It seems that every year, aircraft maintenance practices and procedures become more technologically sophisticated. Despite this sophistication, it’s still often necessary to simply look at aircraft engine parts and mechanical assemblies via visual inspection. That’s not to say technology doesn’t enter into visual inspection. Borescopes, based on technologically advanced optical systems, are a frequent choice in inspecting mission-critical parts or components.
For most combustion chamber and turbine blade inspection needs, borescopes are an invaluable tool. Borescopic inspection becomes imperative when certain factors exist:
- Limited access: Deep bores, cross holes or undercuts, or parts located in inaccessible sections of an aircraft, require using borescopic inspection to determine key characteristics.
- Critical components: The part or component is mission critical — for instance, mating surfaces with a gasket between them that might prevent leakage in a fuel system.
- Unspecified troubleshooting: When specified characteristics are not well defined — corrosion, blockages, debri, and the like, there’s no substitute for the human brain to identify what can be defects.
A scope for all uses
The two most common types of borescopes used in aircraft inspection are rigid and flexible borescopes.
Rigid borescopes offer the highest resolution and brightest images, and are more durable and more economical. These scopes use traditional lens-relay systems to transmit images. The lenses are usually assembled inside a rigid stainless-steel tube. If access into the part to be inspected allows the use of a rigid borescope, it is the best investment. Rigid borescopes are used when the area to be inspected is reachable in a straight path, as in many machined parts, tubes, and some molded parts. For many turbine blade and combustion chamber inspections, a rigid borescope is the best bet.
If the pathway to the inspection area is indirect — through a curved pipe, bent tube, or complex casting, for instance — users should opt for a flexible borescope. These scopes employ a fiber-optic image bundle housed inside a flexible sheath. The resolution of a flexible borescope is defined by the number of optical fibers in the instrument’s imaging bundle; this number can range from 10,000 to 30,000 pixels. Flexible borescopes are not the answer to all user needs. Because of the fiber optics, the image quality is not as good as rigid borescopes. Also, flexible borescopes can be expensive, more fragile, and sometimes hard to handle. However, even with these limitations, excellent-quality flexible borescopes are available and the need for flexibility is often the most important factor.
Scopes in maintenance
In the aircraft maintenance world, mechanics use borescopes primarily as part of scheduled maintenance. Specialty service and repair facilities, in particular, use borescopes frequently. Also, a number of manufacturers and the FAA require borescopic inspection of certain parts, and some manufacturers specify borescopic inspection to get baseline data for trend/condition monitoring.
Licensed A&P-rated mechanic Jim Thayer is director of maintenance for USAirports Air Charter and USAirports Flight Support Certificated Repair Station. He notes that borescopes often help him “confirm what I already suspected.” For instance, he says, “If the engine starts to lose power, you might expect inlet or exhaust issues. Using a borescope, I have found damage in those areas that I otherwise would only have found by disassembly.”
A typical turbine engine offers several avenues for borescopic inspection. In the combustion chamber, manufacturers specify inspection criteria, but issues include corrosion, cracking due to heat, and shifting of the chamber. More particularly, borescopes are frequently used to inspect:
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