CS39 Chainsaw Safety Rules (Aerial Standards)
CS39 covers the competent use of a chainsaw from a rope-and-harness system during aerial tree work; it is not a general climbing or ground-felling qualification. Work only within your current training, assessment, and local rules. Check the saw manual, plan rescue and communication, and stop if your position, control, or cutting plan is not safe.
Inspecting a tree and finding a dead limb can make a chainsaw seem like the quickest fix. But how do you know the work is within your training, and that a cut will not put you, your climber, or the tree’s structure at risk? A wrong cut can cause serious injury, and a poorly placed pruning cut can leave lasting damage.
CS39 is a UK chainsaw qualification commonly associated with use from a rope-and-harness system. Qualification names and assessment requirements may change, so confirm the current requirements with the relevant awarding body. A certificate does not make every aerial cut safe. Site conditions, equipment, training, and the task all matter.
What CS39 covers, and where its limits are
Treat CS39 as evidence of assessed chainsaw skills, not permission to take on any aerial cutting job. The operator must stay within current competence and follow the rules that apply where the work takes place.
Aerial chainsaw work combines cutting hazards with work-at-height hazards. The saw can injure the operator or another person, while a falling limb can strike people, damage property, or load the climbing system. A cut can also trap the bar or cause wood to split unexpectedly.
The work may be guided by applicable workplace safety rules and arboriculture guidance, such as ANSI Z133 in the United States. In the UK, follow current Health and Safety Executive guidance and relevant industry practice. The right requirements depend on location, employer, and work type.
Decide whether to proceed
Proceed only when the task, site, rescue plan, and equipment fit your training and the work can be controlled. If any of those conditions are missing, stop and seek qualified help.
Do not treat a certificate as a substitute for assessing tree condition. Look for deadwood, hanging branches, decay, weak unions, cracks, and limbs under tension. Consider how the planned cut could affect the climber’s position and the remaining tree. Complex, heavily loaded, or hung-up limbs may call for specialist planning.
If the work involves a tree near power lines, do not guess at a safe distance. Requirements depend on the voltage, work, and local rules. Contact the utility or a qualified line-clearance professional where needed.
Equipment setup and site safety
Check the saw, climbing system, communication, and rescue plan before ascent; do not rely on equipment labels alone. Compatible, well-maintained gear is part of safe work, but it cannot correct poor positioning or a weak cut plan.
Match the saw to the work
Use a saw, bar, and chain combination approved by the saw manufacturer. There is no universal safe bar-length-to-engine-size ratio. A bar that is too long or a chain that does not suit the saw can reduce control and raise kickback exposure.
Check the wood and the cut, not just whether the tree is hardwood or softwood. Species, diameter, defects, and the forces within a limb all affect how it may respond. Follow the exact saw and chain manuals for fuel, chain, bar, and maintenance specifications.
Inspect and plan before climbing
Inspect the saw’s chain brake, throttle lockout, stop switch, chain catcher, handles, anti-vibration mounts, bar, chain, and guards. Check that climbing and positioning gear, connectors, and the saw attachment system are compatible and undamaged. Remove defective equipment from service.
Before ascent, confirm who is climbing and who is on the ground, how they will communicate, and how rescue will be carried out. Set a drop zone and keep other people out of it. Check for overhead hazards, escape options, and public access.
Wear the protective equipment required for the task and jurisdiction. This commonly includes head, eye, hearing, hand, leg, and foot protection suited to chainsaw work. PPE reduces some risks; it does not replace safe work positioning, a controlled cutting plan, or a workable rescue plan.
A safe aerial chainsaw work sequence
Use a planned sequence: check the site and equipment, function-check the saw on the ground, climb and position, make controlled cuts, then lower and secure the saw. Stop if communication, footing, saw control, or rescue support becomes inadequate.
- Complete the checks. Confirm PPE, saw condition, climbing system, roles, communication, rescue arrangements, drop zone, and hazards. Assess the limb’s condition and load before choosing a cut.
- Start and test the saw on the ground. Follow the saw manual and use a stable, controlled position. Check that controls and safety features work. Do not start the saw aloft or while climbing.
- Climb and establish a stable work position. Keep control of the saw and its attachment system. Plan how the limb will move and where it will go before engaging the chain.
- Make only cuts you can control. Keep the saw below shoulder height where practicable, use a secure stance, and maintain a clear retreat path. Avoid cutting with the bar tip, which can increase kickback risk.
- Stop before moving. Stop the saw before repositioning, passing it to someone else, or leaving the cutting position. Never climb with the saw running.
- Finish safely. Lower and secure the saw, shut it down, let it cool, and clean it as directed by the manual. Report defects and keep damaged equipment out of service until repaired and checked.
Keep the chain, bar, and falling material away from climbing lines, positioning systems, and other people. Use a cut sequence suited to the limb’s weight and tension so the chain is less likely to bind and the limb is less likely to move unexpectedly. Do not force a cut when the wood’s response is unclear.
Stop for barber-chair risk
Stop and reassess if a loaded stem or limb may split lengthwise during cutting. This failure, often called a barber chair, can make wood rise or split violently and can strike the climber or damage climbing equipment.
Check the wood’s load, attachment points, cut plan, and retreat route. Use only a method that is within your training and allows the forces to be controlled. If you cannot assure control, do not make the cut; get task-specific help.
Maintenance, sharpening, and storage
Follow the exact saw and chain manuals for all settings; generic figures are not safe substitutes for model-specific instructions. A poorly maintained saw can cut unpredictably and increase the effort needed to complete a cut.
- Fuel: Use the gasoline and oil ratio specified by the saw manufacturer. A 50:1 mix is common, but not universal. Never use straight gasoline in a two-stroke saw that requires mixed fuel.
- Chain fit: Match pitch, gauge, drive-link count, bar, and sprocket to the manufacturer’s specifications. A 3/8-inch low-profile chain with a .050-inch gauge is one possible configuration, not a CS39 rule.
- Depth gauges: Set them with the chain maker’s specified tool and instructions. There is no single safe setting for every chain.
- Bar oil: Use suitable chain-and-bar oil for the saw and operating conditions. Check oil delivery using the manufacturer’s method.
- Engine speed: Idle and maximum no-load speed vary by model. Use the specified procedure and a tachometer where required. Do not tune by sound or exceed the stated limit.
- Sharpening: Use the specified file size, angles, and depth-gauge tool. Replace a damaged or excessively worn chain, bar, sprocket, or safety part. Do not increase engine speed to make a dull chain cut.
Before service, shut down the saw and let it cool. Clean the chain-brake area, bar groove, oil ports, air filter, and cooling fins as directed by the manual. For storage, follow the maker’s fuel-system guidance; store fuel in approved containers away from ignition sources. Cover the chain and secure the saw against unauthorized use. Recheck chain tension and lubrication before the next job.
Field reference: checks that matter
Use this table as a quick reminder, not as a replacement for training, the saw manual, or a full site assessment.
| Check | What to confirm | Stop-work sign |
|---|---|---|
| Operator and task | Work fits current training and assessment | Task exceeds competence |
| Site and tree | Deadwood, decay, unions, tension, hazards, and drop zone assessed | Uncontrolled limb or public exposure |
| Climbing and saw gear | Compatible, intact, and suitable for the task | Damage, poor fit, or failed safety part |
| Start-up | Saw started and function-checked on the ground | Faulty controls or safety devices |
| Cutting position | Stable position, controlled reach, clear retreat path | Unstable footing or poor saw control |
| Communication and rescue | Roles, signals, and rescue plan confirmed | Lost communication or no workable rescue |
Field experience and common errors
In municipal tree work, I have seen a modest dead limb turn into a complex job because decay or a weak union changed how it was supported. The lesson is not that every suspect limb needs special equipment. It is that a limb’s appearance alone does not show how it will move when cut.
A common mistake is to focus on the saw while overlooking the climbing position and the limb’s load. Another is to assume that a familiar fuel mix, chain setting, or bar size suits every model. Check the manual rather than relying on memory.
For homeowners, a dead or decayed limb is a reason to seek an assessment, not to try aerial chainsaw work without the right training and rescue support. If pruning is appropriate, the cut should preserve the branch collar rather than damage the trunk or leave a long stub. Timing and cut placement depend on species, condition, and the purpose of pruning; they do not make unsafe aerial work safe.
Frequently asked questions
Is CS39 a general tree-climbing qualification?
No. CS39 is associated with chainsaw use from a rope-and-harness system, not a general qualification for all climbing or ground-felling tasks. Training and assessment names can change, so check current awarding-body requirements. You must also follow local safety rules and stay within your current competence for each job.
Can I start a chainsaw while climbing or once I am in the tree?
No. Start and function-check the saw on the ground, following its manual, before climbing. Do not start it aloft or while climbing. Stop the saw before repositioning, passing it, or leaving the cutting position. If safe control is not possible, stop work and reassess.
What bar length is safe for aerial chainsaw work?
There is no universal safe bar length for aerial work. Use only a bar and chain approved for that saw, then consider the cut, wood diameter, and your ability to control the saw. A longer bar does not make a task safer and can increase control and kickback concerns.
Is 50:1 fuel mix correct for every chainsaw?
No. A 50:1 gasoline-to-oil mix is common, but the correct ratio depends on the saw and oil instructions. Check the exact model manual before mixing fuel. Using the wrong mix can damage the engine, while straight gasoline is unsafe for a two-stroke saw that needs mixed fuel.
What should I do if a limb is under tension or starts to split?
Stop cutting and reassess the limb, its support, the forces in the wood, your position, and your retreat route. A barber-chair split can rise violently and put the climber or climbing system at risk. Do not continue unless a competent, task-specific method can control the cut.
Does CS39 mean I can prune any tree with a chainsaw?
No. A qualification does not make every tree or cut suitable for you. Assess the tree, the load, nearby hazards, access, and the pruning objective. A homeowner should use a qualified professional for aerial chainsaw work, especially where decay, unstable limbs, utilities, or public access create added risk.
(This article was written by one of our staff writers, Nathan Pierce. Visit our Meet the Team page.)
