Discontinuous testing procedures will be repeated in contrast to on-line-monitoring (chapter
25.1) in specified time periods or in a special activity (Fig. "Short time periodic non destructive testing"). To this belongs non destructive testing
(= NDT). Its application (chapter 18.104.22.168) 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:
Routine checks of
Special tests like ultrasonic testing, eddy current testing or pentrant inspection at rotor blades, grooves for blade roots in disks (Fig. "Short time periodic non destructive testing") 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.
Fig. "Short time periodic non destructive testing" (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.
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.
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.