Bjorn’s Corner: Aircraft engine maintenance, Part 3


By Bjorn Fehrm

March 17, 2017, ©. Leeham Co: In the last Corner,  we showed graphs of the yearly flight hours for engines on single-aisle aircraft. Now we will deduce the market for engine overhauls from these graphs.

These will show which engines generate a maintenance volume that is interesting for engine overhaul companies and which engines are niche.

Figure 1. Principal picture of a direct drive turbofan. Source: GasTurb.

Based on the market size, we will then go through how an engine is maintained when new, mature and at end-of-life.

Removal rates of single aisle engines

To understand the engine maintenance market, we need to deduce the number of shop visits per year from the different engines we graphed last week. The graphs were in Engine Flight Hours (EFH). To get to Engine Flight Cycles (EFC, which is the primary driver for engine overhaul for single-aisle engines), we establish the ratio Engine Flight Hours to Engine Flight Cycles.

EFH to EFC ratios vary from below one to up to four for single-aisle engines. Our scope is not to conduct an accurate engine maintenance survey but to get a feel for the maintenance market. We can therefore assume an average flight hour to flight cycle ratio of 1.5 without doing a large fault.

We can also assume that engine families with only mature engines in the market are now in a phase were they typically stay on wing for 7,500-10,000 cycles (time on wing decrease for later shop visits). The cycles on wing within a family will vary, with the highest-rated engine reaching EGT limits quicker. Low-rated engines will stay on wing until the first LLP limit will force the removal for first shop visit.

The time on wing is also dependent on the environment. Hot and dusty areas (like North Africa, Middle East) will deteriorate the engine’s compressor parts. This will reduce the flight cycles before a removal or change to a lower rated use on a smaller aircraft variant

Shop visits

With the described assumptions, the flight hour tables of last week translates to the shop visits per year table in Figure 2.

Figure 2. Engine shop visits for different engine families. Source: Leeham Co. Click to see better.

I have clustered engine families as maintenance shops certify to handle a whole family with its variants. There is a physical difference in the CFM56-5 and -7 engines, but the technologies used for the engines are the same.

Shop structure

It’s obvious there is a large difference between the CFM56/V2500 engine overhaul markets and the rest.

If we assume that an engine maintenance company would like to have at least 50 overhauls of an engine family per year, with an absolute minimum of 10-20, there can’t be too many shops competing for jobs for the PW2000 and RB211-535 engines. The PW6000 is only maintained by MTU in Hannover. The BR715 family is present on Business jets, which improves the situation.

For the CFM56 and V2500, there exists a real engine maintenance market. Competing for overhaul work are engine OEM shops (pure OEM or Joint Ventures with airlines/industrial partners) and independent shops.

Independent shops divides in airline-affiliated shops (like Delta TechOps, United Services,  Air France Industries/KLM Engineering & Maintenance, Lufthansa Technik..) and technology companies (like MTU Germany/China, BEDEK Israel, GA Telesis…).

Overhaul strategy

The  choice of the overhaul strategy is dependent on the age of the engine family and the individual engine. When an engine family is new, most jobs will be around OEM guarantee cases. There are also few shop visits, as the engine is in its honeymoon phase. There are no scheduled shop visits until past 10,000 flight cycles (and then only for highest rated variants).

With typical 1,500 flight cycles per year for a single-aisle engine, there will be 10 years or more before the an overhaul market is created for engines like CFM LEAP and Pratt & Whitney GTF. During this phase, the OEM is the maintenance partner and power-by-the-hour maintenance agreements are norm.

As an engine family matures, the engines have typically passed the first shop visit and time on wing is shorter. The engines are now in the main overhaul cycle, with shop visits planned to coincide with LLP limits for major modules. Duration of shop visits is an important factor; this places the jobs with engine shops that are geographically close.

When engines passed the mature phase and are in their last years, either as a family or as individuals, the maintenance strategy changes. It’s time to match the component life of the LLPs to the engine’s remaining life. Spare parts and complete engines are used from part-out aircraft.

Workscopes are planned with used parts so the engines different parts have a matched life until next visit or scrap time. As used engines are available on the market, these can be swapped for the engine going off wing. The removed engine might be restored to go into the replacement cycle or dismantled for spares.

9 Comments on “Bjorn’s Corner: Aircraft engine maintenance, Part 3

  1. Hi Bjorn. I find your series on aircraft engines fascinating but also “scary” when thinking of the times I flew out of short strips in the Andean at around 4000m altitude in 40 year old 727-100’s (sometimes more chickens than pax in the plane).

    Wanted to ask you a question/s for a long time but could never find the right platform. These are “theoretical”.

    Firstly, not sure if this is technical possible? The MTOW of the 350-900 with XWB84’s looks like around 280T.

    Firstly what will the field performance of a 350-1000 fitted with XWB84’s be taking of at 280 Tons,

    What will the fuel consumption be on say 4000Nm sectors with the XWB84’s compared to when fitted with XWB97 taking off at 280T and,

    What will the maintenance cost/requirements of the XWB84’s be in long run compared to the XWB’97 if the assumption is made that all take-offs are at 280T (also assuming taking off at at strips with altitudes of <400m).


    • Most of that stuff can be found on the internet, with reliable or authoritative sources.

    • Hi Anton, what you are checking if could one build a shorter range/regional A350-1000 using the weaker engines? Yes, at 280t the TO distance is essentially the same as the A350-900 because engine thrust is the same and the longer fuselage doesn’t make a lot of difference at low speed (induced drag is the dominant low speed drag, not friction drag). Fuel consumption will be no different as both engine variants the have the same TSFC, ie the have the same fuel consumption per lbf of thrust. Maintenance costs for the fuselage is the same as for the A350-1000 and engines the same as TXWB84.

  2. The key to improved EFH/EFC is EGT. Reduced EGT improves this ratio. The DUGAN KINETICS mod to the MD80 variant reduces EGT by augmenting thrust and by BL injestion. This is a patented but recent mod without much history.

  3. How has the minimum level changes over time Bjorn? Has it been 10-20 for some time now or have complexity and tolerances pushed it up?

      • Should have been changed, sorry.

        You wrote “If we assume that an engine maintenance company would like to have at least 50 overhauls of an engine family per year, with an absolute minimum of 10-20”. Has the minimum been 10-20 for some time now or have complexity and tolerances pushed it up?

        • Thanks Woody,

          I got help from Alistair Forbes, Rolls-Royce, to answer this one. And you are right. The present generation of turbofans are more demanding to overhaul due to tighter tolerances and more complicated parts. This puts the limit at something like 20 overhauls per year to make a viable business. There is a chance for a smaller shop to focus on only module swaps but that is not a real overhaul workscope. With that a lower rate per year can be OK.

          Finally, there is a proficiency limit from EASA/FAA of minimum two visits per year and engine type.

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