Non-destructive testing (NDT) used in the aerospace industry plays a critical safety role in the manufacturing process of flight-critical parts and components. Fluorescent Penetrant Inspection (FPI) is one of these NDTs that allows the detection of defects in metal products. In this article, we discuss this inspection technique to better understand the important role it plays for aircraft manufacturers and how it fits into the manufacturing process.

What Is Fluorescent Penetrant Inspection?

Fluorescent penetrant inspection is a non-destructive testing process in which dye is applied to the surface of a material to examine for surface defects. Many metallic products are suitable for FPI inspection because the parts’ surface shall be smooth and have small, tight pores. The fluorescent dye contrasts with the dark background, allowing inspectors to find flaws that could cause the part to fail in operation. FPI inspection is often used throughout the manufacturing process, after key fabrication stages. This ensures the metal meets all specifications of the aircraft manufacturer and industry.

Role of FPI Inspection in the Aerospace Industry

FPI is one of the Non-Destructive Testing (NDT) methods used in the aerospace industry. The materials used in making aerospace components are susceptible to fatigue, vibration, and corrosion. FPI allows inspectors to find cracks and gouges on the surface of welded, sheet metal and CNC-machined products that aren’t visible to the eye. By capillary action, the fluorescent dye finds its way into flawed areas on the metal’s surface to expose discontinuities.

The aerospace industry has developed three main types of dye penetrants that are suited for use in aerospace applications.

  • The first type is a colour-contrast penetrant (visible dye). The dye is applied all over the part and allowed the required time for the product to permeate into the part. The excess material is then cleaned, and a developer is applied. Since this penetrant is composed of an intense red dye, the inspection can be performed using a white or fluorescent light.
  • The second type, the often-preferred choice, is called fluorescent penetrant. Because it uses a fluorescent dye, it is far more sensitive to small imperfections.
  • A third type is known as a dual-mode or dual-sensitivity liquid penetrant. True to its name, this penetrant includes dyes that appear coloured under white light and under ultraviolet fluorescent light. The limitation is that these visible colours are not as intense as those generated by the single-mode visible and fluorescent liquid penetrant.
Bell Helicopter Swashplate

FPI Inspection Process (ASTM E1417, Nadcap Approved)

Because it is crucial that all critical flight components are faultless, the FPI inspection process must conform to industry standards. The inspection standards are defined in ASTM E1417, which has been approved by Nadcap (National Aerospace and Defense Contractors Accreditation Program). This FPI inspection standard addresses every possible aspect of the process, from personnel and equipment requirements, and every step in the preparation of parts for inspection

How to Read Defects Using Fluorescent Penetrant Inspection

Using FPI inspection to identify flaws, involves several steps:

  1. The surface of the parts shall be cleaned prior to the penetrant step. There can be no contaminants on the surface, such as oil, paint, or dirt as this will hinder the process’s effectiveness.
  2. The penetrant is applied to the surface and left to permeate into the defects. This can take up to 30 minutes.
  3. After the penetrant has had time to seep into the surface, which is called dwell time, the excess penetrant is removed from the surface. The way the penetrant is removed depends on the type of penetrant. Some penetrants can be washed off with water, while others are removed by a solvent. The key is not to remove too much of the penetrant.
  4. The next step is applying a contrast to the surface, called a developer. The developer may also have a dwell time to allow the background sufficient time to absorb into the metal for full contrast.
  5. The inspector examines all surfaces of the metal with UV lighting, which must be performed in a dark room. Areas in question are marked so that they can be identified and interrogated where possible. This stage of reading the inspection results and analyzing the identified defects, if any, is critical. Inspectors will then classify and interpret the type, size, and suitability for service of the part. Based on these results, the aerospace manufacturer will decide if the defect meets the specified requirements.
  6. Finally, the part must be cleaned for the next step in the process. The FPI dye can cause corrosion if it is not cleaned off properly. If the part is deemed defective, it must be documented and provided a disposition to rework or scrap the unit(s).

Again, after each fabrication, the FPI process may need to be repeated to test the quality of the part following the manufacturing process. A part may go through multiple testing before it is deemed ready to be used on an aircraft. Following the inspection, a substrate coating followed by a primer and top coat can be applied on the part.

Key Advantages and Disadvantages of FPI


  • With the FPI process, only one work process is necessary to find the errors on the surface.
  • FPI inspection can be used with non-magnetic materials and electrical insulators, as well as non-porous metals.
  • It’s also suitable for a high-volume production line, even though it can take time to get through the process. (Some levels of automation may be developed in the preparation of parts prior to inspection)
  • FPI is one of the more affordable processes for finding defects in products.


FPI inspection does have a few disadvantages, but they are avoidable by trained technicians.

  • If an object is not cleaned properly, the surface may show imperfections.
  • This method can only find surface defects.
  • The inspector must be trained on how to observe imperfections.
  • The penetrant does stain skin and clothes.
  • Some penetrants and cleaning materials are toxic and/or hazardous.

FPI Inspection: The Method of Choice for the Aerospace Industry

A complete set of special processes is required to support welding and manufacturing capabilities in the aerospace industry. Organizations in this field are expected to achieve precision and there is little to no tolerance for defects in products, parts, and processes. FPI inspection stands out as the non-destructive method of testing for welding, sheet metal and CNC machining fabrication for the aerospace industry. Affordable and precise, this method allows highly trained technicians to detect non-visible defects on critical aircraft components.

As an aerospace manufacturer, Avior is committed to delivering products that meet the highest industry standards using rigorous controls that include FPI inspection and UT inspection. Our welding, sheet metal and CNC machining fabrication lines are Nadcap certified and meet the specifications of many aircraft manufacturers including Boeing, Bombardier and BellFlight. Visit our “What We Do” section to learn more about our manufacturing and testing expertise.