NDI/NDT Guide for EASA Part-66: 6 Methods from Modules 6 & 7

What Is NDI/NDT and Why Is It on the Module Exams?

Non-Destructive Testing refers to inspection techniques that evaluate the properties of a material, component, or system without causing damage. In aviation, NDT is used to detect cracks, corrosion, disbonds, inclusions, wall thinning, and other defects in aircraft structures and components — without cutting, drilling, or disassembling them.

The EASA Part-66 module exams test NDT knowledge because every certifying engineer must be able to select the right inspection method for a given defect and understand the limitations of each method. A cracked engine mount caught during a visual inspection could prevent an in-flight failure. A delaminated composite panel missed by an inexperienced technician could lead to structural collapse. NDT knowledge is literally life-saving, and the module exams reflect that.

NDT content appears primarily in Module 6 (Materials and Hardware — non-destructive testing techniques in the materials syllabus) and Module 7 (Maintenance Practices — inspection techniques and NDT procedures), with some overlap into the propulsion modules for engine component inspection. Expect several NDT-specific questions in these modules, plus NDT in your practical training and assessments. In the EASA system, NDT used as a basis for release to service is performed by personnel qualified in accordance with standards such as EN 4179 / NAS 410 within the maintenance organisation's quality system — a requirement that also appears on the exams.

The Six NDT Methods You Need to Know

The Part-66 syllabus focuses on six primary NDT methods. The table below summarises each method, what it detects, where it is commonly used in aircraft maintenance, and its key limitations.

Method-by-Method Breakdown

1. Visual Testing (VT) — The Foundation

Visual inspection is the most basic and most commonly used NDT method. It is the first step in virtually every aircraft inspection — from daily walk-arounds to heavy maintenance checks. The module exams expect you to understand that visual inspection is not just "looking." It involves:

• Using proper lighting (natural or artificial, at the correct angle)
• Using aids such as borescopes, mirrors, and magnifying glasses for inaccessible areas
• Knowing what to look for — cracking, corrosion, loose hardware, fluid leaks, chafing, deformation
• Documenting findings in the maintenance record or inspection report

Visual inspection detects a large percentage of all defects found in aircraft maintenance. However, it is limited to surface conditions only. If a crack is hidden beneath a paint layer or inside a structure, visual inspection alone is insufficient.

Exam tip: Questions about visual inspection on Part-66 exams often focus on what additional tools are used (borescope, mirror) and when visual inspection must be supplemented by another NDT method.

2. Dye Penetrant Testing (PT)

Dye penetrant inspection is used to detect surface-breaking defects in non-porous materials. The basic process is: (1) clean the surface, (2) apply penetrant and allow dwell time, (3) remove excess penetrant, (4) apply developer, (5) inspect for indications.

Key facts for the exam:

• Works on metals, ceramics, and some plastics — but not porous materials like wood, concrete, or unsealed composites
• Cannot detect subsurface defects — only defects open to the surface
• Surface must be clean and dry — oil, grease, paint, or corrosion products will block the penetrant from entering defects
• Two types: fluorescent penetrant (requires UV light — more sensitive) and visible dye (red dye, white developer — does not require UV light)
• Temperature range affects dwell time and sensitivity — most penetrants are designed for 10–50°C (50–122°F)

Exam tip: The exams love to test the limitations of dye penetrant. A common question asks why PT cannot be used on a specific component — the answer is almost always "because the material is porous" or "because the defect is subsurface."

3. Magnetic Particle Testing (MT)

Magnetic particle inspection uses magnetic fields to detect surface and near-surface defects in ferromagnetic materials (steels and irons). When a magnetic field is applied to a ferromagnetic part, a crack or discontinuity causes magnetic flux leakage at the surface. Fine iron particles (wet or dry) are attracted to the leakage, forming a visible indication.

Key facts for the exam:

• Ferromagnetic materials only — does not work on aluminium, titanium, magnesium, or composites
• Can detect both surface and subsurface defects (up to approximately 6 mm deep, depending on magnetising current)
• Two magnetisation methods: circular (current passed through the part — detects longitudinal cracks) and longitudinal (magnetic field along the part axis — detects transverse cracks). Both orientations must be used for a complete inspection
• Parts must be demagnetised after inspection — residual magnetism can attract steel debris in service, causing wear or bearing failure
• Dry particles are better for rough surfaces; wet particles (in liquid suspension) are better for smooth surfaces and automated inspection

Exam tip: The most common MT exam question involves crack orientation versus magnetic field direction. Remember: circular magnetisation (current through part) detects longitudinal cracks parallel to the current. Longitudinal magnetisation detects transverse cracks perpendicular to the field.

4. Eddy Current Testing (ET)

Eddy current inspection uses electromagnetic induction to detect defects in conductive materials. An alternating current in a probe coil induces eddy currents in the test material. Defects disrupt the eddy current flow, changing the impedance of the probe coil — which is displayed on the instrument.

Key facts for the exam:

• Conductive materials only — works on all metals but not composites or non-conductive materials
• Depth penetration is controlled by frequency: lower frequency = deeper penetration (but lower sensitivity to small defects). Higher frequency = shallower but more sensitive
• Excellent for detecting cracks in and around fastener holes — a very common aviation application
• Can measure conductivity (useful for detecting heat damage in aluminium) and coating thickness
• Surface finish, probe lift-off, and part geometry all affect the signal — operator skill is critical
• Requires reference standards (calibration blocks) with known defects for setup

Exam tip: Eddy current questions on Part-66 exams often focus on the relationship between frequency and penetration depth. Expect at least one question asking: "To detect a deeper defect with eddy current, should you increase or decrease the frequency?"

5. Ultrasonic Testing (UT)

Ultrasonic inspection uses high-frequency sound waves (typically 1–10 MHz) to detect internal defects. A transducer sends pulses of sound into the material, and reflections (echoes) from the back surface and any internal defects are displayed on a screen as a pulse-echo display (A-scan).

Key facts for the exam:

• Works on all sound-conducting materials — metals, plastics, composites, ceramics
• Excellent for detecting subsurface defects: cracks, delaminations, disbonds, inclusions, wall thinning, and corrosion
• Requires a couplant (water, gel, grease) between the transducer and the part — air gaps block sound transmission
• Two main techniques: pulse-echo (single transducer sends and receives) and through-transmission (separate transmitter and receiver on opposite sides)
• Angle beam transducers are used for detecting cracks oriented perpendicular to the surface (e.g., fatigue cracks in bolt holes)
• Reference standards with known reflectors (flat-bottom holes, notches) are essential for calibration and sizing

Exam tip: UT questions frequently test the concept of the dead zone (the near-surface area where the initial pulse masks echoes from shallow defects). Also, know that higher frequency gives better resolution but less penetration; lower frequency penetrates deeper but with less sensitivity to small defects.

6. Radiographic Testing (RT)

Radiographic inspection uses X-rays or gamma rays to create an image of the internal structure of a component. Dense areas (solid metal) absorb more radiation and appear lighter on the film or digital detector. Voids, cracks, and inclusions absorb less and appear darker.

Key facts for the exam:

• Can inspect any material — metals, composites, honeycomb structures, welds
• Produces a permanent record (film or digital image) that can be reviewed later
• Two-sided access required — the radiation source must be on one side and the detector on the other. This makes RT impractical for some in-situ inspections
• Crack detection depends on orientation — a crack aligned parallel to the radiation beam is easily detected; a crack perpendicular to the beam may be invisible
• Radiation safety is a major concern — requires certified operators, controlled areas, dosimeters, and shielding
• Gamma ray sources (Ir-192, Co-60) are portable and do not need power, but have fixed energy and cannot be turned off. X-ray generators can be adjusted but require electrical power

Exam tip: A classic exam question asks: "Which NDT method would you use to inspect a weld for internal porosity in a steel fitting?" Answer: Radiography. Another classic: "What is the primary disadvantage of radiographic inspection compared to ultrasonic?" Answer: Radiation hazard / two-sided access required.

Comparison Table: Choosing the Right NDT Method

Part-66 exams often ask you to select the right NDT method for a given scenario. Here is a quick-reference decision guide:

Sample NDT Exam Questions

How NDT Is Tested on the Part-66 Module Exams

Based on Part-66 Appendix I and analysis of question patterns, here is how NDT content breaks down across the module exams:

Study Strategy for NDT Questions

Here is the most efficient way to master NDT for your Part-66 module exams:

1. Learn the six methods by their acronyms. Visual (VT), Dye Penetrant (PT), Magnetic Particle (MT), Eddy Current (ET), Ultrasonic (UT), Radiographic (RT). Know which materials each method works on (ferrous vs. non-ferrous, conductive vs. non-conductive, porous vs. non-porous).

2. Master the limitations. For every method, ask yourself: "What can it NOT detect?" This is where examiners focus their questions. If you know the limitations of each method, you will correctly answer the "which method is most appropriate" questions.

3. Create a decision tree. When given a scenario question, work through: Is the defect surface or subsurface? Is the material ferromagnetic or non-ferromagnetic? Is two-sided access available? Is this a regulatory question or a technical question? Each branch leads to the correct method.

4. Practise with exam-style questions. The Skylicence Europe practice question bank includes 50+ NDT-specific questions with detailed explanations, covering method selection, procedures, regulatory requirements, and limitations across all six methods.

5. Review the key syllabus areas. NDT appears in the Module 6 and Module 7 subject areas of Part-66 Appendix I, plus the airframe and propulsion modules. Make sure you know which syllabus subject covers which method, and understand how NDT qualification (EN 4179 / NAS 410) works inside a Part-145 quality system. The Part-66 study guide has the full breakdown of modules and references.

See the aviation maintenance glossary for NDT terminology →

NDT Certification Levels

The Part-66 licence does not in itself certify you for NDT work. Within a Part-145 approved maintenance organisation, NDT is performed by personnel qualified in accordance with the applicable standards — in Europe, typically EN 4179 (aerospace series — qualification and approval of personnel for non-destructive testing), which is harmonised with NAS 410. The three levels are:

• Level 1: Performs specific NDT under supervision. Can set up equipment and record results but cannot interpret or evaluate indications
• Level 2: Performs and interprets NDT. Can select the technique, calibrate equipment, evaluate results, and prepare reports. Level 2 certification is typically the minimum requirement for NDT used as a basis for release to service
• Level 3: Develops techniques, approves procedures, and trains/certifies Level 1 and 2 personnel. Provides ultimate technical authority for NDT within an organisation

Exam tip: Level 2 is the certification level most commonly referenced in exam questions. Remember: Level 2 can interpret and evaluate; Level 1 cannot.

Common NDT Myths — and What the Exam Actually Tests

Myth: Dye penetrant can detect any surface crack.
Fact: PT only detects defects open to the surface. A crack covered by paint, corrosion, or sealant will not be detected unless the coating is removed first.

Myth: Magnetic particle testing works on all metals.
Fact: MT only works on ferromagnetic materials (iron, steel, nickel, cobalt). It does not work on aluminium, titanium, magnesium, or austenitic stainless steel.

Myth: Ultrasonic testing does not require a couplant.
Fact: UT requires a couplant (water, gel, grease) between the transducer and the part to transmit sound energy. An air gap reflects nearly all the sound energy.

Myth: Radiographic testing detects any crack orientation.
Fact: RT sensitivity depends heavily on crack orientation. A crack aligned with the radiation beam is easily detected; one perpendicular to the beam may not be visible at all. This is why RT and UT are often used together for critical inspections.

Myth: Eddy current can inspect any conductive material at any depth.
Fact: ET penetration depth is limited by the skin effect. Higher frequencies inspect shallower depths; lower frequencies penetrate deeper but with reduced sensitivity. Maximum practical depth is typically 6–10 mm in aluminium.

Final Advice

NDT is one of the most "learnable" subjects on the Part-66 exams. Unlike legislation questions that require memorising specific regulation references, NDT follows logical rules — each method works on certain materials, detects certain defect types, and has specific limitations. If you understand the physics and logic behind each method, you can reason your way through NDT questions even if you have not memorised every detail.

The most effective approach is to create a simple reference card with the six methods and their materials/limitations, then practise applying them to scenario questions. Once you can quickly rule out methods that do not match the material or defect type, you will consistently land on the correct answer.

For more targeted practice, use the Skylicence Europe platform to generate custom practice sessions focused on NDT questions. The adaptive difficulty ensures you see harder questions as you improve, and the answer explanations reinforce the material, method, and regulatory reasoning for each question.