What a damage category actually is
Nearly every inspection provider grades findings on a scale, usually 1 to 5 or similar, from cosmetic to critical. The scales look alike and are not the same. One provider's category 3 is another's category 4, the definitions are written by the provider, and they are often not printed in the report. Before comparing two reports, or trending a fleet over time, the first question is whether the same scale was used, and by whom.
As a working translation, most five-point scales mean roughly this:
- Category 1. Cosmetic. Coating marks, light crazing, dirt. No action beyond routine maintenance.
- Category 2. Minor. Early leading edge erosion, small coating cracks, receptor marks. Repair at the next planned campaign.
- Category 3. Moderate. Erosion into the laminate, open coating cracks, small trailing edge splits, minor lightning damage. Repair within a defined period, typically months.
- Category 4. Serious. Laminate exposed and damaged, trailing edge cracks near maximum chord, delamination, longitudinal cracks along the spar cap. Engineering assessment and prompt repair; operation may be restricted.
- Category 5. Critical. Transverse cracks over the spar cap, structural lightning damage, root cracking, open bond lines. Stop the turbine until an engineer has assessed it.
The boundaries between 3, 4 and 5 are where the money is, and they are the ones inspectors most often disagree on.
What a good report contains
For each finding, a report that can be acted on gives:
- Blade identity. Turbine, blade position, and the blade serial number, not just A, B, C.
- Location. Distance from the root in metres, the side (pressure or suction), and the chordwise position (leading edge, spar cap, trailing edge). "Mid-blade" is not a location.
- A photograph with scale. Close enough to see whether fibres are exposed, with something in frame that gives the size.
- Damage type. Erosion, crack, delamination, lightning, impact, repair defect, using a defined vocabulary.
- Size. Length and width, and for erosion the stage.
- Category, with the definition. The scale printed in the report, not on a separate document.
- Recommended action and timeframe. Monitor, repair within N months, assess before operation.
- Comparison with the previous inspection. New, unchanged, or grown, with the previous photograph beside it.
A report missing the last item cannot tell you the thing that matters most about any crack: whether it is growing.
Where reports go wrong
In our experience the same failures recur across providers.
"Monitor" as a default. A finding graded 3 with the action "monitor" is often a finding the inspector could not classify. Monitoring is a decision that requires a next inspection date and a method that can see the change. Without both, it is a deferral.
Category inflation and deflation. Inspectors under time pressure grade towards the middle. Inspectors who also sell repairs have an interest in the upper half of the scale; inspectors paid per turbine by an owner have an interest in the lower half. Neither is dishonest; both are structural.
External only. A drone report covers the surface. It says nothing about bond lines, webs, the root laminate or anything inside the blade, and it should not be read as a statement that the structure is sound. A report that gives a clean bill of health from drone imagery alone is reporting on the coating.
No method statement. How the blade was positioned, what camera and resolution, what lighting, and whether all four faces of every blade were covered. Without it, a missing finding cannot be distinguished from a missed one.
Prior repairs unrecorded. A repair done by a different contractor two years ago will appear as a surface anomaly, get graded, and either alarm or reassure depending on who is reading. The repair history belongs beside the report.
Categories compared across scales. A fleet trend built from three providers' reports over five years is trending the providers, not the blades.
Reading a report as a decision, not a list
The findings that need attention first are not the ones with the highest count. Read the report in this order:
- Every category 4 and 5, individually, with the photograph, the location relative to the spar cap, and the previous inspection's view of the same spot.
- Every finding that has grown since last time, whatever its category.
- Every finding on the spar cap or the trailing edge inboard of maximum chord, whatever its category.
- Every blade type or plant that appears more than once in the 4s and 5s. A pattern across blades of one type is a serial defect question, and it changes what the report is telling you.
- The rest, for the repair campaign.
The inspection checklist sets out what should have been looked at; the report tells you what was. The gap between the two is a finding in its own right.
What to ask the provider for
Before acting on a report, and especially before commissioning repairs from the same provider:
- the category definitions, in writing
- the method statement: positions, resolution, coverage, weather
- the full image set, not the selected images in the PDF
- the previous inspection's images of each category 3 and above finding
- whether any finding was reclassified between the field and the report, and why
- the inspector's experience with this blade type
None of these are unreasonable requests, and a provider who resists them is telling you something.
Frequently asked questions
Are blade damage categories standardised?
No. Each inspection provider defines its own scale. Two reports using "category 3" may mean different things, and a fleet trend built across providers is unreliable unless the scales have been reconciled.
Can I rely on a drone inspection report to say a blade is structurally sound?
No. A drone sees the surface. Bond lines, webs, the root laminate and internal delamination are invisible to it. A drone report with no findings means the surface was clean on the day.
What is the most important thing a report should tell me?
Whether each finding has grown since the previous inspection, with the two photographs side by side. Growth is what separates a defect that can wait from one that cannot.
Should the company that inspects also do the repairs?
It is common and it is a conflict. If it cannot be avoided, the report should be reviewed by someone independent before the repair scope is agreed.
How Apex Wind can help
We are an independent blade engineering consultancy in Denmark. We do not inspect blades and we do not repair them, so we have no interest in what a report finds. For teams who commission inspections and act on the reports, our blade training teaches how blades are built, how they fail, what each category means for the structure and how to challenge a classification. In a transaction, our blade technical due diligence reads the inspection record for what it shows and what it could not have seen. If you have a report you are not sure how to act on, contact us.