One of the most powerful non-destructive inspection tools available to an aviation maintenance technician (AMT) is the borescope. This optical instrument allows a trained technician to look deep inside an aircraft engine—examining cylinder walls, piston crowns, valve faces, turbine blades, and combustion chambers—without removing a single cylinder or splitting a turbine case. The result is a faster, lower-cost inspection that preserves the integrity of the engine and its seals while delivering meaningful data about internal component condition.
For the AMT General knowledge test, you need to understand not just what a borescope is, but how to use one correctly, what defects to look for, and how to document and interpret your findings. This article walks through all of that in practical, exam-focused detail.
Types of Borescopes
Borescopes fall into three broad categories, each suited to different access geometries and budgets.
- Rigid borescopes use a straight metal tube containing a series of precision relay lenses. They deliver excellent optical clarity and are ideal when the inspection port and the target component are in a straight line—such as looking through a spark plug hole into a reciprocating engine cylinder. Because they cannot bend, their application is limited to straight-line access paths.
- Flexible (fiberoptic) borescopes route light and the return image through bundles of optical fibers, allowing the instrument to follow curved passages. They are used extensively in turbine engines where inspection ports may not align with the component of interest. Image quality is somewhat lower than rigid versions because the fiber bundle introduces a pixel-like pattern, but the maneuverability advantage is significant.
- Video borescopes (videoscopes) place a miniature charge-coupled device (CCD) or CMOS camera chip at the tip of an articulating probe. The live image appears on an attached display screen and can be recorded digitally for documentation. Articulating tip control—typically operated by a joystick or thumb wheel—allows the camera to be steered around bends, making videoscopes the preferred tool in modern turbine engine maintenance. Many units include measurement software that can estimate crack length or pit diameter by comparison with a known reference.
How Borescope Inspection Works
Access Points
The inspection begins by identifying the correct access port. On reciprocating engines, the spark plug holes are the most common entry points—the threads match the borescope adapter, and the port gives a direct view of the piston crown, cylinder walls, and valve faces when the piston is positioned correctly. On turbine engines, manufacturers designate specific borescope ports at each engine stage: compressor entry, individual compressor stages, combustion liner, high-pressure turbine, and low-pressure turbine. These ports are identified in the engine's maintenance manual (often called the Engine Maintenance Manual or EMM), and using an undocumented port risks damaging seals or FOD-ing the engine.
Lighting
The borescope must carry its own illumination deep into the engine. Rigid and flexible fiberoptic borescopes typically transmit light from an external source (a halogen or LED light generator) through the fiber bundle alongside the image-return fibers. Videoscopes integrate LED lighting directly at the camera tip, providing consistent illumination regardless of probe length. Without adequate lighting, subtle cracks, erosion, or deposits become invisible, so the technician should verify light output before each use and keep the light-transmitting fibers clean and undamaged.
Piston Positioning on Reciprocating Engines
To inspect a cylinder's internal surfaces thoroughly, the piston must be moved through its stroke with the borescope in place. The technician typically uses the starter (with ignition disabled and the mixture at idle cutoff) or turns the propeller by hand to rotate the crankshaft. This positions the piston crown at the top of the bore, then moves it to expose the lower cylinder walls. Both top dead center (TDC) and bottom dead center (BDC) positions should be examined. The engine must be completely de-energized before inserting the probe and before rotating the engine by hand—a critical safety step.
Turbine Engine Inspection Procedure
Turbine borescope inspections follow a carefully scripted sequence defined by the manufacturer. The engine must be shut down, cooled to the temperature specified in the EMM (typically below a defined threshold to prevent thermal shock or technician burns), and the relevant access port plug removed. The probe is guided in to the marked depth, and the technician systematically rotates the borescope tip to scan each blade row. On many turbofan engines, the fan or low-pressure turbine can be manually rotated using an approved barring tool so that each blade passes in front of the probe, allowing 100% blade inspection without moving the probe to a new location.
What to Look For: Common Defects
The AMT must know the difference between acceptable serviceability limits and rejectable findings. Manufacturers publish specific limits in their maintenance manuals; the following are the general categories that the FAA and industry recognize:
- Cracks: Crack rejection criteria for turbine blades are manufacturer- and location-specific—some coatings or non-critical areas may have published permissible limits, so cracks should always be evaluated against the specific engine manual rather than treated as automatically rejectable regardless of length. On cylinder walls, cracks in the barrel are generally cause for removal from service, but the disposition should always be verified against the specific engine manufacturer's serviceable limits and overhaul manual, since some manuals distinguish crack types and locations. Distinguish cracks (sharp, irregular edges) from scratches (smooth, reflective bottoms).
- Erosion and tip rubs: Turbine blade tip erosion from rubbing against the shroud, or leading-edge erosion from ingested particles, reduces aerodynamic efficiency and can propagate to structural failure. Compare observed erosion to the manufacturer's serviceable limits, usually expressed in maximum depth or missing material area.
- Hot section burning and oxidation: Combustion liner burn-throughs, turbine nozzle guide vane oxidation, and blade leading-edge melting indicate over-temperature events. These are almost universally rejectable findings that trigger additional inspections per the EMM.
- Carbon deposits and coking: Heavy carbon buildup on piston crowns, combustion chambers, or turbine nozzles can restrict cooling air passages and alter fuel spray patterns. Excessive deposits may indicate a fuel system or lubrication problem.
- Corrosion: Surface pitting on cylinder walls, compressor blades, or combustion chambers. Technicians assess pitting depth and surface area against manufacturer limits. Active corrosion on aluminum compressor blades may appear as white powdery deposits.
- Scoring and scuffing: Vertical scratches on cylinder walls from inadequate lubrication or abrasive particles. Minor honing marks are normal; deep scoring is not.
- FOD damage: Foreign object damage on compressor or fan blades typically appears as nicks, dents, or bent tips and must be compared to the manufacturer's permissible damage limits before blending or rejection.
Why Borescope Inspection Matters
Aviation regulations under 14 CFR Part 43 require maintenance to be performed using methods, techniques, and practices acceptable to the FAA, typically those described in the manufacturer's maintenance manual or approved equivalent data. Borescope inspection is specifically called out in engine manufacturer Time Between Overhaul (TBO) programs and on-condition maintenance programs as a primary means of determining airworthiness without teardown. Catching a developing turbine blade crack or a cracked cylinder wall during a scheduled borescope inspection can prevent an in-flight engine failure—arguably one of the highest-consequence events in aviation.
From an economic standpoint, removing and overhauling an engine on a time-based schedule without inspecting it first often results in scrapping components that have serviceable life remaining. Borescope inspection supports the on-condition philosophy: parts are removed only when they show evidence of deterioration beyond limits, maximizing asset life and reducing cost.
Key Numbers and Rules
- Always de-energize the ignition system and verify the mixture is at idle cutoff before inserting a borescope into a reciprocating engine cylinder.
- Allow turbine engines to cool to the manufacturer's specified temperature before opening borescope ports—required cool-down times vary widely by engine model and must be taken from the specific EMM rather than assumed from a general range.
- Record all findings with date, engine total time, cycles, description of finding, location (port number, clock position, blade number), and disposition in the maintenance record per 14 CFR §43.9.
- Use only manufacturer-approved borescope ports and follow probe insertion depth limits; forcing a probe past internal structure can cause FOD or seal damage.
- If a videoscope's measurement function is used to size a defect, calibrate the software using the manufacturer-specified reference target before recording measurements as factual data.
- Serviceability limits vary by engine model and must be looked up in the specific EMM—never apply limits from one engine type to another.
Documentation and Disposition
Every borescope inspection must be documented regardless of outcome. A finding-free inspection is still recorded, noting the date, engine time in service, which ports were inspected, the scope of the inspection (all blades, specific stages, etc.), and the technician's name, certificate number, and signature. When defects are found, the record must describe the defect in enough detail that another technician reviewing the records later can understand exactly what was seen and where. Photographs or video captures from a videoscope should be retained in the maintenance file. The disposition—serviceable within limits, repaired by blending, or removed from service—must be clearly stated along with the reference (manual section and revision) that supports the decision.
Common Test Traps
- Mixing up borescope types: Rigid borescopes cannot navigate bends—they are straight-line instruments. Do not confuse them with flexible or video borescopes when an exam question describes a curved access path.
- Skipping the cool-down: Exam questions may test whether you know that a turbine engine must be cooled before a borescope inspection; entering a hot engine risks burns and instrument damage, and some EMMs prohibit it explicitly.
- Applying generic limits: The FAA knowledge test may present a scenario where a technician applies limits from a different engine model. The correct answer is always to use the specific engine's maintenance manual.
- Forgetting documentation: A borescope inspection that is not recorded is as if it never happened. 14 CFR §43.9 requires a maintenance record entry for all maintenance, preventive maintenance, rebuilding, and alteration.
- Ignition not disabled: On a reciprocating engine, rotating the propeller or using the starter with the borescope inserted and ignition live can cause engine start—catastrophic for the technician and the probe. Always verify ignition off and throttle/mixture in the safe position.