Blade inspection · Coverage planning
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Blade Inspection Coverage: Limits and Planning
A coverage figure is a share of something. Name that something, the regions it covers and how you will accept or reject the result.
Two values, not one
| Value | Stated as | Conditions carried in the source | What it does not say |
|---|---|---|---|
| Internal coverage | Up to 70 per cent | Documented internal coverage value | That this is achieved within any particular run time |
| Timed scenario | 70 m in 60 minutes or less | One cavity, 70 m internal area within a 95 m blade, documented setup, unobstructed path | That a run under these conditions delivers the coverage figure |
The product document behind the WIT-BR60 states two figures that are often quoted in the same breath: a documented internal coverage value of up to 70 per cent, and a timed scenario of 70 m in 60 minutes or less.
This site presents them separately and footnotes them separately, and that is deliberate. They are separate product-document values. The timed scenario carries its own conditions — one cavity, a 70 m internal area within a 95 m blade, the documented setup, and an unobstructed path. Nothing in the source states that a run under those conditions delivers the coverage figure, and combining them produces a claim the document does not make.
What a coverage figure is scoped to
An internal crawler inspects where it can travel and where its cameras can see from that path. A coverage figure is therefore a statement about reach, and reach is bounded by the geometry of the cavity it is deployed into.
That has a consequence worth stating plainly: coverage is not a property of the robot alone. It is a property of the robot in a particular blade, entered from a particular access point, in a particular condition. The same equipment can return different practical coverage in two blades of similar length if their internal structure differs.
This is why an up to figure is the honest form. It describes the upper bound the document supports, not a result guaranteed in an arbitrary blade.
Name the denominator
A percentage is a share of something, and “coverage” on its own does not say of what. Four denominators can be meant, and a single run gives a different figure against each.
| Denominator | What the percentage is a share of | What it makes checkable |
|---|---|---|
| Identified surfaces | A listed set of surfaces in a named cavity | Each listed surface has, or lacks, suitable evidence |
| Travel interval | Distance along the cavity from an access point | The crawler reached the stated distance; not that every surface was imaged |
| Accessible cavity | Whatever the crawler could physically reach | Almost nothing: the denominator shrinks as the run gets obstructed |
| Planned inspection path | The route agreed in the work package | Route completion against plan; not surface condition |
The coverage value published on this site is stated without a denominator for any particular blade, so it cannot be tested against a specific asset. Ask the supplier which quantity it is stated against, and write your own denominator into the scope. Distance travelled is not surface coverage, and a successful traversal does not prove that every required feature was visible.
What the remaining volume is
The question that follows a coverage figure is always the same: what about the rest? It has three legitimate answers, and a campaign plan should say which one applies.
Some of the remaining volume is reachable by other methods — external imagery for surface condition, ultrasonic techniques for sub-surface features in specific regions, or endoscopic access where a crawler cannot travel. Some is structurally inaccessible without disassembly. And some is simply outside the scope of the campaign, because the campaign is answering a specific question rather than surveying everything.
All three are defensible. What is not defensible is leaving the reader of a report to infer which one applied, because that inference is usually made in the operator's favour and discovered later.
Reading the timed scenario
The 70 m in 60 minutes or less scenario is useful for planning, provided its conditions travel with it.
It applies to one cavity, over a 70 m internal area in a 95 m blade, with the documented setup and an unobstructed path. Each of those bounds does work. One cavity is not a whole blade. A documented setup assumes preparation time that the figure does not include. And an unobstructed path is an assumption about condition, not a property of the equipment.
Used carefully, the figure supports a scheduling estimate. Used as a productivity claim across a fleet campaign, it will be wrong in the direction that costs money, because obstruction, access preparation and repositioning are exactly the things it excludes.
Define the inspected region
The practical fix is to stop specifying coverage as a percentage and start specifying it as a region with evidence attached.
A requirement that can be checked on completion states which cavity is in scope, which access point is used, how far along the internal length the inspection has to reach, which surfaces within that reach must be imaged, what evidence has to exist for each interval of that reach, and what happens when the path is obstructed before the required distance. That last clause is the one most often missing, and it is the one that decides whether an interrupted run counts as a partial result or a failed one.
| Field | Example entry |
|---|---|
| Cavity and access point | Blade A, leading-edge cavity, entered at the root opening |
| Reach required | From the root to a stated distance along the internal length |
| Surfaces to image | Listed shell and web surfaces facing the crawler path |
| Evidence per interval | Location-referenced original images meeting the agreed quality criteria |
| If obstructed | Stop, record the location and cause, and classify the zone as partial |
Percentages are convenient in a proposal and unverifiable in an acceptance meeting. Regions are the reverse, which is why they are worth the extra sentence. Obstacles, openings and surface questions that decide whether a region can be reached belong in the access pack described in theaccess and compatibility checklist.
Accept or reject coverage on completion
Acceptance needs the same vocabulary on both sides. Give every zone one of four statuses, and require the report to show all four rather than only the favourable ones:
- Inspected with evidence suitable for the agreed review
- Captured, but limited by visibility or image quality
- Accessible, but not inspected during this visit
- Inaccessible or outside the agreed scope
Then set the test: planned zones against achieved zones, by status, with a stated rule for each outcome. For example, which statuses are accepted, which trigger a repeat visit at whose cost, and which are referred to another inspection method. “No finding recorded” and “not inspected” must stay distinguishable, or a gap reads as a clean result.
The delivered record carries these statuses; what a blade inspection report should includelists the fields, and the sample inspection report structure shows them laid out.
Where to go next
Next steps from this guide
Keep reading
- Internal Blade Inspection Robot: Access ChecklistCheck opening, route and surface fit for your blade before writing zones that assume the robot can reach them.
- Blade Inspection Report: What It Should IncludeTurn the four coverage statuses into report sections you can accept or reject.
- Blade Inspection: Drone vs Internal RobotWhere a crawler cannot reach, see what exterior imagery and other methods can evidence instead.
Product
WIT-BR60 internal blade inspection robot
The documented coverage and timed values with the conditions attached to each, and the standards context for blade inspection.
Next step
Send blade, access point and acceptance needs
A coverage scope can then be written against regions and evidence rather than a percentage.
Part of the wind turbine blade inspection guide path.
Evidence register
Sources and limitations
The two figures on this page are supplied product-document values, kept separate and footnoted separately. The external references below cover where a blade inspection requirement comes from in the first place.
- DNV-ST-0376 — Rotor blades for wind turbines: principles and technical requirements for onshore and offshore rotor blades, including provisions for in-service inspection and maintenance. DNV states the standard was aligned with IEC 61400-5 with input from other certification bodies.
- IEC 61400-5:2020 — the blade part of the IEC 61400 series.
- UK HSE inspection and NDT guidance — official guidance that a technique must be matched to the damage sought and applied under a procedure whose capability can be assessed. A coverage percentage is one input to that, not a substitute for it.
Limitation: neither cited standard sets a coverage percentage, and neither validates the product figures above. A coverage requirement is written against the blade, the damage mechanism and the acceptance basis for a specific asset; a catalogue percentage cannot be read as compliance with either standard.
See the source and editorial policy for how supplied product manuals, external method sources and claim limitations are kept separate.
Decision support
Frequently asked questions
Does 70% coverage mean 30% of the blade is never inspected?
It means the documented internal coverage figure is scoped to what a crawler can reach inside the cavity. The remaining volume is not necessarily uninspected — it is inspected by other means, or accepted as out of scope for that campaign. What matters is that the specification says which, rather than leaving the reader to assume the figure describes the whole blade.
Can the coverage figure and the 60-minute figure be used together?
They are separate product-document values and this site presents them separately. The timed scenario applies to one cavity over a 70 m internal area in a 95 m blade, with the documented setup and an unobstructed path. Treating that duration as if it also delivered the coverage figure combines two values that were not stated together.
What makes a path obstructed?
Internal structure, shear web arrangement, bonded joints, accumulated debris and section narrowing all reduce where a crawler can travel. Blade designs differ, so the practical reach for a specific model is a question for the campaign plan rather than something a general figure can answer.
What is the denominator of a coverage percentage?
It is whatever the percentage is a share of: identified surfaces, a travel interval along the cavity, the accessible cavity or an agreed inspection path. These give different numbers for the same run. The coverage value published for the WIT-BR60 carries no denominator for your blade, so a project scope has to name its own.
How is coverage checked on completion?
Compare achieved against planned by zone, using the same status for every zone: inspected with suitable evidence, captured but limited, accessible but not inspected, or inaccessible or out of scope. Agree beforehand what happens when a run is obstructed, because that decides whether a partial result is accepted, repeated or failed.
How should coverage be written into a scope of work?
State the region to be covered rather than a percentage: which cavity, from which access point, to what internal distance, and what evidence has to exist for each metre. A percentage without a region is not verifiable on completion, which is when the disagreement usually happens.
Engineering review
Define the coverage your campaign has to prove
Send the blade model, the access point and the evidence your acceptance process needs, and the scope can be written against regions rather than percentages.