Borescope inspection glossary
Plain definitions of the terms that come up in aircraft engine borescope inspections, from airworthiness directives to TBC spallation.
Rules and documents
Airworthiness directive (AD)
A legally enforceable rule from an aviation authority, such as the FAA or EASA, that requires action to correct an unsafe condition in an aircraft, engine or part.
Service bulletin (SB)
A manufacturer's instructions for an inspection, modification or repair. It becomes mandatory when an AD requires it.
Notice of proposed rulemaking (NPRM)
A proposed rule, such as a proposed AD, published for public comment before it's finalized.
Aircraft maintenance manual (AMM)
The manual for maintaining an aircraft and its installed engines, including on-wing borescope tasks and their limits.
Engine shop manual (ESM)
The engine manufacturer's manual for off-wing maintenance, repair and overhaul.
Maintenance program
The operator's approved schedule of inspections and tasks, including scheduled borescope inspections.
Return to service
The approval, by an authorized person, that an aircraft or engine may operate again after maintenance.
Certificate of release to service (CRS)
In EASA rules, the signed statement that maintenance was done properly and the aircraft or component can be released.
EASA Form 1
EASA's authorised release certificate for parts, components and engines.
FAA Form 8130-3
The FAA's authorized release certificate, or airworthiness approval tag, for parts, components and engines.
Dual release
A release that meets both FAA and EASA requirements, issued by an organisation approved by both.
Part 145 (FAA)
The US rules for certificated repair stations, 14 CFR Part 145.
Part-145 (EASA)
The EU rules for approved maintenance organisations.
Part-66 (EASA)
The EU licensing rules for aircraft maintenance engineers, including certifying staff.
A&P mechanic
A US mechanic certificated with airframe and powerplant ratings.
Continuing airworthiness management organisation (CAMO)
Under EASA rules, the organisation that manages an aircraft's airworthiness and its maintenance program.
Maintenance, repair and overhaul (MRO)
A company that maintains aircraft, engines or components.
Aircraft on ground (AOG)
An aircraft that can't fly until a maintenance issue is fixed.
Inspection and equipment
Borescope inspection (BSI)
A visual inspection of an engine's internal parts through access ports, using a borescope.
Borescope
Strictly, a rigid optical tube for straight-line viewing; in aviation, the general name for any probe used for internal visual inspection.
Videoscope
A flexible borescope with a camera at its tip, and the standard tool for engine inspections today.
Fiberscope
A flexible borescope that carries the image through fiber-optic bundles.
Articulation
Steering the probe tip, usually in two or four directions, to aim the view.
Borescope port
A plugged access hole in the engine casing for inserting the probe.
Guide tube
A tube that steers the probe to a specific inspection area.
3D measurement
Measuring a finding's size or depth from the image, using stereo optics or projected patterns.
Engine rotation
Turning the engine's rotor slowly, by hand or with a drive tool, so each blade passes the camera.
General visual inspection (GVI)
A visual check of an area or assembly for obvious damage, failure or irregularity.
Remote visual inspection (RVI)
Visual inspection with optical tools such as borescopes, where the eye can't reach directly.
Non-destructive testing (NDT)
Inspection methods that don't damage the part, such as visual, eddy current, ultrasonic and penetrant inspection.
Blending
Removing minor damage from a blade by smoothing it within manual limits. Borescope blending does this with the engine installed.
Disposition
The decision on a finding: serviceable, serviceable with a reduced interval, or unserviceable.
Repetitive inspection
An inspection repeated at a set interval, often required by an AD or after a finding.
On-wing and off-wing
Maintenance with the engine installed on the aircraft, or removed from it.
Shop visit
An engine's induction into a shop for maintenance that separates major engine case flanges.
Flight cycle (FC)
One takeoff and landing. Many inspection intervals are counted in cycles.
Lease return
Handing an aircraft or engine back to its lessor, usually with inspections showing it meets the return conditions.
Engine sections
Fan
The large front rotor of a turbofan. Most of the air it moves bypasses the core.
Booster (LPC)
The low-pressure compressor stages behind the fan, on the same shaft.
High-pressure compressor (HPC)
The compressor stages driven by the high-pressure turbine, which raise the air's pressure before the combustor.
Combustor
Where fuel burns with compressed air. It includes the liner, the dome and the fuel nozzles.
High-pressure turbine (HPT)
The turbine stage or stages just behind the combustor, which drive the HPC. They're the hottest rotating parts in the engine.
Low-pressure turbine (LPT)
The turbine stages that drive the fan and booster.
Nozzle guide vane (NGV)
A stationary vane in front of a turbine stage that directs hot gas onto the blades.
Stage
One row of rotating blades and its row of stationary vanes.
Hot section
The combustor and the turbines.
Gas path
The route air and combustion gas take through the engine core.
N1 and N2
The speeds of the low-pressure and high-pressure rotors, also used as names for the rotors themselves.
Exhaust gas temperature (EGT)
A key measure of engine health. An exceedance can trigger a borescope inspection.
Damage and findings
Nick
A sharp-edged notch in a blade or vane, usually from an impact.
Dent
A smooth depression in a surface, without missing material.
Erosion
Gradual wear of a surface by particles, often at leading edges.
Crack
A fracture in the material, and usually among the most serious findings.
Burning
Heat damage to metal, from discoloration to melted or missing material.
Burn-through
Burning that has gone all the way through a part, such as a combustor liner or a vane.
Missing material
A piece of a blade, vane or liner that has broken away.
Tip curl
Bending of blade tips, usually from rubbing or impact.
Foreign object damage (FOD)
Damage from objects the engine ingests, such as birds, stones or tools.
Domestic object damage (DOD)
Damage from parts of the engine itself that have come loose.
Thermal barrier coating (TBC)
A ceramic coating that insulates hot-section parts.
Spallation
Loss of a coating layer that flakes away from the surface.
Oxidation
Chemical attack of hot metal by oxygen, seen as discoloration or scaling.
Coking
Carbon deposits, for example on fuel nozzles.
CMAS
Calcium-magnesium-alumino-silicate: deposits from ingested sand, dust or ash that attack thermal barrier coatings.