What is inside a blade
A rotor blade is two composite shells bonded to one or two shear webs, with thick unidirectional spar caps carrying the bending load and sandwich panels stiffening the rest. Inside, the blade is a hollow tube that tapers from a root diameter of a few metres to a tip too narrow to reach a hand into. Along the length run the adhesive joints: the leading-edge bond, the trailing-edge bond, and the web-to-shell bonds on both sides of each web. A lightning down conductor runs from the tip receptors to the root. Near the tip there are drainage holes, and often a balancing chamber somewhere in the first third.
Every one of those elements is a place where a defect can live without leaving a mark on the outside. That is the reason the inside matters.
What an internal inspection looks for
The inspector is looking for anything that changes the way the load path was designed to work:
- Bond-line condition. Missing adhesive, voids, porosity, cracks running along or across the joint, and adhesive that has separated from the laminate. The web-to-shell bond is the one that decides whether the blade keeps its cross-section under load.
- Shear web damage. Cracks in the web laminate, core shear failures in the sandwich, buckling waves, and debonding of the web foot.
- Laminate defects. Wrinkles in the spar cap or root laminate, delamination, dry fibres, and transverse cracks in the trailing-edge laminate.
- Root zone. The laminate around the bolt inserts, the transition from the thick root to the aerofoil, and any signs of movement or cracking at the bushings.
- Lightning protection. The route and condition of the down conductor, its connections, and any burn or arc marks around receptors. See Lightning damage inside a wind turbine blade.
- Water and debris. Pooled water, moisture staining, loose objects and manufacturing leftovers that can move and abrade.
For a field guide to these defects and how they present from the inside, see Wind turbine blade defects that start on the inside.
Why the inside comes first
Most of the failures that end a blade's life are structural, and structural damage grows from the inside out. A trailing-edge bond that has cracked will run for metres before the shell shows a line. A web that has debonded from the shell changes the stiffness of the section long before the blade looks any different from the ground. By the time a defect is visible to a drone or a ground camera, the repair is a large one.
External inspection is still needed. Erosion, surface cracks, lightning receptor condition and coating damage are external problems and are inspected well from the outside. But an external inspection alone gives a false sense of completeness, because it cannot see the parts of the blade that carry the load.
How far a technician gets
An internal inspection by a technician is done from the root. The technician enters through the root hatch, in a confined space, with the turbine stopped and the blade locked. The blade narrows quickly, and the inspection ends where it becomes impossible to move on. In practice that is a short way past the root, and the rest of the blade, the part where the trailing-edge bond is under the highest strain and the section is thinnest, is never seen.
That is why internal inspections are either incomplete or skipped. The cost of a technician in a confined space is high, the coverage is small, and the report is a handful of photographs with no position reference.
What a crawler changes
A remotely operated crawler removes the person from the blade and extends the reach. Detectra enters at the root, drives up to 80 % of the blade length, and records the inside in 8K 360° video under its own light. Capture takes about 15 minutes per blade, and up to 12 blades can be inspected in a day. The operator stays at the root with a tablet.
Three things change with that:
- Coverage. The inspection now includes the middle of the blade, where the bond lines are most loaded and least seen.
- Evidence. A 360° recording captures the whole sphere in one pass, so nothing has to be aimed at while the crawler is inside. Every take carries the distance from the root, so a finding can be located and revisited at the next inspection.
- Review at the desk. The looking happens afterwards, from any angle, with time to compare against the previous recording. In Fenestra, findings are flagged on the exact frame and tracked over time, and the report is generated from them.
When to inspect the inside
There is no single interval that fits every fleet, and the blade manufacturer's maintenance documentation sets the baseline. In practice, internal inspections are worth scheduling at these points:
- Before the end of the blade warranty, so that manufacturing defects are documented while a claim is still possible. See End-of-warranty blade inspections.
- After a lightning strike or storm event, when the outside shows damage or the lightning monitoring system has recorded a strike.
- When an external inspection finds a trailing-edge or leading-edge crack, to see whether the bond behind it is affected.
- As a baseline on a newly acquired fleet, so that later inspections have something to compare against.
- On a fixed cycle for blade types with a known history of internal defects.
Frequently asked questions
What does an internal blade inspection look at?
The shear webs, the web-to-shell and trailing-edge bond lines, the spar caps, the root laminate around the inserts, the lightning down conductor and its connections, and any water or loose material inside the blade. The aim is to find structural damage before it shows on the shell.
Why can a technician not inspect the whole blade from the inside?
The blade tapers, and a confined-space entry from the root ends where the section becomes too narrow to move on. That leaves the outer part of the blade, where the trailing-edge bond is most loaded, uninspected.
How long does an internal inspection with a crawler take?
With Detectra, about 15 minutes of capture per blade, with the turbine stopped and the blade locked. Review of the recordings takes about 3 hours per turbine today.