What blade damage categories mean
Almost every inspection provider grades findings on a numbered scale, typically one to five, from cosmetic to immediate action. The scales are not standardised between providers, which is the first problem: a category 3 from one contractor is not a category 3 from another, and a fleet inspected by two providers cannot be compared without mapping the scales to each other.
What the scales have in common is a progression through four questions:
- Is the damage in the coating only, or has it reached the laminate?
- Does it open a path for water into the structure?
- Does it reduce the section's ability to carry load?
- Is it growing, and how fast?
Damage that fails only the first question is a yield and maintenance matter. Damage that fails the second is a durability matter with a clock on it. Damage that fails the third is structural, and the only remaining question is how quickly.
When damage is genuinely cosmetic
A finding is cosmetic when the load path is untouched and no water can reach the laminate. In practice that is a short list: surface scuffs and handling marks in the topcoat, light pinholing, logo and paint degradation, minor hail marking that has not broken through, and staining.
The trap is that "cosmetic" is a statement about the present, and blades are inspected every year or three. Coating loss that is cosmetic today is a laminate exposure in two winters if it is on the outer third of the leading edge, where the relative velocity and the rain do the work. Cosmetic findings on the leading edge outboard of roughly seventy percent radius should carry a rate, not just a category.
When damage becomes a water path
This is the category that owners under-repair, because the blade still looks intact and the turbine is still producing.
Once the gelcoat is breached and the laminate is exposed, water enters. In a glass laminate it wicks along fibres and into any void or bond line it reaches. It freezes and expands. It degrades the resin interface over time, and it moves the damage from the surface into the structure without anything new being visible from outside. The same is true of an open bond line at the trailing edge, an unsealed repair boundary, a puncture near the tip, and a blocked drain hole that leaves standing water in the root end.
The decision rule here is simple and often ignored: seal before winter. A laminate exposure repaired in autumn is a coating job. The same exposure found the following spring, after a season of freeze and thaw, can be a laminate repair with a scarf and a cure schedule that needs a tent and a heater.
When damage is structural
Structural damage is damage that changes how the blade carries load. The findings that belong here are specific:
- transverse cracks over the spar cap, on either surface, at any length. The spar cap carries the flapwise bending moment, and a crack across it is not a surface feature.
- spar cap wrinkles, delamination or fibre breakage, which cut compressive strength and fatigue life, and which are usually found by ultrasonic testing rather than by eye.
- open or cracked trailing edge bond lines, particularly near maximum chord, where edgewise fatigue drives the crack along the bond.
- shear web debonds, which remove the shear connection between the shells and let the section deform in a way the design did not assume.
- root laminate cracks, moved inserts and lost bolt pretension, at the joint that carries the entire blade.
- any crack that has grown measurably between two inspections, whatever its category on the day it was first seen.
For these, the question is not whether to repair but whether the blade is fit to turn until the repair happens. That is an engineering assessment against the design basis, not a scheduling preference, and it is the point where an owner should have someone independent of the repair contractor doing the assessment.
The four inputs to the repair decision
Category alone does not produce a decision. Four things do.
Growth rate. A 300 mm trailing edge crack that was 300 mm last year behaves differently from one that was 80 mm last year. Growth converts a monitor decision into a repair decision faster than severity does. This is the argument for measuring findings rather than describing them: without the previous measurement, there is no rate, and every campaign starts from zero.
Position on the blade. The same defect matters more outboard for erosion and aerodynamic loss, and more inboard for structural load. A crack at 15 m radius on a 70 m blade sits where the bending moment is high. The same crack at 60 m sits where it is not. Categorisation scales rarely capture this, which is why radius belongs in every finding.
Access cost and season. Reaching a blade is most of the cost of a small repair. That is why the honest decision is often to accelerate a category 2 finding into this year's campaign because the platform is already there, and to defer a category 3 finding that is stable until the weather allows a proper cure. Cure temperature and humidity limits are real constraints in a Northern European winter, and a repair done outside them is a repair that will be done again.
Contract and warranty position. Whether the damage is a warranty claim, a serial defect, an insured event or ordinary wear changes who pays and therefore how the work should be documented. Repairing first and arguing later routinely destroys the evidence. Where a defect may be a manufacturing defect, the blade should be photographed, measured and if possible sampled before the repair removes the fracture surface.
The case for repairing earlier than the category suggests
Blade repairs escalate in cost by roughly an order of magnitude at each step: a coating repair on a platform, a laminate repair requiring a tent and controlled cure, a structural repair requiring engineering sign-off, and a blade exchange requiring a crane. The crane is the cliff. Most of the value in a blade maintenance programme comes from keeping findings below the step where a main component lift becomes necessary.
That argues for treating water paths as urgent even when they are graded low, and for bundling small repairs into a campaign that is already mobilised rather than optimising each finding separately.
Frequently asked questions
Can a blade keep operating with a known crack?
Sometimes, and it is an engineering decision, not a default. It requires an assessment of the crack against the design basis, a defined re-inspection interval, and usually a documented limit on what the turbine may do in the meantime. A crack across a spar cap is not a candidate for this.
Who decides the damage category, and can it be challenged?
The inspection provider assigns it, and it can and should be challenged where the reasoning is not given. Categories drive spend, and a category assigned by the party who will quote for the repair deserves an independent read.
Does leading edge erosion need repairing if production is still normal?
Production loss is only part of it. Once the laminate is exposed, the damage is no longer aerodynamic, and the repair cost rises with the season it is left. Erosion that has broken through the gelcoat should be sealed at the next available access regardless of what the power curve says.
What should be done before any repair on a blade that may have a manufacturing defect?
Record it properly first: photographs with scale, measurements, the blade serial, plant, mould and production date, and where possible a sample or cross-section from the affected area. Once the repair is done, the evidence of what caused the damage is inside the repair.
How Apex Wind can help
We are an independent blade engineering consultancy in Denmark. We assess damage against the design basis and give owners, insurers and lenders a defensible answer on whether a blade is fit to operate, through blade failure investigation and blade design and structural assessment. We review inspection and repair recommendations as part of technical due diligence. We carry out no repairs and take no referral fees, so the recommendation to repair or not is not a quotation. If a finding needs an independent read before it drives spend, contact us.