SPAD repair saves combustion case
Repair on CFM56-5A turbofan
Lufthansa Technik's Engine Parts and Accessories Repair (EPAR) business unit has produced a master-stroke in the form of a repair for the CFM56-5A combustion case. Developed in close cooperation with the manufacturer, the new procedure means that this extremely expensive part can now be repaired economically.
The combustion case of a turbofan is exposed to extreme stress during operation: the CFM56-5A has to endure cyclical pressures of up to 24 bar and temperatures of several hundred degrees. These conditions mean that eventually – after between 20,000 and 25,000 cycles – cracking can occur in the forward section. Due to the size and position of these defects, a weld-repair was not permitted in the past.
Efficient repair process
However, the combustion case is an extremely expensive component. To avoid having to scrap it, Lufthansa Technik therefore joined forces with the manufacturer, GE Aircraft Engines, and developed a SPAD repair (spare part alternate detail). The complex and time-consuming repair has now been officially approved, so that customers now have the possibility of a new, highly efficient repair from Engine Parts and Accessories Repair.
The repair entails replacement of the forward section and the outlet guide vanes (OGV). The first step of the process is to remove the damaged sections. The remaining, functional aft section is now prepared for welding with the new OGV ring and forward section SPAD. The SPADs, which are supplied as blanks, are then brought to the required dimensions using a lathe. All the parts are carefully measured and tested, using x-ray and fluorescent penetrant inspection (FPI), amongst other technologies.
Electron beam welding (EBW) is used to join the two SPADs to the aft section. It was here that the full expertise of Lufthansa Technik came into its own. Although this technique is theoretically capable of minimizing the weld distortion, this depends critically on optimum preparation of the welding seam and a suitable welding jig which guarantees the correct geometry of the part after welding.
The OEM's confidence in Lufthansa Technik was vindicated when the precision of the welding operation was demonstrated with a high degree of reproducibility. The joining operation is then followed by a series of mechanical operations, in the course of which all the functional surfaces, drilled holes and threads are precisely introduced and then deburred.
An extensive test program is then carried out on the combustion case, at the end of which the repaired part is ready to be refitted. To all intents and purposes it is a new part which costs much less than the normal cost of a replacement part. In this way the repair described is an extremely cost-effective solution, despite all the work that goes into it.