Cost-conscious operators are all-too aware that as an aircraft racks up flight hours and cycles its maintenance, costs rise. So keeping shop visits as cheap as possible might seem the natural approach to cost control.
However, a series of research projects into the economics of ageing aircraft carried out by maintenance, repair and overhaul (MRO) specialist Lufthansa Technik (LHT), have identified some areas where, depending on how far an operator's asset management strategy reaches into the future, it makes sense to spend more than immediately necessary to secure net savings further down the line.
The overarching aim of LHT's work has been to reduce total cost of ownership, but some of the initiatives aimed at reducing fuel consumption have turned up some surprising findings.
Working with university researchers, LHT has studied how wear and tear on individual engine parts affects specific fuel consumption. The team undertook test runs with engines, which had been deliberately configured with worn components, to determine what forms of deterioration have the most significant impact.
Some of the results have surprised the engineers themselves and have led to changes in the maintenance regime Lufthansa Group applies to its own fleet. One example is that restoring the optimum leading edge shape of a fan blade is much more relevant to fuel consumption than restoring lost chord width.
Fan blades typically lose aerodynamic efficiency because of erosion on the leading edge caused by particulate matter such as dust, sand, volcanic ash and foreign object debris. Repairs have long been established to restore chord width and re-contour leading edges.
However, LHT has now stopped restoring chord width for performance purposes. This will only be done to prevent premature scrapping - when a blade's chord width falls below an allowable minimum - or if additional material is needed to re-contour the leading edge.
Even though the fan creates about 70% of an engine's thrust, the front-end module has historically attracted comparatively little maintenance effort in LHT's workshops, says propulsion systems engineering director Thilo Seitz. Short of significant damage caused by foreign object debris, for example, regular maintenance work has typically been mostly limited to inspections for cracks, smoothing aerofoil surfaces and correcting excessive tip clearance.
Since 2011, however, LHT has started re-contouring the leading edges of CFM International CFM56-5C fan blades more regularly, which can cut specific fuel consumption by up to 0.25%. As the savings become tangible during cruise, the process makes most sense on long-haul aircraft. The process has been applied to Lufthansa's Airbus A340-300s, and is now being adopted for the carrier's General Electric CF6-powered Boeing 747-400 fleet.
Seitz says an optimally rounded leading edge profile lasts for about two years and LHT has developed an on-wing procedure to re-contour the aerofoils between two shop visits. While this is not as precise as the computer-controlled robotic process employed in the overhaul facility, Seitz says it creates enough savings to warrant the effort.
Another research project focuses on a super-polishing and protective coating for compressor blades, and is being flight tested on two CFM56-5C and two CF6-80C2 engines. The tests started three years ago, when two CFM56-5C engines were overhauled in parallel and re-assembled in the same manner with identical parts. The only difference was that one engine was equipped with a number of compressor blades with the novel surface treatment.
These two powerplants have yet to be overhauled, but technicians inspecting their cores found the surface-treated blades accumulated much less dirt than standard parts. Seitz adds that the leading edge shape and coating remained largely intact, while the profile of untreated blades in the lower compressor stages - where the strain is strongest on the aerofoil's forward section - had almost completely deteriorated.
The full effects have yet to be analysed, but Seitz is confident the surface treatment will create a net specific fuel consumption benefit and, if the coating proves to be durable, should reduce deterioration of the components. In turn, this could lead to lower repair costs and fewer prematurely scrapped engines.
"The aim is not to make the shop visit as inexpensive as possible," he says, "but [to find out] in what areas it makes sense to spend a bit more to create savings on the bottom line over the next four years."
Seitz adds that LHT has changed its hot section MRO philosophy because its CFM56-5C engines spend more time on-wing. While LHT repaired turbine blades in the past to exploit remaining runtime of the life-limited parts, it now installs new aerofoils at every overhaul event. This makes the shop visit more expensive, but the shorter residual life of used life-limited parts is no longer a limiting factor in the engine's on-wing time.
Seitz says that in spite of higher overhaul costs, the "premature" replacement of turbine blades still leads to total ownership cost reductions during the engine's life. The crucial question for operators, however, is whether they want to minimise an engine's full-life service cost - or merely the maintenance cost burden of the much shorter period of an operating lease.