Nondestructive Inspection/Testing (Part 1) | Aircraft Systems

Nondestructive Inspection/Testing (Part 1)

The preceding information in this site provided general details regarding aircraft inspection. The remainder of this site deals with several methods often used on specific components or areas on an aircraft when carrying out the more specific inspections. They are referred to as nondestructive inspection (NDI) or nondestructive testing (NDT). The objective of NDI and NDT is to determine the airworthiness of a component, without damaging it, that would render it unairworthy. Some of these methods are simple, requiring little additional expertise, while others are highly sophisticated and require that the technician be highly trained and specially certified.

Training, Qualification, and Certification

The product manufacturer or the FAA generally specifies the particular NDI method and procedure to be used in inspection. These NDI requirements are specified in the manufacturer’s inspection, maintenance, or overhaul manual, FAA ADs, supplemental structural inspection documents (SSID), or SBs.

The success of any NDI method and procedure depends upon the knowledge, skill, and experience of the NDI personnel involved. The person(s) responsible for detecting and interpreting indications, such as eddy current, x-ray, or ultrasonic NDI, must be qualified and certified to specific FAA or other acceptable government or industry standards, such as MIL-STD-410, Nondestructive Testing Personnel Qualification and Certification or A4A iSPec 2200, Guidelines for Training and Qualifying Personnel in Nondestructive Testing Methods. The person must be familiar with the test method, know the potential types of discontinuities peculiar to the material, and be familiar with their effect on the structural integrity of the part. Additional information on NDI may be found by referring to Chapter 5 of FAA AC 43.13-1, Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair.


Advantages and Disadvantages of NDI Methods

Figure 1 provides a table of the advantages and disadvantages of common NDI methods. This table could be used as a guide for evaluating the most appropriate NDI method when the manufacturer or the FAA has not specified a particular NDI method to be used.

Method

Advantages

Disadvantages

Visual

• Inexpensive

• Highly portable

• Immediate results

• Minimum training

• Minimum part preparation

• Surface discontinuities only

• Generally only large discontinuities

• Misinterpretation of scratches

Penetrant Dye

• Portable

• Inexpensive

• Sensitive to very small discontinuities

• 30 minutes or less to accomplish

• Minimum skill required

• Locate surface defects only

• Rough or porous surfaces interfere with test

• Part preparation required (removal of finishes and sealant, etc.)

• High degree of cleanliness required

• Direct visual detection on results required

Magnetic Particle

• Can be portable

• Inexpensive

• Sensitive to small discontinuities

• Immediate results

• Moderate skill required

• Detects surface and subsurface discontinuities

• Relatively fast

• Surface must be accessible

• Rough surfaces interfere with test

• Part preparation required (removal of finishes and sealant, etc.)

• Semi-directional requiring general orientation of field to discontinuity

• Ferro-magnetic materials only

• Part must be demagnetized after test

Eddy Current

• Portable

• Detects surface and subsurface discontinuities

• Moderate speed

• Immediate results

• Sensitive to small discontinuities

• Thickness sensitive

• Can detect many variables

• Surface must be accessible to probe

• Rough surfaces interfere with test

• Electrically conductive materials

• Skill and training required

• Time consuming for large areas

Ultrasonic

• Portable

• Inexpensive

• Sensitive to very small discontinuities

• Immediate results

• Little part preparation

• Wide range of materials and thickness can be inspected

• Surface must be accessible to probe

• Rough surfaces interfere with test

• Highly sensitive to sound beam discontinuity orientation

• High degree of skill and experience required for exposure and interpretation

• Depth of discontinuity not indicated

X-Ray Radiography

• Detects surface and internal flaws

• Can inspect hidden areas

• Permanent test record obtained

• Minimum part preparation

• Safety hazard

• Very expensive (slow process)

• Highly directional, sensitive to flaw orientation

• High degree of skill and experience required for exposure and interpretation

• Depth of discontinuity not indicated

Isotope Radiography

• Portable

• Less inexpensive than x-ray

• Detects surface and internal flaws

• Can inspect hidden areas

• Permanent test record obtained

• Minimum part preparation

• Safety hazard

• Must conform to federal and state regulations for handling and use

• Highly directional, sensitive to flaw orientation

• High degree of skill and experience required for exposure and interpretation

• Depth of discontinuity not indicated

Figure 1. Advantages and disadvantages of NDI methods

General Techniques

Before conducting NDI, it is necessary to follow preparatory steps in accordance with procedures specific to that type of inspection. Generally, the parts or areas must be thoroughly cleaned. Some parts must be removed from the aircraft or engine. Others might need to have any paint or protective coating stripped. A complete knowledge of the equipment and procedures is essential and, if required, calibration and inspection of the equipment must be current.

Visual Inspection

Visual inspection can be enhanced by looking at the suspect area with a bright light, a magnifying glass, and a mirror. Some defects might be so obvious that further inspection methods are not required. The lack of visible defects does not necessarily mean further inspection is unnecessary. Some defects may lie beneath the surface or may be so small that the human eye, even with the assistance of a magnifying glass, cannot detect them.

Surface Cracks

When searching for surface cracks with a flashlight, direct the light beam at a 5 to 45 degree angle to the inspection surface towards the face. [Figure 2] Do not direct the light beam at such an angle that the reflected light beam shines directly into the eyes. Keep the eyes above the reflected light beam during the inspection. Determine the extent of any cracks found by directing the light beam at right angles to the crack and tracing its length. Use a 10-power magnifying glass to confirm the existence of a suspected crack. If this is not adequate, use other NDI techniques, such as penetrant, magnetic particle, or eddy current to verify cracks.

Nondestructive Inspection/Testing
Figure 2. Using a flashlight to inspect for cracks

Borescope

Inspection by use of a borescope is essentially a visual inspection. A borescope is a device that enables the inspector to see inside areas that could not otherwise be inspected without disassembly. Borescopes are used in aircraft and engine maintenance programs to reduce or eliminate the need for costly teardowns. Aircraft turbine engines have access ports that are specifically designed for borescopes. Borescopes are also used extensively in a variety of aviation maintenance programs to determine the airworthiness of difficult to reach components. Borescopes typically are used to inspect interiors of hydraulic cylinders and valves for pitting, scoring, porosity, and tool marks; search for cracked cylinders in aircraft reciprocating engines; inspect turbojet engine turbine blades and combustion cans; verify the proper placement and fit of seals, bonds, gaskets, and subassemblies in difficult to reach areas; and assess foreign object damage (FOD) in aircraft, airframe, and powerplants. Borescopes may also be used to locate and retrieve foreign objects in engines and airframes.

Borescopes are available in two basic configurations. The simpler of the two is a rigid type, small diameter telescope with a tiny mirror at the end that enables the user to see around corners. The other type uses fiber optics that enable greater flexibility. [Figure 3] Many borescopes provide images that can be displayed on a computer or video monitor for better interpretation of what is being viewed and to record images for future reference. Most borescopes also include a light to illuminate the area being viewed.

Nondestructive Inspection/Testing
Figure 3. Rigid and flexible borescopes

Liquid Penetrant Inspection

Penetrant inspection is a nondestructive test for defects open to the surface in parts made of any nonporous material. It is used with equal success on such metals as aluminum, magnesium, brass, copper, cast iron, stainless steel, and titanium. It may also be used on ceramics, plastics, molded rubber, and glass.

Penetrant inspection detects defects, such as surface cracks or porosity. These defects may be caused by fatigue cracks, shrinkage cracks, shrinkage porosity, cold shuts, grinding and heat treat cracks, seams, forging laps, and bursts. Penetrant inspection also indicates a lack of bond between joined metals. The main disadvantage of penetrant inspection is that the defect must be open to the surface in order to let the penetrant get into the defect. For this reason, if the part in question is made of material that is magnetic, the use of magnetic particle inspection is generally recommended.

Penetrant inspection uses a penetrating liquid that enters a surface opening and remains there, making it clearly visible to the inspector. It calls for visual examination of the part after it has been processed, increasing the visibility of the defect so that it can be detected. Visibility of the penetrating material is increased by the addition of one or two types of dye: visible or fluorescent.

The visible penetrant kit consists of dye penetrant, dye remover emulsifier, and developer. The fluorescent penetrant inspection kit contains a black light assembly, as well as spray cans of penetrant, cleaner, and developer. The light assembly consists of a power transformer, a flexible power cable, and a hand-held lamp. Due to its size, the lamp may be used in almost any position or location.

The steps for performing a penetrant inspection are:
  1. Clean the metal surface thoroughly.
  2. Apply penetrant.
  3. Remove penetrant with remover emulsifier or cleaner.
  4. Dry the part.
  5. Apply the developer.
  6. Inspect and interpret results.

Interpretation of Results

The success and reliability of a penetrant inspection depends upon the thoroughness that the part was prepared with. Several basic principles applying to penetrant inspection are:
  1. The penetrant must enter the defect in order to form an indication. It is important to allow sufficient time so the penetrant can fill the defect. The defect must be clean and free of contaminating materials so that the penetrant is free to enter.
  2. If all penetrant is washed out of a defect, an indication cannot be formed. During the washing or rinsing operation, prior to development, it is possible that the penetrant is removed from within the defect, as well as from the surface.
  3. Clean cracks are usually easy to detect. Surface openings that are uncontaminated, regardless of how fine, are seldom difficult to detect with the penetrant inspection.
  4. The smaller the defect, the longer the penetrating time. Fine crack-like apertures require a longer penetrating time than defects such as pores.
  5. When the part to be inspected is made of a material susceptible to magnetism, it should be inspected by a magnetic particle inspection method if the equipment is available.
  6. Visible penetrant-type developer, when applied to the surface of a part, dries to a smooth, white coating. As the developer dries, bright red indications appear where there are surface defects. If no red indications appear, there are no surface defects.
  7. When conducting the fluorescent penetrant-type inspection, the defects show up (under black light) as a brilliant yellow-green color and the sound areas appear deep blue-violet.
  8. It is possible to examine an indication of a defect and to determine its cause as well as its extent. Such an appraisal can be made if something is known about the manufacturing processes that the part has been subjected to.

The size of the indication, or accumulation of penetrant, shows the extent of the defect and the brilliance is a measure of its depth. Deep cracks hold more penetrant and are broader and more brilliant. Very fine openings can hold only small amounts of penetrants and appear as fine lines. [Figure 4]

Nondestructive Inspection/Testing
Figure 4. Dye penetrant inspection

False Indications

With the penetrant inspection, there are no false indications in the sense that they occur in the magnetic particle inspection. There are, however, two conditions that may create accumulations of penetrant that are sometimes confused with true surface cracks and discontinuities.

The first condition involves indications caused by poor washing. If all the surface penetrant is not removed in the washing or rinsing operation following the penetrant dwell time, the unremoved penetrant is visible. Evidences of incomplete washing are usually easy to identify since the penetrant is in broad areas rather than in the sharp patterns found with true indications. When accumulations of unwashed penetrant are found on a part, the part must be completely reprocessed. Degreasing is recommended for removal of all traces of the penetrant. False indications may also be created where parts press fit to each other. If a wheel is press fit onto a shaft, penetrant shows an indication at the fit line. This is perfectly normal since the two parts are not meant to be welded together. Indications of this type are easy to identify since they are regular in form and shape.

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