Bjorn’s Corner: Aircraft engine maintenance, Part 5

By Bjorn Fehrm

March 31, 2017, ©. Leeham Co: In the last Corner, we showed flight hour graphs for wide-body engines. Now we will deduce the market for engine overhauls from these graphs.

It will show which engines are still in engine manufacturer care, in their main maintenance cycle and in the sun-set phase.

Figure 1. Principal picture of a three shaft turbofan. Source: GasTurb.

The phase the engine is in and its future flight hour development will decide the attractiveness of the engine for overhaul organizations.

Removal rates of wide-body engines

To understand the engine maintenance market for the wide-body engines, we will now deduce the number of shop visits per year from the engine flight hour graphs we showed last week.

The use of wide-body engines is different from the single-aisle. Most wide-body aircraft have an average flight time of over six hours. This means the deterioration from the flight hour is a more important trigger for maintenance actions than flight cycles.

For the early life of engines, the deterioration from, for example, erosion might trigger a shop visit before the first Life Limit Part (LLP) is forcing the engine off the aircraft. As engine families mature, most workshop visits will be the result of LLP limits being reached, unless the engines are operated in a harsh way (short flight times or polluted air areas).

The time on wing for an engine will depend on the flight time-to-cycle ratio. Engines which operate in domestic traffic with short flight times (like ANA’s Boeing 787) will have much shorter time on wing than engines operated mainly on long-haul routes.

Overhaul strategies

The notes about overhaul strategy made in Part 3 for short-haul engines is also valid for wide-body engines. Engines which are in the introduction phase are handled by the OEMs, on power by the hour contracts.

Mature engines which are popular create an overhaul market with several players competing for the jobs. The cost for overhauls in the wide-body market is high. A complete LLP stack for an Airbus A330 engine cost around $8m and for the Boeing 777 around $12m. With LLP flight cycle limits varying from 3,500 flight cycles to 20,000 cycles and the typical wide-body flying around 700 flights/year, the engines will have consumed the low-rate LLPs several times over before the complete stack has been changed.

A typical shop visit for an A330 engine will cost between $5m to $8m dependent on engine and use, whereas a GE90 visits cost $10m or more.

Engines in the sunset phase are maintained by shops which certified on the engine during its hey-day. Overhaul strategies for such engines will be time and material. Used parts will be built into the engine, so when the engine goes end of life, the LLP value is at a minimum.

Shop visits

If we assume the average time on wing for long-haul engines is around 20,000 flight hours, we get the shop removal picture in Figure 1.

Figure 2. Wide-body engine flight hours and shop visits during 2016. Source: Rolls-Royce. Click to see better.

Assuming 20,000 flight hours for an engine between shop visits is a rough measure. Typically, two-shaft engines are below that and three-shaft above (to avoid discussion, this is not coming from Rolls-Royce, but from independent engine MRO surveys that we have access to).

For our purposes, a global mean of 20,000 flight hours to generate the shop visit picture is OK. 

Shop structure

It’s clear the wide-body engine overhaul market is smaller than the single-aisle. We have less than half the number of shop visits. Single-aisle engines had a total of 3,665 shop visits; for wide-body we count 1,440 for the past year.

If we again assume that an engine maintenance organization would like to have at least 50 overhauls of an engine family per year, with a minimum of 10-20, there can’t be too many shops competing for jobs outside of Trent 700 and GE90.

We will discuss the engine shop structure for wide-bodies more in detail in the next corner.

10 Comments on “Bjorn’s Corner: Aircraft engine maintenance, Part 5

  1. The widebody Engines are running much hotter at Take Off Thrust hence they are very much cycle limited, often 1500-2500cycles. This is why widebody Engines are not suitable for 1-2 hr hops 5-12 times a day like narrowbody Engines. They can be cycled up to 20 000 cycles Before a shop visit.
    That is why the 797 will be unique if it can take +15000 cycles and being a light widebody.
    RR Sir Stanley Hooker &co: was smart using the RB211-22/-524 cores for the much lower thrust RB211-535’s on the 757.

    • I’m sure you’re right in your numbers of cycles, but I think the reason behind it is slightly different (although I think we end up in the same place) – fuel is a greater component of a long haul’s costs than it is for short haul, while the number of cycles per year is lower, so the maintenance burden is lower. Therefore it makes economic sense to run the engines hotter (to improve the thermodynamic efficiency and lower the fuel burn) and have less margin between overhauls. the benefit of lowering the fuel cost is greater than the cost of increasing in maintenance costs (especially as the cycles per year are lower due to the flight length). Like most things in jet engines, it is a balance

      • I agree, with 1-2 cycles per day it takes many years to reach the 2500-3000 cycles when the hot section is out of AMM limits or EGT margin is getting close to 0 C. Hence the engine designers go for “hot” engines for widebodies. The interesting happens when you take a “narrowbody engine” like the RB211-535E4 and run it 1-2 cycles per day. It will stay on wing +30 000 hrs and with proper engine wash the LEAP-1A, -B can reach those numbers. The PW1135G will come as well to those numbers on wing when PWA has poured the typical numbers of ATA72 SB’s into their engine. The economics for trans Ataltic flying will be changed and many airlines will opt to fly narrowbody transatlantic routes with the MAX and Neo.

          • The Aircraft Maintenance Manual “AMM” defines allowed damages and when to replace an Engine. To make the Engine run a bit cooler and last longer on wing many operators use Engine compressor wash Equipment to Clean the blades and vanes in the compressor and hence get longer time on wing especially in areas where the dirty air leave deposites in the compressor, Service Bulletines are modifications or inspection instruction in addition to the Aircraft and engine manuals as Engines and Aircraft are not built per year model standards like cars but improvements are constantly incorporated as Service Bulletines. PWA needed to improve some earlier models with many SB’s and the PW1100G family seems to be similarly effected as the JT8D, JT9D, PW2000 and PW4000 models were in its days.

  2. Any thoughts on how the trend to having only 1 engine provider per airframe type will play out for aircraft owners Bjorn? Obviously reduces competition up front (and so could raise prices) but once past the intro phase, volume of sales dependent, could mean more indies playing and playing longer into the sun-set.

    Similarly for Airbus and Boeing. If the consequently higher volumes are sufficient to keep a tighter lid on costs come sun-set, any thoughts on how this might affect airframe product lifecycles?

    • Competition is always the best cap on OEM margins. When it comes to engines it’s a bit more complicated as the whole picture is skewed.

      For whatever reason no engine OEM can get the correct price for a new engine (production cost + amortization of development + reasonable return on investment) so they all rely on the use of the engine to consume spare parts (with high margins) to pay back the development and production/sales costs of the engine.

      This back-loaded earning creates large risks for the engine OEMs. Their business plan for a new engine closes much later than for the aircraft. If the aircraft is sold with competition (like most do), the up-front cost and later operations costs of the aircraft (where the engine is about 1/3 of up-front cost and 2/3 of maintenance costs) will be under competition even if there is a sole source situation for the engine. So the competitive pressure is there. For very attractive and high volume programs (new single aisle…) more than one available engine OEM can be justified. For average volume programs no more. For these the overall competitive pressure for the airframe will keep engine costs in line, perhaps not to the level as with multi-source, but on a reasonable level.

  3. I didn’t realize that used parts could be used in an engine overhaul. It seems that this would make the cost a little lower for the customer. You mentioned that an overhaul for a A330 engine will cost between 5m to 8m. How long does it take to overhaul an engine of that size?

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