. The Inspection Report dated December 12,
2016, noted a deficiency regarding the need to submit to the Board a software security procedure and additional as-built documentation of the modification. The Area submitted a report to the Board on December 14, 2016, correcting the deficiency (See attachment H).
In addition to the aforementioned deficiency, Inspector Allen noted a comment on the Inspection Report. A comment is not a deficiency of the required standards, but rather a way of informing the Area of a specific item that may need attention in the future. Inspector Allen noted “The new motor drive’s proof of torque circuit is not able to maintain static torque or forward motion on a start with a loaded lift due to the proprietary nature of the existing control system and brake control module. A very slight rollback does occur on all loaded lift starts, and should be rectified when the new control system is installed in 2017.” Inspector Allen further noted the proof of torque complied with the ANSI B77.1 requirement 188.8.131.52(b) and the ANSI “proof of torque” requirement (See attachment I). The comment was reviewed by the investigative team and since it is unrelated to dynamic effects on the lift, it was not considered relevant to the incident or investigation.
The Area submitted all of the required documents for the modification and testing of the new drive and the lift was licensed on December 15, 2016, following the December 5 and 8, 2016 inspection and review of the December 12, 2016 Acceptance Test Report.
The investigative team also reviewed the initial Acceptance Test Report dated December 12, 2016, from the December 5 and 8, 2016 inspection (See attachment G). In the case of a lift modification such as this, the design engineer Joe Gmuender, P.E., is responsible for the programming, adjustments and fine-tuning subject to the modification. The Board inspecting engineers are not responsible for review of particular programmed settings such as drive parameters, but observe the test procedure to verify performance complies with the Board Rule 21.3.11 and the ANSI B77.1-2011 Standard. The investigative team found no deficiencies, unusual dynamics, or anomalies in the December 12, 2016
Acceptance Test Report.
The investigative team reviewed the witness statements gathered by the Granby Police Department and the Granby Ranch Ski Patrol for information. In addition, the investigative team was notified of a post to the Area’s “Facebook” page regarding QDE operations on a day prior to the incident. These statements provided a wealth of knowledge to allow the investigation team to get a better picture of the dynamics as the incident unfolded and possible indications of input variables into the cause of the incident
The statements allowed the investigation team to deduce the loaded characteristics of the three loaded carriers ahead of the incident chair and four chairs back. Witness statements indicate Carrier
58 was the lightest loaded carrier in the grouping of affected carriers.
The witness statements also tell of Carrier 58 swing beginning at Tower 4 and continuing to increase until contact was made at Tower 5. The statements also indicate the amount of carrier swing of the other nearby carriers.
Witness statements also verified that there was no outside influence of energy, i.e. wind, tree contact or other force inducing additional energies into the cable system, and indicated there was no passenger misconduct.
One specific witness comment regarding the noticeable increase in dynamic cable movement during the first 10 days of this season was further investigated by the STE. According to the witness, the line was much more dynamic than in the previous 10 years they had been skiing at this area. During that visit, these observations were reported to area representatives. The witness further stated the dynamics occurred along the entire line.
Several witness statements allege multiple stops immediately prior to the incident. 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.
All the information gathered from the witness statements allowed the investigation team a better
understanding of the overall situation surrounding Carrier 58’s contact with Tower 5.
The site investigation and testing were conducted by the investigation team from December 29, 2016 to January 5, 2017.
The major components of the lift were inspected and found to be within industry standards and complied with Board Rules and ANSI B77.1-2011 Standard. The investigative team verified that the Carrier 58 contacted the Tower 5 tube. The marking on the Tower 5 tube was consistent with marks on the outside surface of the Carrier 58 bail. In addition, there were matching marks on the grip rollers and the outside flange of the lead-in sheave. The tensioning system was tested and found to be functional with an expected amount of carriage movement under varying loading conditions.
While the investigation team was unable to recreate events exactly as they occurred on the date of the incident, the investigation was able to reasonably simulate the variables present at the time of the incident. To begin the testing, the empty lift was operated at various speeds with multiple changes of speed.
During one of the tests, there was some instability in certain carriers as they passed over and uphill of Tower 12. No other indications of instability were observed. The investigative team noted that Carrier 58 was removed from the line following the incident. The gap left by this missing carrier was slowly closing as the carriers were automatically repositioned in the terminal to be a uniform distance apart. The carriers are spaced in the upper terminal to be a pre-set distance apart based on 67 carriers on line. As with all detachable lifts, there is a gap that remains in the line between Carrier 67 and Carrier 1 during normal operation.
Carriers 55 through 63 (without Carrier 58) were loaded to simulate the estimated weight of passengers as described in the witness statements. The eight loaded carriers were positioned on the down-going line at the lower terminal. The lift was started and accelerated without noticeable carrier movement at Tower 5. After repositioning the loaded carriers to the same starting location of the previous test, the lift was again accelerated to design speed and given a random series of speed change commands. There was significant carrier movement between Towers 2 and 4, but little movement near Tower 5. However, carrier swing between Towers 3 and 4 became so violent that the test was stopped before the lift was damaged. After load repositioning, the test was repeated with rapid speed changes and yielded similar results. Tests were suspended for the day.
On January 2, 2017, there was a meeting of all parties to be involved in additional testing. After lengthy discussions, the investigation team agreed the tuning within the newly installed drive was a probable contributor to the observed carrier instability. The investigative team inspected the new electronic drive. While reviewing the new electric drive parameters, it was noted that one particular setting of the speed controller’s Proportional-Integral-Derivative (PID) parameters, TiS, was set outside of the expected range.
When the lift is operating under electrical power, the lift motor converts electrical energy into mechanical energy to accelerate the lift, and converts mechanical energy into electrical energy to decelerate (slow down) the lift. The electrical energy is managed by the electronic “drive”. The PID portion of the speed regulator controls the motor speed based on the measured real-time motor speed, and the motor speed command. When a new motor drive is installed, the motor drive PID parameters must be programmed based upon the overall mechanical and electrical properties of the individual ski lift. The desired end result of programming the speed controller PID parameters is to have smooth acceleration and deceleration, a constant steady state speed, and minimal speed overshoot. Also, the PID must not overreact to disturbances such as a change in speed command or a change in load. If the PID does overreact, the torque applied to the drive bullwheel can cause line dynamics or surging.
The investigation team noted another drive setting of interest. The deceleration time (“DecTime1”) was set at 6.5 seconds. The DecTime1 makes the lift decelerate rapidly from “fast” to “slow.” The investigation team developed a new test procedure to verify the hypothesis that these important variables (TiS and DecTime1) could have contributed to the carrier instability. A series of six tests were developed to determine the effect of increasing the TiS and DecTime1 parameter values to more traditional values6 used for detachable chairlifts.
The testing continued on January 3, 2017, with the same loaded carrier configuration as used previously. The weather had changed to include a light snow and temperatures ranging between three and five degrees Fahrenheit during the testing with winds less than five mph. The test procedure called for the lift to be operated in the same sequence of speed changes used on the previous tests that resulted in instability. The test procedure that produced the dramatic carrier swing the previous day was muted on the second day of testing due to decrease in temperature. However, the same type of carrier swing was evident at the lower terminal. The team monitored the speed and current at the upper drive terminal. The team observed that increasing the TiS parameter to a typical setting did indeed remove a vast majority of the instability as observed at lower terminal and on the drive display screen at the top terminal. In addition, the investigative team observed that setting a larger deceleration time (DecTim1 changed to 10 seconds) also resulted in decreased torque and carrier dynamics. Following the extensive testing, the investigative team notified the Area that the new electronic drive was unsafe for public operation. The STE further opined that the lift would be safe for public operation using the Diesel Prime Mover since the Diesel Prime Mover operates independent of the electronic drive.
The Board met on January 9, 2017, to review the initial findings of the investigation team. The Board agreed to allow the Area to reopen the QDE lift under a Non-Disciplinary Interim Operation Agreement (“Agreement”) requiring the Area operate the lift under the power of the Diesel Prime Mover as long as there was no interface between the modified drive and control of the lift. In addition, Area personnel had to monitor the lift to watch for any unusual lift dynamics and report any unusual dynamics to the Board. The lift was reopened to the public under this Agreement on January 10, 2017.
On January 18, 2017, the Area personnel contacted the STE when acceleration drive faults occurred with the Diesel Prime Mover. The acceleration rate of the empty lift at times was exceeding the 2.0 feet per second squared (“fps2) limit allowed by the ANSI B77.1 Standard (ANSI 184.108.40.206).
The STE arrived on site the following morning, January 19, 2017, prior to normal daily operations and observed the acceleration faults and some indications of lift instability. The initial slow speed setting of the diesel was reduced and resulted in the acceleration rate being reduced, and within the required ANSI B77.1 Standard (ANSI 220.127.116.11). Following this adjustment, however, the lift began having “parity faults” when the lift was started. This fault is an indication that the return bullwheel had rotated in the reverse direction as determined by the encoder that is mounted on the PTO sheaves at the lower terminal. The STE recommended that the Area close the lift pending further investigation. The Area voluntarily closed the QDE lift for public use through an agreement with the Board, on January 19, 2017.
The STE investigated the dynamic response of the lift. Upon further testing, the STE found that the high deceleration for the diesel and a quick restart was contributing to carrier instability of the lift. The dynamic effects experienced during all speed changes was greatly reduced when there was a time delay between any stops or changes in speed. This time delay allowed the lift dynamic energy to naturally decay and the lift to become steady before adjusting the speed or starting. When a slow command is initiated, the lift must go to that speed and remain there for a minimum of 15 seconds before a fast command can be actuated. Also, the STE found that when the lift comes to a stop as a result of pressing the stop button, if the STE imposed a 15 second delay before the lift could be restarted, the dynamic energy decayed allowing the lift to steady. After additional testing, the STE recommended that the lift was safe for public operation if the deceleration rate was below the ANSI B77.1 requirement of 5.0 fps2 and there was a time delay after a speed change.
After the Board reviewed initial findings of the diesel shutdown, the Area signed an Amended Non- Disciplinary Interim Operation Agreement (“Amended Agreement”) with the Board that required the Area to “take appropriate measures to mechanically modify the lift to stabilize and remove the dynamics from the lift.” The Area contacted Leitner/Poma to complete the modifications to the lift. Leitner/Poma increased the tested time delay between speed changes from 15 seconds to 30 seconds. Following the modification, the STE and Board Inspector Chris Hale, P.E., inspected and tested the lift with multiple load case scenarios, including the eight chairs loaded to simulate the December 29, 2016 incident. The STE and Inspector Hale observed no lift dynamics. Based on the inspection, the Board permitted the Area to reopen the lift for public operation, in accordance with the terms of the Amended Agreement, on January 27, 2017. The terms of this Amended Agreement required reduced speed and Area monitoring of the line. The Area operated QDE under the terms of this Amended Agreement through April 2, 2017, the date the lift closed for the season
On March 30, 2017, a member of the investigation team re-visited the site to gather more information from the lift’s data logger, and to confirm measurements of several components of the line equipment and tension cylinder. The lift is equipped with a data logger which records every fault and produces a line graph of the motor current and lift speed for every start and stop since the QDE lift was installed in 1999. The investigative team reviewed the data to determine the role of the motor drive and lift operator in the incident. On the day of the incident, Record 38 of the data logger shows that the QDE lift accelerating to a “Fast,” the lift’s maximum licensed design speed, and the motor current waveform shows strong oscillations of 400 to 500 amperes (“amps”) with roughly a four second oscillation period. The on-site team only observed current oscillations of this magnitude during testing when commanding an increase in speed while the lift was in a deceleration or motor drive regeneration mode. The current oscillations indicate that the operator had not allowed the QDE lift to reach its steady state speed before commanding a Fast. Record 38 further documents a Tower 5 derail fault when Carrier 58 struck Tower 5 and the lift decelerated to a stop.
The investigation team assembled by the STE included seven Licensed Professional Engineers with combined ropeway experience of over 250 years. After reviewing the physical evidence, the investigation team found that Carrier 58 collided with Tower 5 at an approximate 40 degree angle from horizontal. Witness statements verified that the passengers were ejected from Carrier 58 when it swung and hit Tower 5. The investigation confirms there was no passenger misconduct on the lift, Carrier 58 did not collide with any external fixtures to cause it to swing into Tower 5 and the weather conditions were not a factor. No one on the investigative team has ever witnessed or heard of a similar event. Likewise, literature does not describe such an event.
At this stage of the analysis, the investigation team was able to isolate the two main contributing factors to the incident. In addition, the investigation team identified multiple factors that may have also contributed to the incident. The investigation team is in agreement that the lift should not be operated for public use using any of the primary electric motor drive system that was modified, repaired or replaced.
The investigation team found the following factors contributed to this event: Contributing Factors:
1. Drive Modification. The testing confirmed two of the drive parameters may have created pulses of energy that could explain the rope instability. The first parameter was the TiS parameter with the PID part of the speed controller. During testing the TiS was increased from the setting of 97 to 3200. The amount of lift dynamics was significantly reduced with the higher value.
The second drive parameter (“Speed Filter Time” ABB 50.06) that was evaluated directly influences the actual lift speed measured by the drive. At the time of the incident, it was set at 50 milliseconds (“ms”) compared to a more typical setting of 5 ms. It is probable that the combined effect of this setting along with the TiS setting may have resulted in the drive trying to respond too aggressively to lift demands when changing from “Fast” to “Slow” and back to “Fast” again. With the Speed Filter Time at 50 ms, the motor speed measurements may lag behind when conditions are changing rapidly. In this case, the drive may over-react because it does not detect that the motor is keeping up with corrections.
2. Influence of Speed Changes. Unstable lift dynamics could only be re-created by making rapid changes in the lift speed with the newly installed drive. Many of the witness statements mention multiple changes of lift speed that occurred just prior to the incident. According to the operator, he did not remember making any speed change prior to the incident. Although the data logger does not record speed changes, there is supporting evidence indicating that rapid and significant accelerations/decelerations of the lift occurred immediately prior to the incident. Given the information provided by the data logger, the investigation team believes a speed change occurred immediately prior to the incident.
The investigation team found the following factors may have contributed to this event: Potential Contributing Factors
1. Control System Complexity. The new drive was the first major modification made to the lift’s electrical control system since installed in 1999. The new ABB DCS-800 drive replaced the original Control Techniques drive. The new drive required an interface with the original Pilz/Leitner control system, which also controls the regulated service brake application. At this stage of the analysis, it has not been possible to confirm that the interface was made correctly due to the proprietary nature of the original Pilz programming.
2. Control Board Replacement. Maintenance personnel reported that in February 2016, one of the control boards in the old DC drive was replaced. After the control board was installed, there appears to have been a period where the lift operated with speed/current graph showing signs of motor current instability similar to that seen during the testing. The graphs indicate that the drive was tuned within a week and the instability was reduced.
3. Electric Motor Encoder Possible Damage. During the reinstallation of the rebuilt electric motor, the motor had an internal lead that was not bolted down. Upon energizing the motor, the unbolted lead resulted in a catastrophic failure of the main electrical circuit breaker. The company that rebuilt the motor was consulted and came on site to make repairs to the electric motor. The rebuild and re-installation of the electric motor occurred while the old drive was still in place but prior to public operation in the fall of 2016. Without a complete analysis of the electric motor and encoder, the possible connection or contribution is unknown.
4. Unknown Second Electrical Cycle. The data logs show a consistent low energy pulse into the system at about a 3.7 second interval for the past 18 years. The timing is not speed dependent. The source or significance of this “noise” has not been determined.
5. Tension System Verification. Rope stability is directly related to tension. A single hydraulic cylinder is used in a “push” style application and is near full extension. It is possible the cylinder may be “binding” due to the full extension and not operating consistently to maintain constant rope tension. Any binding of the cylinder could affect the overall rope tension. Detailed forensic testing of the tension system and the hydraulic cylinder would be required to determine if the cylinder is binding and operating correctly when fully extended and the significance to the incident OR a statement that it is determined that it could/would be of little/considerable/no significance.
6. Natural Instability of the Profile. The incident was highly unusual in that the dramatic rope and carrier instability appears to be somewhat localized between Towers 3 and 5. There appears to have been a very unique combination of rope tension, carrier spacing, tower spans, tower height, carrier loading and natural carrier movement that led to the transverse carrier swing that resulted in Carrier 58 hitting Tower 5. Carriers 57 and 58 were relatively lightly loaded with one adult and two children each amongst several fully loaded Carriers 55, 56, 59 and 60. Both Carriers 57 and 58 experienced excessive carrier swing prior to the collision of Carrier 58. Further analysis is required to determine the influence of each of these variables.
7. Natural Harmonic Response of the Haul Rope. Fast/Slow speed commands by the operator in rapid succession along with oscillating bullwheel torque from the selected tuning of the drive combined to excite the natural harmonics in the lower spans of the lift. These factors resulted in excessive carrier swing leading to the collision at Tower 5.
The rope position is primarily controlled by the tower sheave wheels at the end of each span. The suspended carriers can swing on all three axes to create transverse displacement of the rope which must be converted back to the longitudinal rope energy at the tower sheave wheels or otherwise be absorbed by the towers and/or carriers. Tower 5 being relatively tall and more flexible may not have been able to absorb much of the translational energy. Standing transverse wave patterns occur in suspended cables with defined harmonics.
The purpose of the investigation was to comply with sections 25-5-704(b) and 25-5-715(4), C.R.S., and Board Rule 23.3 that require an investigation be conducted “when a death or injury results from a possible malfunction of a passenger tramway.”
Ropeway, or lift, malfunctions resulting in casualties are extremely rare. The incident that occurred on December 29, 2016 at Granby Ranch was unprecedented. Although many factors may have combined to amplify the effect of the rope instability leading to Carrier 58 colliding with Tower 5, as outlined above, the performance of the new drive is considered to be the primary cause of the incident.
Electronic drives are not manufactured specific to ropeways. In fact, the same drive may be used in hundreds of other industries from paper mills and food processing to gravel quarries. In addition, each lift is a one-of-a-kind, unique in its design, terrain, vertical profile, length, tower spacing, tower sheaves, carrier spacing, and type of carrier-detachable or fixed grip. As a result, there are no “standard” settings for the drive, but rather a recommended range of settings for the drive to power a lift. 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.
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 Carrier 58 contacting Tower 5. 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. Witness statements confirmed that the carrier was swinging as it approached the tower. The combination of rapidly changing rope speed amplified the lateral swing to the point that the carrier struck Tower 5. The data logger provides supporting evidence for this conclusion (See attachment J).
The investigation team deemed the drive unsafe for public operation and the lift was powered by the Diesel Prime Mover from the time of the incident until the end of the ski season in accordance with the aforementioned Agreements designed by the Board to ensure public safety. However, the investigation team remains steadfast in its opinion that the existing configuration of the electronic drive and the original, pre-modified low-voltage control system were unsafe for public operation.
Full report here: