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.
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