Advanced Recontouring Process (ARP) saves material and fuel costs

Longer Service Life for Engine Compressor Blades
A procedure that is one-of-a-kind worldwide

The compressor blades in modern jet engines are exposed to continuous wear. Dust particles entering with the airflow cause heavy erosion at the leading edges. The deformation of the compressor blades, in particular, results in enormous performance losses. In the past, these expensive components were ground by hand and could be used only for another two rotations, before they had to be scrapped. In 1994, in a collaborative effort, Lufthansa Technik AG and the University of Aachen developed a procedure that is one-of-a-kind worldwide, in that the worn compressor blades are first electronically analyzed and then re-contoured in a precision method using robot technology. This increases the service life of the components from previously three, now to four rotations. Moreover, the improved profile also reduces fuel consumption.  

The Advanced Recontouring Process (ARP) initially has been offered for the CF6-50 and CF6-80C2 engine models that are used mainly in the Boeing B747-200 and B747-400 aircraft types. Now the offer has been extended to CFM56-All and V2500 engines. For performance reasons, engine manufacturer General Electric recommends on the CF6-50 engine that the chord-length losses on each blade, compared with the new component, should not exceed three percent. Using ARP, however, it is possible to extend this limit to eight percent without deterioration of engine characteristics – consequently the service life is improved considerably. Given these data, Lufthansa Technik has adopted it as the standard procedure in engine overhaul.  

Calculating of the best possible aerodynamic profile

The restored profile of the engine compressor blades is not identical to the profile in the new part. Rather, it is calculated as a factor of the reduced chord-length of the worn blades so that the best possible aerodynamic profile is obtained. A computer-controlled inspection system initially inspects the dismantled engine blades: depending on how badly worn the parts are, they are assigned to one of three clusters that are stored for each compressor stage. Then, using an ink-jet printer, a code corresponding to the results of the measurements and weight is printed on each blade.

A grinding robot then applies an optimized profile to the components. In contrast with traditional manual grinding of the engine blades the quality obtained by robot grinding is reproducible again and again and only the necessary minimum material is removed. The chord-length loss is in the area of 0.2 millimeters. The procedure is used to process the first 30 percent of the blade length. Different grinding wheels are used, depending on the engine blade material, which can be titanium or steel. 

Reduced fuel consumption

performance test was used to demonstrate the benefits of the Advanced Recontouring Process: in two series of tests an engine was first refurbished using engine blades that were manually rounded off and then with blades optimized using ARP; in both instances there was approximately eight percent chord-length loss. Analysis of the data shows that in the ARP-ed blades the manufacturer-recommended exhaust gas temperature (EGT) margin was increased by three to four degrees Celsius. By comparison: for every 1,000 flight hours this value deteriorates by approximately three degrees Celsius. Consequently, the ARP-serviced engines can remain on-wing more than 1,000 hours longer, which considerably reduces the costs per flight hour. Along with the improved EGT values, fuel consumption is also reduced. Taken together with the increased compressor performance and the longer service life of the engine blades, significant cost reductions are achieved: The Advanced Recontouring Process saves the Lufthansa fleet more than 0.6 million Euros per year.