Aircraft Engineering: NDT of Outer Starboard Main Plane

1. INTRODUCTION

The inspection process as specified by the United States of Defence for testing of airplane components is recommended. Initially a preliminary survey of the damage has to be done and then the appropriate inspection process has to be followed (USDF, 28 August 2001).

1. Preliminary survey of the damage: The component is inspected using techniques such as visual inspection; by touching and by using mirrors, magnifying glasses, borescopes, optical micrometers, camera, and depth gauges.
2. Selection of Inspection Process. Several factors are considered when choosing the suitable penetrant inspection process. These are as follows: required sensitivity of test, surface condition of test areas, part configuration, testing equipment whether part would be damaged by the dye.

Remaining steps for the process are explained in later sections.

1.1. About NDT

NDT – Non Destructive Testing is a type of testing products and components without damaging the parts. Some parts such as aircraft structural components, ship hull, even construction structures are very expensive and alterations or damages to the components during inspection are not tolerated. If a component has hidden cracks inside, discontinuities, voids, etc. then NDT is used to test the local area for such defects. The component is not damaged and the fool-proof result gives sufficient evidence about the nature of defect. The management can then take the required decision about the component (Bray, 1997). In this paper, the procedure for a repair report for NDT testing of an aircraft component is provided. The upper part of an outer starboard main plane section is found to be damaged and is suspected of having a hidden crack. The Dye Penetrant Test is used in this case to find the defect.

1.2. Dye Penetrant Test

The Dye Penetrant Test is extensively used to trace and find out the location and extent of porous, near surface defects of components. The methods can be applied to find surface breaking defects such as porosity, cracks, leaks and other microscopic defects that cannot be found out by a visual inspection. The method is used to find defects in non-ferrous components such as castings and forgings. In this method, a dye is applied on one side of the component after cleaning the surface. The component is then stored for some time in a dry place. The dye flows into the void or cracks and then appears on the surface. The dye colour is usually dark red, dark blue or other such contrasting colour and it is thus possible to clearly identify the defect (Shull, 2002). This method has been proposed for the outer starboard main plane section since the component cannot be cut and inspected for the defect. By using the dye penetrant test, the crack can be detected if present and then repaired. The process for carrying out the test is as given in the next section.

2. PROCEDURE FOR TESTING

The procedure for inspection and testing is as given below (USDF, 28 August 2001)

1. Pretesting. This is needed to ensure that the part will not be damaged by the inspection process, to establish emulsifier dwell times (if applicable), and to provide a reference sample. The following procedures are used for pretesting: Select the penetrant inspection process to be used. Select a part identical to the part to be placed under test. Perform penetrant inspection on the part selected in step 2 in the same manner in which all parts will be inspected. After each inspection process procedure, check the part for damage or adverse effects. If damage occurs, or adverse effects are noted, select an inspection process which will eliminate the problem
• Select the penetrant inspection process to be used.
• Select a part identical to the part to be placed under test.
• Perform penetrant inspection on the part selected in step 2 in the same manner in which all parts will be inspected.
• After each inspection process procedure, check the part for damage or adverse effects.
• If damage occurs, or adverse effects are noted, select an inspection process which will eliminate the problem.
2. Precleaning. The material is carefully cleaned first of water, carbon, engine varnish, dirt paint, oxide, plating, oil or other contaminants, otherwise the penetrant will not be able to get into the flaw and misleading indications may result.
3. Drying. This process is used to assure the evaporation of any water, solvents, or other cleaning solutions, which might be loaded in a crack.
4. Penetrant Application. Penetrant is applied to a part under test in a manner appropriate to the facilities available. Sufficient dwell time should be allowed for optimum penetration. The penetrant is allowed to remain on the surface for a sufficient time to allow penetrant to seep through the defects by capillary action.
5. Dwell (Draining). The part is set aside to drain after the part has been coated with the penetrant. The penetration (draining) time, in minutes, ranges from 5-240 minutes depending on the material of the part and the characteristic of the damage as well as the type of the penetrant (whether water-washable or post-emulsified penetrant). For a post-emulsified penetrant there is usually the additional step of applying the emulsifier after penetration time has elapsed.
6. Emulsifier application, draining and dwelling. This step is optional depending on the type of penetrant used.
7. Developer Application, Draining and Dwell Time. Apply developer to the part. There are several methods but spraying aqueous wet developer is preferred due to its high sensitivity. Wet developers are allowed to drain prior to drying. Developer is applied to the part under test as appropriate to the process being used and the configuration of the part. Developer dwell time will depend on the type of penetrant developer. Sufficient time is allowed for an indication to form, but the penetrant is not allowed to bleed into the developer in such quantities to cause a loss of definition. The part is ready for inspection under appropriate lighting to detect the presence, location, and size of crack.
8. Inspection. The part should be inspected and the crack interpreted. When the surface has been sufficiently developed it will then be examined to see cracks formation.
9. Removal Methods. The developer is removed after inspection by spraying with water or by washing.
10. Repair. The part should be repaired based on the size, depth and nature of the crack. If necessary, repaint the part before returning aircraft to service.

3. CONCLUSION

The paper has presented an overview of NDT and Dye Penetrant Test. The same test has been used on the outer starboard main plane section to detect a suspected crack. The structured procedure specified for the inspection was presented and discussed and the steps to be followed during the testing have been examined.