Engine overhauls:
the search for material damage

Penetrating oil uncovers all

Once cleaned, about 95 percent of all engine parts proceed to fluorescent penetrant inspection (FPI), a technique for examining external cracks and damage. The remaining five percent are suitable for magnetic particle inspection (MPI) due to the ferromagnetic properties of their material. MPI reveals not only damage on the surface but also abnormalities concealed just below the surface. Both techniques are classed as methods of non-destructive material testing.

Before fluorescent penetrant inspection can be undertaken, the components to be tested must be absolutely dry. They are therefore dried in the forced-air oven at a temperature of 120 degrees Celsius before being introduced to a vacuum furnace with a negative pressure of 20 millibars. This ensures that even the smallest cracks remain free of moisture.

During testing with FPI, electrostatic attraction is used to spray a penetrant, that is, a penetrating oil containing fluorescent paint pigments, onto the parts. Capillary action causes small particles to be deposited in the invisible surface cracks. They can then be detected under ultraviolet light from their poison green shimmer.

The penetrant permits four levels of damage to be distinguished, depending on the extent of fluorescence. Components with a coarser surface require a less sensitive penetrant as otherwise any cracks would be difficult to make out against the coarseness of the surface. Only penetrating oil from the same batch is employed during crack inspection at Lufthansa Technik Engine Services.

After the oil has been in contact with the parts for 30 minutes, it is carefully removed. The first step is to clean the parts with a waterjet at a temperature of between 20 and 32 degrees Celsius and a maximum pressure of 2.8 bar. Next comes immersion in an emulsifying agent for exactly 90 seconds, followed by a water bath. To ensure that all the visible remains of the penetrant on the surface are removed, the engine parts are rinsed under ultraviolet light with a shower head. After being dried at 60 degrees Celsius, a coat of developer, a synthetically created powder, is sprayed on under electrostatic attraction. This draws the remains of the penetrant out of the tiniest cracks. Under ultraviolet light, an experienced technician then inspects the component centimeter by centimeter. Any concealed cracks now reveal themselves in the form of hairline green dots and lines: the paint pigments of the penetrant in the powder.

To proceed cleanly, all the equipment has to be set with the utmost precision. The first job carried out in the morning is therefore to test the ultraviolet lamps to ensure that they are working properly and that their light composition is correct: their white light intensity must not exceed 20 lux, while their black light intensity must be at least 10 watts per square meter. The system is tested with a test block which has artificial defects and surfaces of different degrees of coarseness. 

Fine powdered iron with paint pigments for magnetizable components

The steel parts with ferromagnetic characteristics can be examined for abnormalities with magnetic particle inspection (MPI). This entails placing the component under tension between two contact points in a test bench and rinsing it with a non-reflective magnetization oil, a suspension.

The "oil" contains very fine iron powder with paint pigments, which begin to light up under ultraviolet light. After the component has been well moistened, it is exposed for between three and five seconds to an electrical current between 100 and 8,000 amperes. The strength at which the regulator is set depends on the size and material strength of the component. This is specified in the relevant test instruction.

The current, which now flows through the part in a longitudinal direction, induces a magnetic field around the test item with field lines which run perpendicular to the direction of the current. These in turn force the iron particles into position.

As soon as the smallest change on the surface or just underneath occurs, a stray field forms above the defect, around which the iron filings eagerly gather. Such clusters are rendered visible by the ultraviolet light which is then used to inspect the component centimeter by centimeter. If any abnormality in the longitudinal direction is detected, it is properly documented.

Applying the same principle, the team now turns to the detection of abnormalities in the transverse direction. After the component has been demagnetized from the first test, another magnetic field is now introduced into it, this time using a coil or yoke. The coil is manually run along the test object, the iron filings align themselves once again and under ultraviolet light any abnormalities in the transverse direction can now be detected.

Additional training for cleaning and crack inspection

The cleaning and crack inspection work carried out in the engine shop is performed by specialist technicians (level 1 and 2). Level 3 is the NDT supervisor, whose tasks include the release of test instructions, qualifying staff and supervising the various stages of work.

Before being allowed to do any cleaning and crack inspections, staff have to complete training as a materials technician or chemist. They receive basic courses on engines and engine types, regulations and materials science. Like Airbus's similar facility in Bremen, the training workshop has to be approved by the National Aerospace NDT Board Germany. After about five days of theoretical and practical instruction, the course participants are tested and receive their qualification and license for non-destructive testing in accordance with DIN EN 4179, which applies to the aerospace sector.

The "trained eye" plays an important role in the test procedure. It takes quite a time for the operators to obtain the level of proficiency required to conduct FPI and MPI tests under their own responsibility, given the thousands of different components encountered in the engine shop: a good technician takes a whole year to acquire the skills involved in FPI, following which several years' further experience are needed to become competent to test all the components. Every year the staff have an eye test, and after a long absence of several months they are given refresher training. The legal requirement is that their competence should be tested every three to five years.