Base Machine Pulled Over by Steep Slope Machine


On a winter day in the Pacific Northwest, a harvester operator on a steep slope machine (SSM) operation had a near miss when he pulled over a base machine tethered to his harvester.

The operator had over 43 years of logging experience, including shovel logging, cutting, tower logging, and management.

The operator located the base machine on a gravel road with a four percent grade. He buried the bucket to half its depth behind a stump. The tracks of the base machine were in line with the bucket. The road was crowned, which allowed for the base machine to rock. The sheave was at 35 degrees, and the harvester was on a 60% slope. He only had six trees to fall, and in this location, the trees could have been felled without the aid of the line.

As he was working, the over-speed sensor was activated on the harvester. This indicated that one of the two drums on the base machine was releasing line quicker than the other and not in unison. On this machine at that time, when the over-speed alarm was initiated, the parking brake would engage, locking the drums. The alarm would have to be reset before the base machine could operate again.

After the alarm was activated, the operator stopped to fell a tree. When he completed the cut, he began traveling downslope. He had forgotten to reset the over-speed alarm, so the parking brake remained engaged. When the operator moved the harvester downslope, the back of the base machine was lifted, causing it to pivot and fall over on its side.

In this instance, the over-speed alarm was activated prior to the incident, and the parking brake engaged, locking the drums. This was not by design, and there had been issues with this occurring previously on this machine. Although the operator was aware of the issue, he did not reset the alarm to disengage the parking brake before traveling downslope to continue work. With the base machine located on the crowned road and the parking brake engaged, it took little force for the base machine to be pulled over when the harvester began to move.

Fortunately, there were no injuries resulting from this incident.


  • When first setting up the base machine on a steep slope operation:
    • Bury the bucket completely.
    • Extend the boom past 90 degrees.
    • Set up the base machine, so the pull is as straight as possible.
  • After repositioning the base machine, ensure it is stable prior to working. This can be accomplished by moving the SSM to a safe distance from the base machine but where it is still visible. Travel the SSM back and forth a few times at higher tension. This will stabilize the position of the base machine.
  • If the location for the base machine is not level, place cribbing on the low side and walk the base machine on it to level out or slightly elevate the front of the machine.
  • Before operating the system, ensure that all safety systems are functioning properly.
  • Even very experienced feller buncher operators can be caught off guard by some aspects of tethered systems. The technology is new, and industry best practices are being developed. Extra attention and caution should be used as operators familiarize themselves with the differences presented by tethered systems and SSMs.

Crowned Road
Figure 1: Diagram of Incident Scene