Bjorn’ s Corner: New aircraft technologies. Part 49. Engine Maintenance

By Bjorn Fehrm

March 8, 2024, ©. Leeham News: We are discussing the different phases of a new airliner program. After covering the Design and Production, we now look at the Operational phase of a new airliner family.

For the operational phase, the airplane must pass scrutiny for Continued Airworthiness. The biggest item in a regulator’s Instructions for Continued Airworthiness is the required Maintenance program to keep an airliner airworthy. We discussed airframe maintenance in the last article. Now, we look at engine maintenance.

Figure 1. The CFM56-7 engine for the Boeing 737NG. Source: CFM.

Engine Maintenance requirements

The engines on an airliner are the most important system for its safe flight. They are also the most expensive part of the aircraft’s maintenance, with engine maintenance costs normally exceeding the airframe maintenance costs over an aircraft’s lifetime.

Engines undergo strict certification programs, where rules for overseeing the engine and its maintenance are established. The maintenance concept combines On-Condition maintenance with Hard-limit maintenance.

On Condition Maintenance

On-condition maintenance constantly oversees critical engine parameters, such as engine use, temperatures, vibrations, oil deposits, and wear, as seen during optical inspections of the engine’s internals through borescopes.

The most important parameter is the temperature of the engine’s turbine section. The turbine section has an Exhaust Gas Temperature, EGT, sensor placed in the low-pressure turbine area (Brown module in Figure 1).

The EGT is only allowed a certain highest value; otherwise, parts of the hot section of the engine can be damaged. As an engine wears, the engine’s control computer (FADEC) injects more fuel into the combustor to keep the rated thrust level at takeoff and climb. This gradually raises the EGT as the engine gets worn by use.

Figure 2 shows the typical EGT profile during the engine’s highest stress point, takeoff. During the rest of the flight, temperatures are lower, and engine wear is less. The diagram shows a new engine with the largest margin between the maximum allowed EGT (red line) and the peak value during takeoff (Blue curve).

Figure 1. EGT values during takeoff for an engine during different phases in its life. Source: Leeham Co.

A worn engine gradually narrows the margin between the EGT and the maximum allowed EGT. At a certain point, the engine is removed from the wing and sent for a hot parts performance restoration shop visit (Orange curve).

An engine that has passed its first engine performance restoration shop visit does not regain the full EGT margin of a new engine. Thus, the number of takeoffs until the next shop visit due to EGT margin loss will be lower than for a new engine (Cyan curve).

The time of shop visits due to EGT margin is determined by flight safety concerns (engine damage) and economic factors, as the restoration cost increases nonlinearly beyond a certain engine deterioration.

Hard timed limits

During the design and certification of the engine, the OEM and regulator agree on the life limits of critical parts of the engine. Most of the limits are fatigue limits for shafts, discs, and blades in engine compressors and turbines.

An engine is designed to handle the loss of a fan blade due to a bird strike or similar event (it has a fan case that can contain a lost fan blade), but it’s not designed to contain a compressor, turbine blade, or parts of a burst disc. Thus, such an event must not happen. Therefore, the regulators and industry have developed methods to calculate the fatigue limits of engine parts.

The OEM proposes such limits to the regulator as part of the engine certification, which the regulator accepts as the start limits for the engine. Gradually these limits are increased as in-service experience is gained with the engine.

These parts are called Life-Limited Parts, LLPs. At a certain Flight Cycle limit, these parts must be replaced with new ones. The cost of a complete LLP stack can be up to a third of the new engine price. Thus, an engine’s LLP limits are an important safety and cost parameter.

Short Haul versus Long Haul engine limits

Short-haul engines are predominantly worn by takeoffs, thus the Flight Cycles (FC). They are takeoff stressed up to 10 times a day, with flight times of an hour or two.

Typical for a mature engine like the CFM56 is that the engine has a performance shop visit at half the LLP Flight Ccyle limit, and then a full engine overhaul is done when the LLPs are due for replacement.

These limits vary between engines and how mature the engine designs are, but typical short-haul values for mature engines are 10,000 to 15,000 flight cycles for the Performance Restoration shop visit, and then the full overhaul with LLP stack changes at 20,000 to 25,000 Flight Cycles. New engine types have early lives with limits as low as half these values.

Long-haul engines live a different life. They take off one or two times a day and then spend an average of six to eight hours at lower-temperature cruise conditions. Here, engine erosion and corrosion, especially from salty or dusty air, dictate engine wear. These engines stay on the wing for 5,000 to 8,000 Flight cycles when they have reached a mature state, representing 30,000 to 50,000 Flight Hours. Such engines often only have one LLP change cycle.

The influence of the environment

When discussing how long an engine will stay on the wing, the environment where the aircraft flies has a large influence. An inland climate without hot temperatures is a benign environment, while a hot and dusty Gulf Area is a Harsh environment. A seaside environment like an island or coast area is also a Harsh environment. The time on the wing can change by a factor of two between these environments.

Shop visit costs

About two-thirds of the shop visit costs are for replacement parts like turbine/compressor blades, stator, or combustor parts. The parts costs are even higher for the visits where LLPs are changed. Restoration visits are in the single-digit million dollars for engines like the CFM 56, whereas the full visits, including the LLPs, cost close to $10m.

Long-haul engine shop visits exceed the $10m mark, with the largest engines costing double that. Add the LLP stack, and we talk even higher values.

9 Comments on “Bjorn’ s Corner: New aircraft technologies. Part 49. Engine Maintenance

  1. Actually engines contain blade and vane losses but not disks, spools, blisks or rotating air seals that all have Life limits. A narrowbody engine usually has plenty of EGT margin and hot section damage is the main driver off wing. A widebody engine has less EGT margin and its wear are mainly driven by T-O and climb thrust, hence engine derate and compressor washes are essential to keep them on wing. Some design mistakes like poor corrosion coating inside cooled turbine blades cause problems in salty environments or vibration modes causing cracking at certain rpm’s.

  2. I understand the engine controller using EGT for fuel input

    Do they use fuel flow under certain known temps and climb to determine it need to increase the EGT to maintain thrust?

    • @TW

      Will give you a backhanded answer. EGT limit usually contemplates takeoff at full load and high temp conditions. If an airline chooses to limit operating conditions to circumstances of a borderline engine it can technically go beyond normal EGT considerations. Restricting ops to shorter-haul flights or flying out of destinations that have a lower ambient takeoff temp. These operational choices might buy a few months of utilization but is helpful if an operator is having a surge of removals and is running short of spare engines.

      • Casey:

        How do they determine the Removal as EGT is not a given, you need something else to relate to it.

        Just EGT would not do it. Is it fuel flow and EGT or do they have other performance aspects that do that or a combination?

        Or, a wild example, you have an arbitrary out of the hat 1000 lbs an hour fuel use and an EGT of say 1000 deg.

        You then need 1500 lbs an hour to get the same EGT.

        • @TW

          “EGT margin” will drive engine removal most of the time. If you are close enough to the line where it is an issue then final engine pull can only be delayed “by a matter of months.” The most important factor is simply not red-lining the engine. The plots of EGT margin follow a predictable loss rate. The takeoff conditions of a given aircraft will indicate whether there is sufficient margin for operating the aircraft without redline. If an engine is restricted to lighter duty that is a way to extend life. Common examples of extending would swapping installations from an A321 to A320 installation.

          As also alluded earlier…engine washes are helpful with regaining EGT margin and squeezing the last bit out of an engine run.

          On the other side of things, engines can be pulled earlier than absolutely necessary for reasons as simple as a shop induction slot is available.

          • Or like the guy in China who threw lucky coins into the engines on an A350!

            No damage found but wow.

  3. Good piece.
    You are correct that EGT margin is ONE of the key trends that drive engines off wing.
    Another very important factor that drives engines off wing is oil consumption, especially with ETOPS operations and why the airlines focus is this key element, this includes the APU as well.
    Oil consumption is primarily driven by main bearing degradation and/or the seals.
    Airline operators today use ECM (engine condition monitoring) data science to allow them real time reliability and safety on these important elements.
    There are many ECM elements and vibrations is also paid attention to. Flight crews have vibration alerting on the flight deck.

    All the OEM’s offer Power By the Hour (invented by P&W) that offers a cost per flight hour contract for full engine monitoring and spares availability to keep the fleet in service and limit ESV costs.
    Rolls Royce operates a 24/7 operations center in Derby that monitors contracted engines world wide.

    Boeing and Airbus (other OEM’s as well) offer their own set of real time data monitoring and alerting with tools such as Airplane Health Management (AHM) and Airbus’ Airman. These are also a contracted service usually for the whole airframe systems and engines cost/flight hour.

    All of these are outstanding tools for enhanced performance, reliability and most importantly safety.

    • @Airdoc: absolutely oil can drive removal. And not just oil consumption but oil leakage through bearing compartments into the gaspath.

      LLP and “hot section durability” are going to drive the vast preponderance of removals. One of the things that drives hot section durability is operating temps of the latest engines are higher than previous engines. And along with that is the tradeoff between fuel efficiency and cooling air for the airfoils. Cooling holes that also can be block by particulates or sand.

      As it relates to secondary removal reasons, bearings play a nagging role especially in the turbine locations. Vibration indicators are another factor that can play a role. Latest generation engines are operating a higher RPMs than with previous design experience.

      • Internal Oil leakage is calculated in the oil consumption algorithm.
        Excessive external leakage is easy to verify by mechanics on their walk around and RON checks.

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