FISC safety alert: Yarder stabilising ram failure incident
Friday 27 Mar 2026
The forestry sector has long relied on Thunderbird TY50 and TY70
tower yarders—machines designed to be light, transportable, and
efficient. However, a serious harm injury of a worker, has highlighted a
critical safety concern that demands immediate attention.
The yarder was being prepared to shift. Running lines and guys had been
retrieved. The tower was pushed into vertical position and the
stabilising ram locked. The guy ropes were hooked on the chassis and
ratcheted up to remove excessive slack.
However, during the rope retrieval the tail rope over-spooled or
“birdnested”. A plan was then made to untangle the rope and
run the tail rope out so it could be correctly fleeted back onto the
drum. A grapple operator was positioned in front of the hauler to pull
the rope out and then manage rope tension as it was wound back on.
Another worker was deployed to sit beside the drum and assist fleeting
the bottom layer and a 3rd worker, the
hauler driver, had the tail drum engaged, and idling dead slow in 1st
gear. The yarder driver was observing the drum and stabilising ram
through the cab side window. The rope was being wound slowly onto the
drum, and the first layer was half completed when without warning the
spear on the stabilising ram snapped.
The broken spear shot forward and landed on the workers hand who had
been positioned near the drum. The injury was severe requiring four
fingers to be amputated
Investigation findings
- The investigation revealed that the incident was the result of a
stabilising ram failure.
- It identified that the Thunderbird manual prescribes the slackening
of all guylines before rope recovery, therefore the tower was not
supported by guylines when the stabilising ram failed.
- Also, the stabilising ram spear had been replaced with induction-hardened steel rather than the original mild steel design. Induction
hardened steel provides excellent wear resistance and strength but does
not have the same ductile microstructure of mild steel, which allows it
to deform (bend or stetch) significantly under stress before fracturing,
it absorbs energy well.
Recommendations for TY50 and TY70 tower yarders
- Guyline safety: While Thunderbird manuals allow slackening all
guylines and locking the pole before rope recovery, it is recommended to
keep at least one rear guyline in place to prevent tower collapse if the
stabiliser ram fails.
- Stabilising ram spear: Owners must verify the steel grade of
stabiliser ram spears. It is recommended that a replacement spear should
be consistent th the original mild steel composition with good
energy absorbing ductile properties.
- Lagged drums: Rope fleeting exposes workers for 15–20 minutes.
Lagging drums reduces this risk. Owners should consider replacing at
least the front drum when machines undergo major servicing.
- Annual inspections: Towers must be inspected annually by CBIP-certified inspectors. Inspectors should be briefed on the Kohurau
incident and the importance of spear verification and ram integrity.
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Source & image credit: New Zealand Forest Industry Safety
Council
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