IEC 61400-5 Annex H: In-Service Blade Inspection

What the standard covers

The annex covers periodic inspection of operating blades, including the primary structure that Section 11 covered at build, plus the damage modes that appear in service and did not exist at the gate. Leading edge erosion, lightning damage and crack growth sit in scope, and the reference is the same volumetric approach rather than a visual walk-down.

What it demands

• Periodic inspection of operating blades against the same NDT methods the blade was built to
• Coverage of in-service damage modes, including leading edge erosion and lightning damage
• A record that compares with the as-built evidence rather than standing alone
• A method that reaches the primary structure, not only the surface, on a standing turbine

How it lands on a blade scan

The annex makes the field method matter as much as the factory method. A rope-access scan reaches about 60 % of the blade surface, and of that the UT coverage lands near 50 %, so the true coverage is around 30 %. A robotic crawler that tracks the curve at 300 mm/s covers its whole programme path and repeats the same indication within plus or minus 0.5 mm. On a marine site the numbers get sharper still, because rope access is limited to wind below 10 m/s, near 140 days a year in the North Sea, where a crawler works to 15 m/s and adds roughly 40 % to the window.

How to apply it to a blade

Run these checks in order. By Step 5 the scan plan has the threshold, the geometry and the record it needs.

1. Fix the acceptance threshold. Start from the threshold the zone carries, not from the method you happen to own. IEC 61400-5 Section 11 asks for volumetric detection of indications 6 mm and larger in the laminate plane, and DNV-ST-0126 tightens the leading edge to 3 mm, so the threshold is set before the probe is chosen.
2. Read the geometry off the drawing. Measure the thickness and the curvature the probe has to travel. A spar cap up to 80 mm of unidirectional laminate and a root transition of changing radius are two different scan problems, and the radius decides how the array is coupled.
3. Choose the modality pair. Pick modules by defect geometry, not by habit. PAUT images planar separations that lie parallel to the plies, TOFD catches volumetric clusters whatever their orientation, and running both on a 256-channel system puts the two mechanisms in one pass.
4. Automate the scan path. Put the probe on an encoder-driven carriage and let the path, not the technician, set coverage. A 600 mm spar cap falls to one longitudinal pass at 500 mm/s on a linear array, where a single-element probe needs 20 parallel passes and runs about five times longer.
5. Lock the evidence chain. Write the coverage map, the daily calibration and the indication log to the blade serial while the scan is running. It is the difference between a 3-day type-certification audit and a 3-hour document review, and it is what ISO 9712 qualification is there to sign.

The numbers below are drawn from the equipment records and field campaigns this site already publishes, not from a generic catalogue.