Inspection levels and nondestructive inspection (NDI)

Find damage before it affects airworthiness

Not every defect can be found with a quick look, and not every inspection requires advanced equipment. Aircraft maintenance uses defined inspection levels and specialized nondestructive inspection (NDI) methods to ensure defects are detected before they become safety issues. The key is choosing the right inspection for the material, the location and the type of damage being sought.

The purpose of any inspection is simple: find damage before it affects airworthiness. Some defects are visible on the surface. Others may be hidden inside metal structures, beneath fasteners or within composite materials. Understanding inspection levels and NDI techniques allows technicians to inspect aircraft effectively without causing damage to the component itself.

Inspection levels

Transport-category aircraft maintenance programs use standardized inspection levels so that technicians, inspectors, manufacturers and regulators have a common understanding of the required depth of inspection.

General visual inspection (GVI)

A general visual inspection (GVI) is a visual examination of an area, installation or assembly to identify obvious signs of damage, wear, leakage, deterioration or missing parts. A GVI is often the first step in identifying potential problems. Characteristics include:

• Performed from a normal viewing distance
• Uses normal hangar or daylight illumination
• May require opening access panels or doors
• Does not normally require extensive cleaning or disassembly

What a GVI finds

A GVI is intended to detect readily apparent conditions such as fluid leaks, missing hardware, cracked fairings, corrosion, chafing and structural damage. Typical examples:

• Checking landing gear for leaks
• Looking for loose cowling fasteners
• Inspecting an engine compartment for obvious defects
• Examining tire condition during a transit inspection

Detailed inspection (DET)

A detailed inspection (DET) is a thorough examination of a specific component or area where a defect is suspected or known to occur. The inspector looks for subtle conditions that could be missed during a GVI. A DET requires significantly more time and effort than a GVI. Characteristics include:

• Intensive visual examination
• Good direct lighting
• Close access to the component
• Surface cleaning when required
• Use of mirrors, magnifiers, borescopes or inspection lights

What a DET finds

Typical findings include small cracks, corrosion pits, the security of fasteners, wear patterns, minor deformation and heat damage. Typical examples:

• Detailed inspection of a wing attachment fitting
• Examination of a control cable pulley system
• Inspection of turbine blades through access ports

Special detailed inspection (SDI)

A special detailed inspection (SDI) combines a detailed inspection with a specialized inspection technique. These inspections are used when defects cannot reliably be detected by visual means alone. An SDI may involve:

• Nondestructive testing (NDT)
• Borescope inspection
• Ultrasonic testing
• Eddy current inspection
• Radiography
• Partial disassembly

Where SDIs are used

An SDI is often required by maintenance programs, service bulletins, structural inspection documents or airworthiness directives. Typical applications:

• Fatigue crack detection
• Corrosion hidden beneath structures
• Internal engine inspections
• Composite damage evaluations
• Fastener hole inspections

What is nondestructive inspection?

Nondestructive inspection (NDI), also called nondestructive testing (NDT), is the process of evaluating a component without permanently damaging it. Unlike destructive testing, which requires cutting, breaking or loading a part to failure, NDI allows technicians to inspect aircraft structures and return them to service if they meet requirements. The ideal NDI method:

• Detects defects accurately
• Preserves the component
• Minimizes aircraft downtime
• Provides repeatable results

Liquid penetrant inspection (PT)

Liquid penetrant inspection is used to detect defects that are open to the surface. The developer draws dye back out of discontinuities, making defects visible. How it works:

• Clean the surface.
• Apply penetrant dye.
• Allow penetration into cracks.
• Remove excess dye.
• Apply developer.
• Inspect for indications.

Penetrant: strengths and limits

• Best for: surface cracks, porosity and seams, in aluminum alloys, titanium, stainless steel and other non-ferrous materials.
• Limitations: detects only surface-breaking defects, cannot see subsurface flaws, and requires a clean, non-porous surface.

Magnetic particle inspection (MT)

Magnetic particle inspection is used on ferromagnetic materials only. The part is magnetized, creating a magnetic field. If a crack is present, the magnetic field is disturbed, magnetic particles gather at the discontinuity, and the indication becomes visible.

• Best for: surface and near-surface cracks in steel components such as landing gear parts and engine mounts.
• Limitations: only works on ferromagnetic materials. It does not work on aluminum, magnesium or most stainless steels.

Eddy current inspection (ET)

Eddy current inspection is one of the most common aircraft NDI techniques. An electrical coil induces currents into a conductive material, and defects alter the current flow and produce measurable changes in the inspection signal.

• Best for: surface cracks, near-surface cracks, fastener hole inspections, corrosion detection and material thickness changes.
• Common aircraft applications: wheel inspections, fuselage skin inspections, wing structures, fastener locations and lap joints.
• Advantages: minimal surface preparation, high sensitivity and rapid inspection.
• Limitations: requires electrically conductive material, needs proper calibration standards, and interpretation requires training and experience.

Ultrasonic inspection (UT)

Ultrasonic testing uses high-frequency sound waves to detect flaws inside a material. A probe transmits sound energy into the structure, and sound waves reflect from internal defects, back walls, delaminations and corrosion boundaries. The reflections are displayed and interpreted by the inspector.

• Best for: internal cracks, corrosion thinning, thickness measurements, composite inspection and bond integrity checks.
• Advantages: excellent depth penetration, highly accurate thickness measurement, and detection of internal defects not visible externally.
• Limitations: requires trained personnel, surface condition can affect results, and complex geometries can be difficult to inspect.

Radiographic inspection (RT)

Radiography uses X-rays or gamma rays to create an image of the internal structure of a component. Because of the safety considerations, radiography is highly controlled and regulated.

• Best for: internal structural defects, weld inspections, hidden corrosion, foreign objects and casting defects.
• Advantages: a permanent inspection record, the ability to see internal details, and detection of otherwise inaccessible defects.
• Limitations: expensive equipment, radiation hazards, strict safety requirements, and the need for specially trained personnel.

Borescope inspection

A borescope allows visual inspection of areas that cannot be accessed directly. A borescope often bridges the gap between visual inspection and NDI. Modern video borescopes can provide high-resolution imaging, image storage and measurement capabilities. Common uses:

• Gas turbine engines
• Combustion chambers
• Compressor sections
• Turbine blades
• Gearboxes

Tap testing

Tap testing is commonly used on composite and bonded structures. The inspector lightly taps the surface and listens to the sound produced. Different sounds may indicate delamination, disbonding, core damage or internal separation.

• Advantages: quick, inexpensive and portable.
• Limitations: subjective interpretation and limited depth information.

Thermographic inspection

Thermography uses infrared imaging to identify abnormal heat patterns. Damaged areas often transfer heat differently than sound material. Applications:

• Composite structures
• Bonded assemblies
• Hidden moisture intrusion
• Delaminations
• Electrical system inspections

Choosing the right inspection method

The first step is identifying what material is being inspected, what defect is expected, how deep the defect might be, and what access is available. Then match the method to the defect:

• Surface cracks: liquid penetrant, magnetic particle or eddy current.
• Subsurface cracks: eddy current, ultrasonic or radiography.
• Corrosion detection: detailed visual inspection, eddy current or ultrasonic thickness testing.
• Composite delamination: ultrasonic inspection, tap testing or thermography.
• Internal structural damage: radiography, ultrasonic inspection or borescope inspection.

Important limitations

No inspection method can find every type of defect. This is why manufacturers specify the inspection method to be used for each task. For example:

• Penetrant cannot detect hidden cracks beneath the surface.
• Magnetic particle inspection does not work on aluminum.
• Eddy current requires conductive material.
• Ultrasonic methods require proper coupling and technique.
• Radiography requires strict radiation controls.

Qualified personnel and procedures

NDI is not performed based on personal preference. The applicable documents determine the inspection method, equipment type, probe selection, calibration standards, acceptance limits and rejection criteria. Only properly qualified and certified NDI personnel should perform and interpret advanced NDI inspections. These instructions are found in documents such as:

• Nondestructive testing manuals (NTM)
• Structural repair manuals (SRM)
• Aircraft maintenance manuals (AMM)
• Service bulletins
• Airworthiness directives

Key takeaways

• General visual inspection (GVI) identifies obvious defects using normal access and lighting.
• Detailed inspection (DET) provides a close, thorough examination of a specific area.
• Special detailed inspection (SDI) combines detailed inspection with techniques such as NDI or partial disassembly.
• Match the inspection method to the material, defect type and expected defect depth.
• No NDI method works for every situation.
• Always follow the manufacturer's NDT/NDI procedures, calibration requirements and acceptance criteria.
• The goal is not simply finding defects, but accurately determining whether the component remains airworthy.

The bottom line

Aircraft inspections range from a simple visual check to sophisticated nondestructive testing techniques capable of detecting microscopic cracks hidden deep within a structure. Understanding the differences between GVI, detailed inspections, special detailed inspections and the major NDI methods allows technicians to select the right tool for the job. In aviation maintenance, the quality of an inspection often determines whether a defect is found while it is still minor or after it has become a serious safety concern.