Chapter 13. INSPECTION OF INTEGRAL FUEL TANKS FOR CORROSION
111. GENERAL. The existence of micro organisms in the fuel used in turbine-powered aircraft, results in failure of integral fuel tank coatings and corrosion of the wing skins.
Ultrasonic nondestructive testing provides a method of inspection for the detection and degree of corrosion in integral wing fuel tanks.
112. METHOD. Ultrasonic inspection techniques are used for determining and recording the condition of the interior surfaces of wet-wings. Longitudinal waves should be used since shear and plate waves will not accurately establish the definition or extent of corrosion pits.
113. EQUIPMENT. To scan the underneath surface of an aircraft wing, the following equipment is
a. A wheel search unit. To assure a satisfactory coupling, the surface to be scanned is
b. An oscilloscope with a fast pulse repetition This is necessary to conduct a rapid scan of 500 square inches or more in 15
d. An automatic and manually controlled scanning bridge and carriage.
e. A positioning mechanism, scanner support structure, and lift platform. This equipment places the sheel search unit in a position to scan the lower surface of the wing and provides the apparatus necessary to relate the motion of the search unit to the facsimile recorder. This equipment also permits a rapid engagement of the scanner with the aircraft and requires no jacking or other handling of the aircraft, nor is it necessary to drain the
114. INDICATIONS. Intergranular corrosion appears on the C-scan recording as a solid area projecting outward from fastener holes or other locations where there is a transverse cut exposing the edge of the plate. Large or small areas of pit type corrosion may appear anywhere on the recording with high-density areas giving a mottled appearance to the recording. Thus, the extent and the kind of corrosion can be determined from the recording.
c. A C-scan facsimile recorder. 115.-120. RESERVED.
Chapter 14. INSPECTION OF REPRESENTATIVE AIRCRAFT PARTS
121. GENERAL. A few representative inspections by the contact method are discussed.
122. MAIN AND NOSE LANDING GEAR WHEELS.
Figure 72 illustrates the part to be examined. A surface-wave probe is used to scan around the wheel web for cracks occurring adjacent to the bosses of the tie-bolt holes. The surface wave is able to detect cracks which are not always shown up by other methods of examination. The frequency used is 2.5 MHz and the depth of penetration is approximately 0.060".
For cracks occurring in the zone of the tire areas, a second probe, having an angle of 30° refraction, is sometimes employed for testing the bead seat radius where the test is carried out without disassembly of the wheel. The angle is necessary to direct the beam away from any reflecting surfaces, such as changes in contour, into the zone where defects occur.
123. MAIN LANDING GEAR TORSION LINK. A
2.5 MHz surface-wave probe is applied to the member on its surface with the beam directed towards suspected zones. It is normal to scan most of the surface of the torsion link for fatigue cracks in any position occurring from the highly stressed areas. (See Figures 72 and 73).
CLEAN AND INSPECT THIS AREA
SECTION OF WHEEL FIGURE 73.-Main Landing Gear Torsion Link
SURFACE
PROBE / WAVE
124. MAIN LANDING GEAR TORSION LINK LUGS. A surface-wave probe is used on the lug surfaces and on the thickened boss section for fa- CLEAN AND INSPECT tigue cracks occurring in random directions. It is necessary to direct the ultrasonic sound beam
THIS AREA towards the crack at an angle of 90° in order to
CLEAN AND INSPECT
THIS AREA
• PROBE
CLEAN AND INSPECT
THIS AREA FIGURE 72.-Main and Nose Landing Gear Wheels FIGURE 74.-Main Landing Gear Torsion Link Lugs 41
obtain the optimum reflection. This means that careful scanning in all directions and in all positions around the bosses and lugs is necessary to ensure complete coverage of the suspected area. (See Figure 74.)
125. MAIN LANDING GEAR OLEO OUTER CYL- INDER. Figure 75 indicates the zone in which high stresses produce fatigue cracks. The whole of the section concerned needs to be scanned, using a 2.5 MHz surface-wave probe. Scanning is undertaken from the cylinder towards the fork ends, where the surface wave will travel around the changes in contour to locate defects in the zone indicated. Careful scanning is needed around the whole of the cylinder to find defects in any position around the base of the fork.
taking this examination, the area needs to be cleaned and the grease removed from around the bearings. This can be done without removing any parts from the aircraft. Because left- and righthanded parts are to be examined, the probe is made in such a way that it can be located by either of two "wings" to suit the position of the part to be examined.
CRYSTAL
POSITIONING
FIXTURE INSPECTION AREA (SHADED)
INSPECT FOR CRACKS
IN SHADED AREAS
BEARING FIGURE 76.-Main Landing Gear Trunnion Support Structure
6.00 -PROBE FIGURE 75.-Main Landing Gear Oleo Outer Cylinder
126. MAIN LANDING GEAR TRUNNION SUP- PORT STRUCTURE. This examination is particularly difficult due to the geometry of the section to be inspected and due to its location on the aircraft. Very little space is available to apply the probe to the surface of the trunnion support.
Figure 76 indicates the areas to be inspected but does not give a true picture of the complexity of this examination.
The probe is a complex arrangement producing a beam at an angle of 30° to the surface and having locating "wings" which align the probe in the correct position by insertion of the wing between the support structure and trunnion. Before under- TEST ZONE
SCANNING CARRIED
OUT IN TWO DIRECTIONS #-PROBE FIGURE 77.-Nose Landing Gear Outer Cylinder
127. NOSE LANDING GEAR OUTER CYLINDER.
A 45° transverse-wave probe, 2.5 MHz frequency, is used to scan the upper and lower sections of this cylinder in the areas shown in Figure 77. The 42
scanning is undertaken in two directions for the detection of cracks which normally occur on the inner surface but which can start on the outer surface. The two additional diagrams on the sketch show the method scanning.
128. INBOARD AND OUTBOARD NACELLE STRUT FRONT SPAR FITTINGS. Most of the examination of these components is undertaken on the curved sections of the lugs. The angle of the beam is approximately 20° from the tangent of the circle.
To obtain this angle, the probe must have an incident angle of approximately 9°. Inspection is undertaken around the whole of the curved surface which gives more or less complete coverage of the bolt hole by moving a probe in two directions. In order to direct the beam towards any position of the hole in the lug, the probe has to be moved across the lug section from side to side while being rotated around the curved surface. This leaves a PROBES FIGURE 78.-Inboard and Outboard Nacelle Strut Front Spar Fitting small section of the hole at the back of the lug which is not inspected. This is undertaken by a probe having a smaller angle and a slight taper on the lug surface. Both upper and lower parts of the lug have to be inspected by this method so that the entire bolt-hole section is examined. To facilitate examination from the curved surface, the probe surface is also curved. On some aircraft, this radius differs so that two probes would normally be necessary. Practice has indicated that a probe having a larger radius can be used on lugs of the smaller radius. (See Figure 78.)
129. NOSE LANDING GEAR OUTER CYLINDER Figure 79 shows the area of test undertaken using surface-wave probe with a frequency of 2.5 MHz.
The probe is applied to the curved surface of the cylinder and the beam directed towards the lug.
The sound wave follows the surface of the material and penetrates the flat surface of the lug for detection of defects from the corner of the sections into the lug zones marked on the sketch.
TEST AREA PROBE FIGURE 79.-Nose Landing Gear Outer Cylinder 130-135. RESERVED.
43 * U. S. GOVERNMENT PRINTING OFFICE: 1975 O - 569- 931