CFM HPT MTU Maintenance

MTU’s HPT Nozzle Guide Vane Repair Could Have Wider Application

MTU Maintenance is considering offering a proprietary repair it developed for CFM56 engines to other powerplants.

MTU Maintenance is moving toward using its proprietary high-pressure turbine nozzle guide vane airfoil replacement repair for engines other than the CFM56-5B and CFM56-7, for which it was initially developed.

“We have had a lot of interest from customers for extending this repair to cover other engine types and parts, and we are driving this forward at the moment,” says Dr. Frank Seidel, head of repair engineering for the Hanover, Germany-based company.

Marketed as the “MTUPlus Airfoil Replacement Repair,” the company has offered the repair since 2013, according to Seidel. MTU is a licensee of CFM International, but he points out that the repair was developed by MTU’s industrial engineering department, independent of the engine manufacturer.

“We introduced this repair because we saw a need for a solution to the increasing damage to the guide vanes in operations,” he notes.

CFM56-5 and -7 HPT vanes, made of Rene N5, are assembled as twin vanes, explains Seidel. Due to their location within the engine, in combination with operating conditions, they are exposed to high thermal stress, and are prone to erosion and oxidation.

“The current coating of the vanes does not provide enough protection, and damage beyond standard repair limits can be observed,” he says. “Examples include long cracks in base material that may turn into airfoil cracking, burns, massive loss of material, and nicks and dents. This results in performance degradation and a loss in efficiency.”

The MTUPlus Airfoil Replacement repair for the HPT vanes brings the vanes back to serviceable condition and should prevent further damage with an optimized coating technology, incorporating a platinum aluminide coating bond coating, and an electron beam physical vapor deposition thermal barrier coating top coating applied on both suction and pressure sides. However, Seidel reports, new airfoil casting will be used to repair heavily damaged cluster airfoils—if necessary.

“The joining of the airfoils without additional brazing seams in the sensitive airfoil fillet area provides improved structural stability, as compared to standard repairs on the market,” he says. “At the same time, the repair produces zero scrap and increased durability of the parts for longer on-wing times, as well as reduced costs, by fully avoiding the costs of replacement parts.

Those benefits, he predicts, combined with the increasing CFM fleet size and support of aging engines, will likely generate growing demand for this repair.

In that regard, Seidel points out that the CFM56-5B and -7, combined, will be the major share of shop visits going forward. “Until four years ago, the CFM56-3, accounted for about one-third of our shop visits. That has gone down by half, and represented just 16% of our CFM56 shop visits last year, and we expect this share to diminish further within the next few years. The CFM56-B and -7 are the remainder. Specifically, the proportion of our -5B visits increased to about 38% last year vs. 46% for the CFM56-7.”

Seidel adds that the CFM56 product line--including the CFM56-2, -3, -5B and -7—accounts for approximately 200 repair and overhaul shop visits annually at the MTU Maintenance facilities in Hanover, Vancouver and Zhuhai in China.  “That’s about 20% of the our total yearly engine throughput,” he says.

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