Super coating for superior blades

Innovation on CFM56 turbofan
Process with significant improve

New engine repair technology: Lufthansa Technik is permanently developing innovative repair techniques and modifications for the CFM56 turbofan. A recent example is the new "Surface Finish Improvement Coating" (SFIC) for super polished blades in the engine's high-pressure compressor, a process projected to significantly improve the engine's efficiency and fuel burn alongside with reduced maintenance costs.

Improving the surface of compressor blades reduces the inner friction of the engine. Accordingly the idea of super polishing blades is not new, but the possibilities are limited. Providing compressor blades with a super polished surface never lasted long as the surface quickly became eroded from contaminated air flowing through the engine. Thus the benefit of the polishing was soon wearing off, producing detrimental effects such as reduced compressor efficiency and higher fuel burn. 

The right combination

Lufthansa Technik engineers have now developed a special coating to conserve the reduced surface roughness. A challenging task, because coatings available on the market offer an even higher surface roughness than the polished blades. However, engineers kept looking for new solutions that leave the improved surface properties of the blades unaffected. In addition, the coating must have the same maintenance requirements as the engine itself, i.e. at least withstand the same amount of flight hours as the compressor blades themselves.

Finally a combination of polishing and coating combining the advantages of both approaches was found in the newly developed "Surface Finish Improvement Coating" (SFIC). Lufthansa Technik has meanwhile prepared operational tests with the CFM56-5C engine. The high pressure compressor blades of two engines were modified accordingly. The multilayer system features excellent hardness but also provides the flexibility to withstand vibrations, preventing the coating from separating from the airfoil. And due to its low thickness of only a tenth of a regular anti-erosion coating, the smoothness of the polished surface underneath is nearly completely reproduced by the coating. 

Successful certification

The new coating process was developed in cooperation with a UK-based specialist, where the application process has also taken place. Together Lufthansa Technik and the British partner company performed a comprehensive series of tests. Analyses of the oscillation characteristics for example showed no substantial indication of any disadvantages of the coated blades compared with the standard models. Hardness testing and a metallurgic survey also confirmed the calculated properties of the coating, leading the way to its certification which was granted by EASA.

The successful certification was one of the most important milestones in order to begin with tests under real-life conditions. Preparations for the on-wing test program started when two engines underwent scheduled overhauls at Lufthansa Technik's CFM56 shop in Hamburg. During reassembly each compressor stage was nearly completely stocked with super polished and coated blades. Just four airfoils in every stage were left in their original condition for comparability reasons. In June the first test engine was ready for installation on a Lufthansa Airbus A340-300 widebody jet, with the first flight with this modified engine taking place a month later.

To ensure perfect comparability with the standard engines during the test flights, the A340-300 was chosen. It is providing the best possible proof of fuel consumption reduction during cruise because of its long operation range. The standard integrated measuring equipment of the aircraft is used to monitor the exact fuel burn for each engine during cruise. Based on this data, a monthly report will be issued to the engineers to outline any differences in terms of fuel burn or operating temperatures. The advantage in fuel burn is expected to increase with the number of flight hours, with the full potential of the coated blades becoming visible when their uncoated counterparts begin to wear off because of erosion. 

Significant gains expected

The overall potential to reduce an engine's fuel burn is estimated by Lufthansa Technik's experts by 0.3 percent or even higher. A CFM56-5C engine consumes around 2000 litres of fuel per hour during cruise, thus even such a small reduction would equal an overall saving of nearly 120,000 litres for the aircraft each year in a typical airline operation. A reduction of this magnitude would also significantly reduce the carbon footprint of one aircraft by up to 290 tons per year.

Another expected benefit of the new coating process is a significant reduction of maintenance costs through a nearly complete elimination of airfoil erosion. The airfoil geometry is expected to stay true for a significantly longer amount of time, resulting in a slower decrease of the compressor's efficiency throughout an engine's life cycle. If the flight test engines perform as calculated, the airfoil coating technology offers excellent potential to increase an engine's life expectancy and lengthen the period between shop visits.

The complete test program is scheduled over the next three to four years. In one or two years time however, when an engine's efficiency is usually starting to decay, the modified engines are expected to show their potential. And the difference will increase even further with every year. If the results of the test program turn out to be positive, the "Surface Finish Improvement Coating" will be offered to all customers of Lufthansa Technik's CFM56 services and will be available for all subtypes of the engine. Lufthansa Technik is confident about the success of the test. The development outlined above shows that Lufthansa Technik not only provides its CFM56 customers with best-in-class MRO solutions, but furthermore with continuous improvement of its most important engine type. With innovations like the "Surface Finish Improvement Coating" the CFM56 at Lufthansa Technik will continue to be a success story.