Aircraft MRO

Nondestructive Testing

Dipesh Dhital

25 Jun 2018 — 4 min read

Nondestructive Testing (NDT) is the methodology of inspection, testing, and evaluation of materials/structures in order to identify any discontinuities or defects without destroying the usability of the component/structure.

Nondestructive Testing

In the aerospace industry, where most of the parts/components are highly expensive and low-volume, the use of ‘Destructive testing’ methods is not suitable to test a structure or component’s material and structural integrity of such parts.

NDT methods can be used during manufacturing, and fabrication as well as for in-service inspections to test material integrity, impact damages, cracks, fatigues, and so on.

Thus, non-destructive testing is one of the alternatives which can be employed either before the component/structure is put into use or as a part of the maintenance program.

NDT is now a very vital part to determine safety and quality assurance in the aerospace industry, mostly more useful for ‘rotable parts’.

Nondestructive Testing Techniques

There are various methods of non-destructive testing as listed below. However, all of these tests can be classified either as surface or sub-surface techniques. Surface techniques are used to identify surface defects such as cracks, whereas sub-surface techniques such as ultrasonic testing can be used to determine defects that are not on the surface.

Also, based on the testing methodology, these can either be contact or non-contact. In contact methods, the detection sensors are mostly placed on the structure.

Visual Inspections are the most common forms of NDT methods performed on an aircraft. Besides visual inspection, ultrasonic testing, magnetic particle testing, liquid penetrant testing, radiographic testing, and eddy current testing are the five other highly used NDT methods in the aerospace industry.

A few of the widely used nondestructive testing techniques are as follows:

Acoustic Emission Testing (AE),
Electromagnetic Testing (ET),
Guided Wave Testing (GW),
Ground Penetrating Radar (GPR),
Laser Testing Methods (LM),
Leak Testing (LT),
Magnetic Flux Leakage (MFL),
Microwave Testing,
Liquid Penetrant Testing (PT),
Magnetic Particle Testing (MT),
Neutron Radiographic Testing (NR),
Radiographic Testing (RT),
Thermal/Infrared Testing (IR),
Ultrasonic Testing (UT),
Vibration Analysis (VA) and
Visual Testing (VT)

NDT in Aerospace Industry

NDT techniques are used both at the manufacturing stages and after the components enter the service. Thus, NDT is equally important to both aircraft/part manufacturers and aircraft MRO service providers.

NDT for Aircraft / Part Manufacturers (OEMs)

NDT is used by OEMs (original equipment manufacturers) during manufacturing and fabrication. OEMs and their supply chain use NDT at various stages of manufacturing to determine structural integrity and to check that the components are void of any defects or damages before they even enter the service.

NDT for MRO Service Providers / Airlines

NDT methods became even more important after major aviation accidents such as the 1998 Aloha Airlines DC-10 aircraft cabin failure and 1996 McDonnell Douglas MD-88 engine failure, as they highlighted the necessity of such techniques to ensure quality assurance and safety inspections at regular intervals.

Such incidents brought to light that the aircraft undergoes numerous compression and decompression cycles every flight and all the material and components are under high stress, which further increases the chances of defects and component failure.

Thus, in such scenarios detection of damages at a later stage is simply not an acceptable solution as those need to be detected and repaired at an early stage.

Thus, for safety and quality assurance, NDT methods are now mandated in the regular maintenance program which needs to be carried out at certain intervals or as required, by qualified NDT personnel.

For an aircraft, the airframe account for almost 80% of the total NDT carried out, while engines and other components constitute the remaining 20%.

For airlines, NDT is also highly important from an operational point of view as an efficient NDT process will not only ensure safety but also enable a faster turnaround time. Aircraft-on-ground (AOG) are extremely expensive and thus a reliable and efficient NDT process can also help in cost savings.

NDT Personnel

The National ‘Aerospace Standard 410, Certification & Qualification of Nondestructive Test Personnel’ is the most widely recognized and accepted standard for NDT technicians in the US.

American Society for Nondestructive Testing also provides guidelines regarding such requirements and accepted certifications.

Basically, the NDT Technician certification is classified into four levels:

Trainee: Trained under the supervision of a level II or level III technician
Level I: After passing the initial test, the trainee can achieve the level I
Level II: After completing a certain amount of class work and mandated experience, level II is granted.
Level III: NDT technician with level III certification is allowed to carry out classroom instructions and is also considered highly skilled and capable of NDT supervision work.

However, in the US, FAA does not recognize NDT technicians as qualified personnel to return an aircraft to service. It can only be done by the maintenance manager or senior technician.