March 3, 2017, ©. Leeham Co: We will now go through how airline turbofans are maintained. First, we will describe the typical work which is performed, then look into the markets for engine maintenance.
In the markets for engine maintenance, we will look at who the players are, how they are related to the engine OEMs and why the market dynamics are very different between engines for single-aisle aircraft and wide-bodies.
The engine maintenance market calls the typical maintenance operations that it performs the workscopes. We will focus on the standard types which are triggered by either EGT margin or mechanical deterioration, together with Life Limited Parts (LLP) limits.
The typical workscopes on single-aisle engines is LLP limits since new is one or a maximum two performance restoration shop visits before the trigger for taking the engine off wing.
New engines have larger EGT limits than engines after a performance restoration. Dependent on the extent of the workscope, 60% to 80% of the original EGT margin is regained.
A less ambitious workscope for EGT margin restoration would be only a high pressure turbine restoration, with either exchange or repair of worn or deteriorated parts. Those parts could be turbine blades, seals, or stator parts.
A more ambitious workscope would include low pressure turbine jobs and include compressor section repairs. This can include exchanging blades, seals or restoring/adjusting variable guide vane operation. Typically, the first visit has a lighter workscope and any second visit a more elaborate content.
Costs for restoration visits divide between workscope man hours, parts costs and any repair costs for parts that can be repaired. Such repairs are often done by specialist shops which have the tools of, for example, restoration of coatings on turbine parts.
Typical times on wing for high rated variants of the engines would be around 7,000 to 10,000 cycles for historical engines, whereas the latest specifications could stay on wing another 50% cycles. It would depend on the Engine Flight Hour (EFH) to Engine Flight Cycle (EFC) ratio and the derate the operations would allow.
Lower rates engines, especially later specification ones, would stay on wing until the first LLP limit would be reached.
The CFM56-7 engine for the Boeing 737NG has high EGT margins from delivery and would typically stay on wing until the first LLP limit triggers a shop visit.
Typical costs for a performance restoration visit could be $300,000 for a light restoration of the -5 series, with up to the double for a more thorough visit.
LLP limits depend on the different LLP groups. A CFM56 engine has 18 different LLP groups, whereas the IAE V2500 engine has 25 groups. The V2500 has one LLP limit for all the groups, 20,000 flight cycles. The latest specification CFM56 has cold section limits (fan, booster) of 30,000 cycles, compressor limits of 25,000 and hot section limits of 20,000.
Older specification engines typically had lower limits for hot parts, like CFM56-A variants or -5B or -7 variants before “Tech Insertion” specification on components allowed limits to be raised.
A visit where LLP parts are exchanged would mean a more thorough dismantling of the engine, and the LLP stacks are expensive. A high pressure turbine LLP exchange costs easily $0.5m, the fan/booster parts $0.5m-$0.7m and the whole engine stack is priced north of $3m.
Operators divide the LLP stack prices and limits so they can see the LLP price per flight cycle for the engine. Typically the cost is around $150 per cycle.
Together with shop work-hour costs the visit for an LLP time restoration can be close to $3.5 to $4m.