The Colorado Passenger Tramway Safety Board released its 151-page final report on last December’s fatal accident at Granby Ranch this afternoon. The investigative team included seven professional engineers with more than 250 years of combined lift experience with support from Leitner-Poma and Granby Ranch personnel, among others. The team conducted extensive tests on the Quickdraw lift from the afternoon of the accident through January 5th and spent months writing this detailed analysis, identifying contributing factors and making recommendations for changes. Appendices include witness statements, photographs and prior inspection reports but the core of the document is 13 pages which everyone who works on ski lifts should read. I’ve done my best to summarize below.
Chair 58 contacted tower 5 at a 40-degree angle that morning due to two contributing factors. The first was the tuning of a new drive installed last Fall by an independent contractor. Two specific parameters may have created pulses of energy and rope instability, the report notes. “It is probable that the combined effect of [these two settings] 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.” The second contributing factor was the influence of one or more speed changes leading up to the incident.
Other potential contributing factors were:
- Control system complexity resulting from the new ABB DCS800 drive’s interface with older Pilz/Leitner low-voltage controls.
- A control board replacement from February 2016.
- Possible damage to the electric motor encoder.
- Unknown electrical cycle shown in data logs that had occurred at a 3.7 second interval over the entire life of the lift.
- Tension factor(s) that would require more testing to determine.
- Natural instability of the profile. “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,” the document states.
- Natural harmonic response of the haul rope.
Wind was not found to be an outside influence, nor was passenger conduct. “The incident that occurred on December 29th, 2016 at Granby Ranch was unprecedented,” the investigative team wrote. “Although many factors may have combined to amplify the effect of the rope instability leading to Carrier 58 colliding with Tower 5, the performance of the new drive is considered to be the primary cause of the incident.” The report explains electronic drives such as the DCS800 added to Quickdraw last year and used on many lifts are also used in a wide variety of other applications. The tuning and “fine-tuning” of a drive is complex and unique to each application and lift. “It appears the new drive was not comprehensively tuned to this particular lift during installation,” the document says.
Perhaps most important are investigators’ recommendations for changes to the ANSI B77 standard and/or Colorado’s rules moving forward:
- Ensuring tuning and adjusting of a drive to optimum values for correct response under multiple loading conditions, not just a full lift or an empty lift.
- Improving acceptance testing, such as loading a small grouping of carriers, then making multiple speed changes and placing personnel in different places along a lift to observe line dynamics.
- Requiring speed change time delays.
- Requiring stop time delays before a re-start.
- Considering combining return bullwheel speed monitoring with drive sheave speed monitoring to indicate possible dynamic instabilities when values do not match.
- Reviewing requirements for an engineer to consider when an analog drive is replaced with a digital one.
- Considering conflicts when only part of a complex control system is upgraded.
- Reviewing acceleration/deceleration overspeed monitoring requirements.
- Addressing the licensing aspect of tuning an electric drive and considering issuing temporary licenses until “fine-tuning” is complete and can be signed-off on by a professional engineer.
- Considering requiring “black boxes” on lifts that record stops, starts, speed changes, and any parameters that have significant dynamic effects on the haul rope.
All of these recommendations come with trade-offs and will certainly be discussed in-depth by the ANSI committee and state tramway boards. The CPTSB yesterday formally referred the Quickdraw incident report to the Colorado Department of Regulatory Agencies Office of Investigation for review and possible disciplinary action. In the meantime, the lift industry will move forward, learning from the past and working to ensure these machines transport people as safely as ever.