Before the inspection: what the scope has to say

An inspection that is not specified produces findings that cannot be compared with last year's. Fix these in the scope of work before anyone goes up:

  • the damage categorisation scale to be used, named explicitly, and the same one as the previous campaign
  • resolution at the blade surface, not camera megapixels: a figure in millimetres per pixel, and the standoff distance it assumes
  • coverage: all four surfaces, or suction and pressure side only, and whether the trailing edge and the tip are imaged separately
  • whether the inspection is blade-referenced: every finding located by radius in metres from the root, and by surface, not by "near the tip"
  • serial numbers of every blade, recorded from the blade, not from the turbine record
  • what triggers an immediate call: the findings the inspector is to report the same day rather than in the report

That last item matters more than the rest. An open trailing edge found on a Tuesday is not a report item; it is a phone call.

External blade inspection checklist

Ground-based imaging and drone inspection see the same surfaces. Rope access sees less area but can touch, tap and measure.

Leading edge

  • coating loss, pinholes and pitting, with the radius at which it starts and ends
  • exposed laminate: the point where erosion stops being cosmetic, because water now reaches the fibres
  • the leading edge protection, if fitted: lifted edges, missing sections, and the transition where the tape or shell ends
  • erosion depth where it can be measured, not just area, because depth decides whether it is a coating repair or a laminate repair

Erosion is graded by how far through the build it has gone: coating only, primer reached, gelcoat gone with laminate exposed, laminate removed. The first two are aerodynamic and a matter of energy yield. The third and fourth are a water path into the shell.

Trailing edge

  • bond line cracks, recorded as a length and a radius, with the crack tips photographed
  • any opening that is continuous rather than intermittent, and whether light passes through
  • damage at maximum chord, where edgewise strain is highest
  • previous repairs, their extent, and whether a crack has restarted at the repair boundary

A trailing edge crack that has grown between two campaigns is a different finding from the same crack measured at the same length twice. Growth rate is the number that drives the decision, and it only exists if the last inspection was measured rather than described.

Tip

  • lightning receptors: present, secure, unburnt, and the surface around them
  • tip cap condition and any sign of the tip being open
  • burn marks, delamination and puncture holes, which point to a strike that did not reach a receptor
  • drain holes: present and clear

Shell surfaces, suction and pressure side

  • transverse cracks over the spar cap, which are structural until shown otherwise
  • longitudinal coating cracks in a line along the spar cap edges, which often mark the laminate boundary underneath
  • blisters, wrinkles visible through the coating, and any local change in surface profile
  • impact damage, hail marks and debris strikes
  • bond line witness marks along the shell joints

Root end

  • the root face and bolt circle, where visible
  • water staining, corrosion product and any sign of standing water inside
  • cracks radiating from the root transition

Internal blade inspection checklist

An internal inspection, whether by a crawler robot or a technician entering the blade, sees the structure rather than the surface. It is the only way to check most of what actually carries load.

  • shear web bond lines, both sides, along the full accessible length: voids, cracks, adhesive that never wetted the laminate
  • the leading and trailing edge bond lines from inside, where a crack is visible long before it reaches the outer surface
  • spar cap surfaces for wrinkles, ply drops, dry fibres and resin-starved areas
  • the root insert area: adhesive coverage, cracks in the laminate around inserts, and any insert that has moved
  • standing water, its depth, and where it has been sitting, which the staining shows
  • lightning down conductor continuity and the condition of its connections
  • repair records against what is visible: repairs found inside but absent from the log are a finding

Water inside a blade is not a housekeeping item. It adds mass at radius, it freezes, and it tells you the shell has a path in that nobody has found from outside.

Post-event inspection checklist

After a defined event, the inspection is not the routine one.

After a lightning strike, whether logged by the detection network or by the turbine: receptor condition, down conductor continuity measured rather than assumed, internal inspection at the tip, and a check for delamination and puncture away from the receptors. IEC 61400-24 sets the requirements for the protection system and the test methods used to validate it (IEC 61400-24:2019).

After an overspeed, emergency stop or grid event: root bolts, root transition, spar cap for compression damage on the suction side, and the SCADA record for what the blade actually experienced.

After transport, lifting or installation: the areas the slings and supports touched, the root face, and the trailing edge, which is the part that gets damaged in a yard and repaired without a record.

After a blade failure anywhere in the fleet: the neighbouring blades and the blades from the same plant, mould and production window, inspected by a method that can see the defect in question. If the suspected defect is a wrinkle in a spar cap, a visual survey answers nothing and ultrasonic testing is the minimum.

What the report has to contain

A report that cannot be acted on is a cost, not an inspection. Each finding needs:

  • a location: blade serial, surface, radius in metres, and chordwise position
  • a measurement: length, area, depth, not only a photograph
  • a category on the agreed scale, and the reasoning for the category where it is not obvious
  • a cause, or a statement that the cause cannot be determined from the image
  • a recommended action and a timeframe, distinguishing what must be done before the next winter from what is monitored

And the campaign as a whole needs a fleet view: which findings repeat at the same radius, on the same surface, in the same blade type or production batch. A defect that appears once is a repair. The same defect at the same location on eleven blades is a serial defect, and the inspection has just found the most expensive thing on the site.

Frequently asked questions

How often should wind turbine blades be inspected?

The common practice is a full external inspection every one to three years, tightened after a defined event, in a known problem fleet, or where the previous campaign found growing damage. The interval that matters is the one that catches a crack before it becomes structural, and that depends on the observed growth rate, not on a calendar rule.

Is a drone inspection enough on its own?

For the external surfaces, a properly specified drone inspection is the most efficient method available and finds most surface damage. It cannot see inside the blade, cannot measure depth reliably, and cannot find a subsurface defect such as a wrinkle or a bond void. Any fleet with a suspected internal or manufacturing defect needs an internal or instrumented inspection as well.

What is the difference between a damage category and a repair decision?

The category describes the damage. The repair decision weighs the category against the remaining life, the access cost, the season, the warranty position and the growth rate. Two blades with the same category can correctly get different decisions, and a report that conflates the two removes the owner's ability to make that judgement.

Who should specify the inspection, the owner or the service provider?

The party carrying the risk should specify it. A scope written by the provider who will also do the repairs is not independent, and it is the most common reason inspection campaigns are not comparable from one year to the next.

How Apex Wind can help

We are an independent blade engineering consultancy in Denmark. We write inspection scopes of work, review inspection reports, and carry out blade failure investigation and root cause analysis when a finding turns out to be structural. We support technical due diligence on blade condition across a portfolio, including whether a previous inspection campaign supports the conclusions drawn from it. We sell no inspections, repairs or hardware and take no referral fees. If you want a campaign specified so the findings are comparable and defensible, contact us.