Common Causes Of Zipline Failure


Can you guess what's wrong with this picture?

Flying foxes fail for many reasons. For the scope of this article it means failing to pass inspection as well as a real catastrophe. Below are some of the situations we've encountered in the last 12 months. In no particular order ...

  1. Hardware is not rated
  2. Shackles not moused or lack of split-pins
  3. Anchor points are not backed up
  4. Corrosion on cable or hardware
  5. Trees growing over anchors or cable
  6. Poor performing wire grips
  7. Incorrect main cable tension
  8. Broken wires or deformed cable
  9. Missing documentation

Hardware is not rated

All hardware used in the system requires a stamp indicating its Safe Working Load or SWL.

Occasionally this may not be possible such as for the main cable. A certificate of performance is needed instead. An example might be through destruction testing which records at what load a sample of the same cable broke.

Shackles not moused or lack split-pins

Shackles need a way to prevent their pins from working loose (or be fiddled with). Split pins are quick and easy to apply. Otherwise a thin wire wrapped through the pin's eye and around a shackle body is fine too. This technique is called mousing.

Anchor points are not backed up

Where cables support life their anchor points require a secondary attachment system or backup. If the main anchor point fails then the backup anchor saves the day. Both ends of the main zipline must have the anchor points backed up. Many of the ziplines we survey fail on this point.

Corrosion on cable or hardware

Corrosion or rust is normally easy to spot. A slight discoloration may not be significant. Distinct brown patches with a scaley surface should ring alarm bells.

Beware of hidden rust. The photo below shows the cable end wrapped in duct tape. This sweats and traps moisture inside which promotes corrosion. It also makes the fixture impossible to inspect. An instant gong!

Anchor wrapped in duct tape.jpg
Shackle pin is moused but eye is rusty and covered in tape. Failed !

Trees growing over anchors or cable

Zipline cables can be anchored to trees. However, this practice introduces new challenges. The tree must be inspected by an Arborist at least every 12 months. If there has been a bad storm the tree must be inspected again. Where there is no such recent arborist report the zipline has failed.

The big problem with trees is that they grow. The trunk expands and envelopes the cable or anchor. Acids in tree sap can seep into wire strands and rust out hardware. Once grown over the system cannot be verified for safety.

Poor performing wire grips

Many flying foxes use wire grips (bulldog clamps) to secure the main cable. They are convenient in that cable can be cut to length on site. But legally they have on-going maintenance challenges that are not worth the initial saving. Wire grips must be,

  • Tightened to within a specified range - not too loose or too tight
  • Inspected and passed annually by an engineer - additional costs
  • Monitored often by a rigger and retensioned as needed - more expense

Ziplines that fail using wire grips mostly do so through lack of verifiable reporting. They are also responsible for loss of life in real incicents. We recommend don't use them.

Incorrect main cable tension

Zipline tension is critical to avert problems.

When over tight the cable puts stresses on other components. An example is the main image at the top of the page. The cable has physically pulled the anchor bolt through the post. Daylight shows between the seat of the bolt and the timber. In time the system will collapse. If it does there is also no backup to prevent disaster.

If the cable has low tension the system performs poorly as a flying fox. Too much sag inhibits the fun factor for participants. The cable may also drop too low to the ground for safety. Dynamic loads from extreme winds can affect hardware connections.

In fact, all zipline components must be tensioned correctly. Static and dynamic load testing is used to determine the system will not slip when used. During these tests tension is adjusted accordingly. Load testing is required every year.

Broken wires or deformed cable

Cable is easily deformed if not properly installed. Over tightened wire grips are a common culprit. Rubbing of components can lead to wire strands breaking. This in turn lowers the SWL of the cable. If 10% of strands are broken the zipline is no longer serviceable.

Damage is also be caused by,

  • Incorrect trolley size for the cable
  • Wrong sheave diameter for cable diversions
  • Rusting and corrosion

Missing documentation

Last but not least is the requirement to maintain up to date documentation. It is a legal responsibility under the standard to do so. Missing information means the zipline must be decommissioned until everything is in order. Running such a zipline is an offense.

Documents prove all legal obligations have been fulfilled. They can be requested by anybody, including zipline participants. Not just Work Safe inspectors. So what information should be kept? At a minimum,

  • 6 monthly survey report by advanced rigger
  • Annual load testing report by NATA accredited testing authority
  • Annual arborist report (if connected to tree)
  • Engineering plans from construction
  • Destruction testing certificates (of un-rated hardware)
  • 12 monthly engineer inspection report on wire grips (if used)

Wrapping up ...

As you can see, running a zipline operation has considerable responsibility. People trust their lives into the operator's care.

The potential causes for flying fox failure are many. More than have been listed here. But by choosing to operate within Australian standards we ensure the industry remains safe and fun for everyone.