The usual five categories
There is no international standard for blade damage categories; each manufacturer and inspection company publishes its own. Most of them converge on five levels that run from cosmetic to critical, and most of them define the levels by what should happen next:
| Category | Typical name | What it means | What happens |
|---|---|---|---|
| 1 | Cosmetic | No effect on structure or performance. Paint chips, light staining, resin drips. | Record. No action. |
| 2 | Minor | Small, stable, no structural involvement. Early erosion, small adhesive voids, a scratch through the coating. | Record. Check at the next inspection. |
| 3 | Moderate | Could progress; not yet affecting the load path. Coating loss to laminate, short bond-line cracks, moisture staining. | Monitor on a defined interval, or repair at the next planned visit. |
| 4 | Serious | Affects the structure or will within the inspection interval. Cracks in the laminate, web debonding over a length, wrinkle with delamination, damaged conductor. | Repair before the next interval. Engineering assessment. |
| 5 | Critical | Immediate risk of failure. Through-cracks, large debonded lengths, structural damage from lightning. | Stop the turbine. Repair or replace. |
The definitions are the useful part. A category that is defined by an action, "repair before the next interval", can be applied consistently. A category defined only by a description, "moderate", cannot.
Why scales drift
Three reasons, all of them ordinary:
- Different starting points. Drone companies wrote their scales around the shell: erosion, surface cracks, receptor damage. Technician scales grew out of repair work. Neither had the inside of the blade in mind.
- Different incentives. A supplier that also sells repairs has a reason to grade up; one paid per inspection has a reason to grade quickly. Neither is dishonest; both move the numbers.
- Different people. Even within one company, two inspectors will grade the same photograph differently unless the definition tells them what to look for and what to do.
The result is that a fleet's history is written in three dialects, and the question that matters, which findings are getting worse, cannot be answered across them.
Where internal findings fit
Findings from inside the blade sit awkwardly on a scale designed for the shell, for two reasons. Some of the most serious internal defects look small: a hairline crack along a web foot is a category 4 by consequence and a category 2 by appearance. And some of the most alarming-looking things inside a blade are nothing: adhesive squeeze-out, resin drips, factory markings, and the normal roughness of an infused laminate.
The way through is the same as for the outside: grade by consequence and by what happens next, not by appearance. Practical anchors for the inside:
- Adhesive voids and short gaps in a fillet: category 2, checked next time, unless they add up to a length that changes the joint, which is a 3 or a 4.
- Cracks along a bond line: category 3 while short and stable, category 4 once they run or once there is a step at the fillet.
- Web debonding over a measurable length: category 4.
- Wrinkles in the spar cap or root: category 3 as found, category 4 with any sign of delamination, because they grow under fatigue.
- Standing water: category 3 on its own; category 4 with ice or with a mass that unbalances the rotor.
- Loose objects: category 3, because they abrade and block drainage, and because removing them is cheap.
- Conductor damage or a loose fixing: category 4 after a strike, because the next strike will find it.
Wind turbine blade defects that start on the inside describes each of these; the point here is that they can be graded on the same five steps as the shell once the steps are defined by action.
Keeping one scale across a fleet
Four rules make a scale hold across suppliers and years:
- Define each category by the action it triggers, and write the definition down. "Repair before the next interval" survives a change of inspector; "serious" does not.
- Attach the severity to the finding, not to the report. A finding is one defect at one position on one blade. Its severity is a property of that record, and reclassifying it changes one record and every report that draws on it.
- Map every supplier's scale onto yours at import. When a drone report grades erosion as "B", it becomes your category 2 the moment it enters the record, and the original grade is kept alongside for reference.
- Let the position and the history do the grading over time. The same finding at the same distance from the root on two dates, side by side, tells you whether it moved. Movement, not appearance, is what moves a finding from a 3 to a 4.
This is the structure Fenestra is built around: one record per finding with its position and severity, findings and defect history over time, supplier PDF reports imported with AI assistance so older inspections sit on the same scale, and every AI-proposed candidate confirmed, rejected or reclassified by an inspector before it enters a report. The recordings that feed it, from Detectra, carry the distance from the root on every take, so the "same position next year" comparison is possible in the first place. From inspection findings to asset decisions sets out what that makes possible at fleet level.
Frequently asked questions
What are the blade damage categories?
Most suppliers use five levels: 1 cosmetic, 2 minor, 3 moderate, 4 serious, 5 critical. There is no international standard, and definitions differ between companies. Scales that define each level by the action it triggers, from "record" to "stop the turbine", are the ones that can be applied consistently.
How should defects inside the blade be graded?
By consequence and next action, not by appearance. A hairline crack along a web foot is serious; a large patch of adhesive squeeze-out is nothing. Web debonding over a length, wrinkles with delamination, damaged conductors and standing water with ice sit at category 4; short bond-line cracks and small voids at 2 or 3 until they move.
How do you compare damage categories from different inspection companies?
Map every supplier's scale onto one fleet scale at the moment the findings enter the record, keep the original grade alongside, attach the severity to the finding rather than the report, and let the position and history of each finding decide whether its category changes.