Incident Ropeway Overview
Triple Chair was manufactured by CTEC (Cable Transportation Engineering Company) and installed in 1983. Rise: 1000’ Length: 5000’ Line speed is approximately 475’ ft/min with a 6 second loading interval. Carrier capacity is 3 riders. It is a “Left-Hand-Up” non-height adjustable, bottom drive tension. Tension is by a concrete counterweight. At the time of the incident, the carriage was at mid-travel, the counterweight had free travel. The Drive Terminal is a 4-leg design; considered a first generation carryover from the engineers Thiokol ropeway experience. Regarding swing - comparatively speaking the carrier tolerates wind. The carrier design has had little or no changes throughout time. The stem to sheave/assembly clearance is well past the recommended range of degrees of swing. The sheave liner design provides a flat area before the rope contact with the flange. The grip tensioning mechanism also acts as a swing dampener. The control/comm line is affixed to the tower tops with the crossarm clamped at a lower position at all towers. All towers have available height and angle adjustment space.
A rope deflector/catcher is located at each subassembly. Rope catchers have blended profiles for rope passage and have been updated. Design geometry of the primary and subassemblies would have grip contact in red (based on grip location) when a partial derailment occurs.
Newer example shown for clarity.
The grip jaw design is external with raised edges that will impact rope catchers.
Newer example shown for clarity.
No main axel catcher/deflector is included, this could allow for assembly rotation unless a full “catch” occurs. Possible caught rope locations are shown only one direction for clarity. Newer example shown with anti-rotation bolt.
Author's Note: Most ropeways have these design characteristics it is not unique to this manufacturer.
Overview continued... The top terminal is fixed. The unload ramp is a raised wooden structure with the ramp breakover at the guide sheave. The operator station has an unobscured view of the lift-line to the tower below the upper span. The tower below the top span is approximately 375’ from the operator station (picture above). That span is treeless. The station has an unobscured view of the bullwheel. No other large structures exist nearby. The profile has spans that cannot be seen by either station. The ropeway is under USFS oversight, annual inspections are deferred to insurance representatives. No State oversight exists. It has undergone previous load tests. CTEC was partnered by Garaventa then rights were acquired by Doppelmayr. CTEC’s principal remained in the industry and formed another ropeway company. Recent photos and videos show an anemometer is located on the unloading tower approximately 4’ above the crossarm affixed to the tower cap. Older pictures show no anemometer at that location.
Ownership
The resort is owned by JMA Ventures a real estate investment company located in California. They also own Homewood at Tahoe.
It previously owned Alpine Meadows then sold it to Squaw Valley (now Tahoe Palisades) in 2011.
Ski Resort Overview
This 6 chairlift resort is located in the Beartooth range in Montana close to the Wyoming border. The climate is drier in the winter and wetter in the summer. It has 30% snowmaking coverage, but large portions of steeper/rougher terrain still need sufficient snowfall to open. Triple chair feeds multiple lifts, the runs nearby are covered by snowmaking
Incident Weather Description
Barring any mechanical failure that might have precipitated the derailment, it is helpful to present all the weather factors. A starting point for an examination is a press release from the resort…”a gust of high wind was the cause” …
Gust Definition: a short duration or burst of wind.
We can clarify that definition to say: a gust is a burst of wind above existing or “steady-state” wind.
It would be wrong to say gust speed always parallels steady-state wind. Gusts do have a correlation to atmospheric conditions, daily heating cycles and weather fronts. Gust direction is not forecasted due to the chaotic nature of the atmosphere. Gust occurrence and speed have been forecasted successfully by weather models for the last decade. Reviewed below are the current and forecasted weather the day of the incident to give the reader a broad weather picture.
Jet Stream
The bulk of the jet is over Montana with a convergence west of the resort. The jet should not be considered a compact stream but a river of great depth with many off-shooting fingers that will influence lower elevation weather.
Frontal Locations
Forecast chart of Front’s produced around midnight – no Fronts cross or approach the resort at 8AM, but one obviously exists on Montana’s northern border. Moving to the 11AM picture, notice there is a lot of “messiness” with the Fronts just north of the resort. At 2PM this trend continues. Fronts in these pictures are simplified for the viewer. They can be segmented or missing or quite strong. Any time between 8:01 and 12 noon this forecasted Front system is very close the resort. As a rule of thumb, the air is colder on the northside of a Front. The black lines are wind flow.
Front Locations 11 AM
Approximate front location at the time of the incident, pink lines are idealized flow to low pressure areas.
Temperature
This is a temperature plot of a station that is located at the resort. It shows a sharp change the morning of the incident - this is more than likely associated with daily heating, as other days show the same steps.
Barometric Pressure
A 7 day pressure plot for the region. Pressure drops, especially steep drops are associated with higher wind as the atmosphere is trying to balance the different pressure regions. Notice the trend started the day before. As mentioned above, gust speed is greater than steady-state wind.