Smart solution for ISS repair
High-Detail-Repair for CFM56
Problem of abrasion
With a new repair method for the Inner Stationary Seal (ISS) of the CFM56 turbofans, Lufthansa Technik has once again managed to combine a substantial cost reduction with the potential for increased service life.
The Inner Stationary Seal of the CFM56-5A and CFM56-5B/5C/7B engines is a labyrinth gas seal built up as a combination of knife-edges on the rotating part and a honeycomb structure as counterpart in the stationary structure. With the abrasion caused by the knife-edges in the honeycomb material during thousands of hours of operations, the honeycomb structure must be renewed when the engine comes into the shop for an overhaul. As a result of the required turning and grinding operations, the base material is also worn down – often beyond the specified tolerance.
In the past this meant a scrapped part and the necessity to order a new part with a price tag of about 50,000 dollars. No longer: Lufthansa Technik has developed a repair method that is a very economical alternative. In addition, the EASA/FAA DER-approved repair method provides the potential for an increased service life making smart use of the potential of the component design.
ISS repair process
The stationary part of the ISS consists of two sections, the mounting flange and the honeycomb base. The latter is cut off as the first step of the repair. Then a pre-turned ring of forged Incoloy 909 is welded to the flange using Lufthansa Technik's Electron Beam Welding (EBW) facility, with older part numbers of the CFM56-5A using the material Incoloy 907 being modified as part of this repair. During the following turning operation the final dimensions of the honeycomb base are manufactured. Making use of the available tolerance, the inside diameter is made slightly smaller, leaving more material for future honeycomb removals. This in turn means a longer life for the part, as more removal operations can be performed within the specified tolerance. As a final step, the new honeycomb material is brazed into the ring. Lufthansa Technik has detected further savings potential to be realized by a repair of the heat shield, which is currently under development.
In addition to engine overhauls, Lufthansa Technik is also preparing to offer the efficient and safe repair to component customers.