What is a borescope inspection: borescope inspection explained, and why the method exists at all
A borescope inspection is a remote visual inspection of the interior of a machine, vessel, pipe or structure, performed by inserting a rigid or flexible optical probe with its own illumination through an existing access port or drilled inspection hole. Nothing is dismantled and the item usually stays in place.
The reason the technique exists is economic and it is worth stating plainly. The alternative to looking through a 10 mm port is stripping an engine, opening a vessel or cutting into a wall, which costs days of downtime, a lifting plan, gaskets and a re-commissioning test. A borescope buys the same sightline for an hour of work. That trade is the entire business case.
How does a borescope inspection work?
- Rigid borescopes use a relay lens train in a straight tube. Best optical quality of the three, restricted to straight-line access.
- Flexible fibrescopes carry the image along a fibre bundle and bend around obstructions, at a cost in resolution.
- Video borescopes, or videoscopes, put a camera chip at the tip and steer it with articulation controls. This is now the default for serious industrial work, and it produces a recordable image rather than an eyepiece view.
- Measurement tips on some videoscopes use stereo or phase-shift techniques to size an indication, which requires calibrated tips and careful technique to be defensible.
Formally this is remote visual testing, and it inherits the rules of visual inspection. ASME Boiler and Pressure Vessel Code Section V, Article 9 allows a remote system in place of direct viewing on condition that it is demonstrated to have resolution and lighting at least equivalent to direct observation, checked against a resolution target (ASME BPVC Section V). Good practice for the technique itself is set out in the Energy Institute's guidance on visual inspection using borescopes, EI 3623 (Energy Institute).
Where it is used: confined and inaccessible spaces
- Gas turbines and aero engines: combustor liners, nozzle guide vanes, turbine blades, compressor sections, inspected on wing through dedicated borescope ports.
- Reciprocating engines: cylinder bores, valves and piston crowns, checked through the injector or spark plug hole.
- Pressure vessels, heat exchangers and boilers: internal corrosion, erosion, scaling, tube condition and weld root profiles.
- Pipework and ducting: blockages, deposits, liner condition and joint quality where no man-access exists.
- Building fabric: cavity walls, voids and floor structures, examined through a small drilled hole rather than by opening up.
- Gearboxes, hydraulics and plant and machinery generally, wherever an internal check would otherwise mean draining and splitting a casing.
Aviation is where the method is most formalised, because there the inspection is mandated rather than chosen. US airworthiness directives on Rolls-Royce Trent 7000 engines, for example, require initial and repetitive on-wing borescope inspections of the high pressure turbine blades to detect axial cracking, with blade set replacement driven by what those inspections find (Federal Register, 2026).
Borescope inspection: a practical example
A gas engine shows a slow rise in oil consumption on one bank. Rather than lift the head, the technician removes the spark plugs and passes a 6 mm videoscope into each cylinder. Four bores are clean. The fifth shows vertical scoring on the thrust side and carbon build-up on the piston crown. The findings are recorded as stills with the cylinder identified in each frame, and the head comes off for that cylinder only. One targeted strip instead of six speculative ones.
Where the method runs out
- It shows only what the probe can reach and light. No port, no inspection, and a shadowed area behind a vane is an area you have not examined.
- It is still visual. Surface condition only, no wall thickness, no subsurface flaw detection. Ultrasonic or eddy current follows where those questions matter.
- Sizing is a skill, not a readout. Perspective at short working distances is deceptive and uncalibrated measurement is unreliable.
- Cleanliness governs the result. Deposits, oil film and condensation on the lens hide the exact features being looked for.
- The environment brings its own regime. Entry into vessels, isolation of rotating plant and hot work near access ports normally sit under a permit to work, and the inspection plan has to respect that.
What it is commonly confused with
A borescope inspection is not endoscopy, although the optics are related, and it is not an alternative to volumetric non destructive testing. It is a way of moving the eye, not a way of seeing further into material. The other confusion worth heading off is treating recorded footage as a finished assessment. The video is evidence. The grading of what appears in it, against acceptance criteria, is the inspection.
Borescope inspection report: what to record
A borescope inspection produces two things: the recorded images and the written judgement about them. The report has to hold both, and it has to let a second person who was not there reach the same conclusion. Formal remote visual testing procedures set the detail; the generic minimum below applies to a turbine, an engine bore or a wall cavity alike.
- Item identity: asset or serial number, component, and the access port or hole used
- Equipment: borescope type and tip, diameter, articulation, light source, and the resolution check made before starting
- Location of each indication: cylinder, blade number, stage, clock position, distance from the port
- Each indication as a still or clip, with the scale reference or measurement method stated if sizing was attempted
- Surface condition at the time: cleaning done, deposits, oil film, anything that limited the view
- Areas not reached, stated as such rather than left blank
- The acceptance criteria applied, the disposition (accept, monitor, reject) and who signed it off
The photo evidence has to carry its own context. A still of scoring with no cylinder number, no date and no probe details is an observation, not a record. Timestamped frames, the order they were taken in, and an unbroken chain from the original file to the report are what make the footage usable later, in a warranty claim or in a dispute about when the damage first appeared. The inspection report and minimum evidence set entries cover the general rules, and inspection photo evidence standards goes into the photo side in detail.
The same rules apply when the person holding the camera is not the inspector. A technician at a remote site, or a customer with a phone, can capture a first look through a plug hole, and that footage is only useful if the identity, location and condition fields arrive with it. Guided capture flows exist to enforce exactly that.