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25.2.2 Discontinuous testing procedures during maintenance

Discontinuous testing procedures will be repeated in contrast to on-line-monitoring (chapter 25.1) in specified time periods or in a special activity (Ill. 25.2.2-2). To this belongs non destructive testing (= NDT). Its application (chapter 25.2.2.2) is also discussed at an other place (e.g., production of new parts, volume 4, chapter17.3.1) of these book series. At this will be pointed in the following summary (Ill. 25.2.2-1) of applications. NDT procedures are also used during special activities at the mounted aeroengine („on wing“). Discontinuous tests serve primarily two targets:
Monitoring:

Routine checks of

  • Filters and sieves: Oil, ILL: 22.3.3.1-2; fuel, Ill. 22.2.2-2.1.
  • Oil analysis: Ill. 22.3.4-3 and Ill. 22.3.4-4.
  • Magnetic plug control and chip evaluation: Ill. 22.3.4-7 up to Ill. 22.3.4-10.
  • Condition determination of components like bladings of compressor and turbine. Static partsin the hot gas stream: Gas ducts of the casings and combusion chamber with the borescope(Ill. 25.2.2.1-4 up to Ill. 25.2.2.1-9).

Special checks/tests:

  • Monitoring of acute deteriorations like LCF crack formation: Thermal fatigue at turbineguide vanes (volume 3, Ill. 12.6.2.4 and Ill. 12.6.2-9) or of the combustion chamber (volume 3, Ill. 11.2.2.2-1). Thermal abrasion of oxidation protection coatings (volume 3, Ill. 12.4–13).
    Typical procedures/processes are:
    • with the borescope (Ill. 25.2.2.1-4).
    • NDT procedures (volume 4, chapter 17.3.1) with liquid penetrant inspection,
    • Ultrasonic inspection (Ill. 23.3.1.2-8) or eddy current inspection.
    • Application of magnetic sondes to control hollow turbine blades for inner sulfidation(Ill. 25.2.2-1 and volume 1, Ill. 5.4.5.2-2.1).
  • X-ray for the identification of hot part deformations (Ill. 25.2.2.2-4) and crack formation/fractures (Ill. 25.2.2.2-5).
  • Thermography for the identification of delamination of coatings (volume 4, Ill. -16.2.2.8-6)

Special tests like ultrasonic testing, eddy current testing or pentrant inspection at rotor blades, grooves for blade roots in disks (Ill. 25.2.2-2) or in a disk bore. The accessibility can demand the temporary disassembly of a component (e.g., fanblade).


Ill. 25.2.2-1 (Lit. 25.2.2-1): In this summary are tests/processes during maintenance work at mounted/on wing aeroengines. Cross references refer to applications and examples.

Ill. 25.2.2-2 (Lit. 25.2.2-2 up to Lit. 25.2.2-5): At the left aeroengine of this business jet (sketch above), a failure with fragment exit occurred. Thereby the region of the fan was separated. However, it could be recovered at the groud with help of radar data from the flight control(!). From the responsible authority (NTSB) an investigation was carried out at the failure components to find the cause. This showed:
A fraction of the rim from the fan disk has been broken out with five blades (sketch middle left). The fan disk had in the failure moment about. 5 200 operation hours with 3300 start-stop cycles. The laboratory investigation showed, that the crack started from an already existing fatigue crack with critical length. It was located at the edge of a blade groove (detail below) at the rear side of the disk. Crack inspections with penetrant and eddy current showed in the neighboring grooves also small fatigue cracks. Flaws (material faults, scores/notches), which would explain the developing of the cracks did not exist. The material data correlated the specifications.

History: Already about five years before, a local failure occurred. Also here a fatigue crack in comparable length existed. In this case the disk had 9 000 cycles. After this the a approved lifetime was reduced by the OEM from 10 000 cycles to 4 100. Disks with a higher lifetime have been tested for cracks. Further five have been found with cracks between 4 000 and 6 300 cycles. After this, the OEM extended for eddy current tested disks without cracks with an additional shot peening the lifetime for further 3 000 cycles.
A year after the current failure, the OEM demanded in a service bulletin the exchange of all 56 disks of the same heat treatment lot. These have been investigated. They showed no cracks or material abnormalities. Also between the five cracked disks, there was no commonality. Because the lifetime till the fracture in the acute case was only 3 300 cycles, the flight safety authority urged a markedly shorter inspection interval. At this the OEM reacted with further airworthiness directives. These demanded very detailed measures for certain disks, which are identified with the serial numbers.

Comment: The cracks must be due to LCF (volume 3, Ill. 12.6.1-6). The crack propagation of this fatigue type, in the case at hand, can be related the start-stop cycles. With this, a life time monitoring with sufficient short time periods and sufficient sensitive tests (penetrant inspection, eddy current inspection) seems reasonable.
The check of the disks from the heat treatment lot from the failed disk can not be explained by the investigation result. So it must have been a suspicion. This does not arise from the available papers. Thinkable is, that the detailled informations in later instructions are connected with further findings.

References

25.2.2-1 A.U.Khan, „Non-destructive Testing Applications in Commercial Aircraft Maintenance”, NDT.net, June 1999, Vol 4 No.6, www.ndt.net, Page 1-9.

25.2.2-2 I.Goold, „Fan-discs cracks discovery“, Zeitschrift „Flight International”, 31 July - 6 August, 1991, Page 20.

25.2.2-3 J.L.Kolstad, National Transportation Safety Board (NTSB), „Safety Recommendation, in reply refer to A-91-25“, March 22, 1991, Page 1 and 2.

25.2.2-4 Civil Aviation Safety Authority, Airworthiness Directive AD/TFE 731/32, „Allied Signal (Garrett/AiResearch) Turbine Engines - TFE 731 Series, Fan Rotor Disc”, 4/2002, Page 1 and 2.

25.2.2-5 AlliedSignal Inc. Airworthiness Directive „Amendment 39-9512, Docket 95 - ANE-54“, 96-04-01, www.tc.gc.ca, Page 1-3.

© 2021 ITTM & Axel Rossmann
25/252/2522/2522.txt · Last modified: 2021/03/16 22:06 (external edit)

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