Granby Ranch HSQ Accident Report
Quickdraw lift at Granby Ranch, Colorado is a 1999 Leitner detachable quad
where a “rare dynamic event” last December caused a a chair to contact
this tower, killing one and injuring two children.
Colorado Passenger Tramway Safety Board released its on last December’s 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
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.
electrical cycle shown in data logs that had occurred at a 3.7 second
interval over the entire life of the lift.
factor(s) that would require more testing to determine.
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
- Natural harmonic
response of the haul rope.
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.