By Bjorn Fehrm

September 30, 2024, ©. Leeham News: We do an article series about engine development and why it has longer timelines than airframe development. It also carries larger risks of product maturity problems when it enters service than the airframe of an airliner.

We have covered the parts of an engine that involve challenging technology and which decide its reliability (dispatch consistency) and durability (time on wing). Now, we discuss why modern engine design is more challenging regarding these parameters than airframe design.

Figure 1. The Pratt & Whitney GTF in cross-section, one of the new engines. Source: Pratt & Whitney.

Airframe reliability and durability

Historically, both the airframe and its systems and engines have caused reliability problems (aircraft not flying). While there are still flights that get canceled because of airframe problems, they are now rare. Most of the time, it’s because of a problem with a system that must have a certain functionality to guarantee a safe flight.

The aircraft’s MEL (Minimum Equipment List) decides whether an airliner can fly when an instrument, equipment, or system malfunction occurs. The aircraft has a certain level of redundancy to allow a continued safe flight after faults in redundant system parts.

Airframe durability is now measured in tens of years. The most critical event for durability is structural fatigue, and rigorous testing at certification establishes the airframe fatigue limits. Typical for a single-aisle jet is more than 50,000 flight cycles of initial certification, which gradually increases during the aircraft’s life.

Different inspections are conducted at each overhaul of the airframe to look for unforeseen fatigue. These inspections, together with a fail-safe design (critical load paths have redundancy), have made crashes caused by structural problems extremely rare.

As a college aircraft program manager said: “The science of making a typical jet airliner is well understood. There are very few unknown unknowns left”.

The latest large change has been the switch from aluminum as a structural material to composite construction. This change was made after 40 years of experience using composite parts in major airliner structures (the first I remember is the A300 vertical stabilizer). There have been no reliability or durability problems associated with it.

Engine reliability and durability

The story for engines is different. The boundaries for applied technology have been continuously expanded. The limits for engine performance, both in thrust-to-weight ratio and fuel consumption (measured as TSFC, Thrust Specific Fuel Consumption, for jet engines), have been set by available material and manufacturing technologies.

Here are some examples:

Fan and Fan Shroud materials

The engine’s propulsive efficiency increases with lower overspeed (in engine speak, Specific Thrust), which is achieved by increasing the bypass ratio. However, increased fan diameter means heavier blades, which means a beefier fan shroud to catch a blade out. Both result in a larger, heavier engine, which forces a larger nacelle, which builds mass and drag.

The available blade materials allowed the first generations (Conway, JT-3D, JT8) to achieve bypass ratios around one. Later, this was increased to four to five (CFM56, V2500, Trent 700, PW4000), and now we are at 10 to 12.

Materials have gone from steel to titanium to composites, where the latter has proven very durable (no blade has been lost from a composite fan to date).

Compressor stability

Compressor designs have been plagued by stalls caused by different in-service deterioration mechanisms. This has beset engines with inherent small margins to stall as they deteriorate in use. Modern materials, 3D aerodynamics, and advanced manufacturing techniques (to make the complicated form of the compressors) have increased the deterioration margins so that compressor stalls are rare today.

Combustor

As described, the turbulent nature of combustor flow makes it difficult to model the processes. It makes it a cut-and-try development, where it takes time to fine-tune a combustor to give good performance and emissions while being durable. The result has been combustors that must be changed at half the wanted lifetime. The interactive work to increase the time on wing frustrates airline customers.

Turbines

As we have described, the turbine temperatures set the performance levels of the engine in many ways. It makes the designer seek the highest possible temperatures, bordering on the possible. Any in-service unknown, unknowns like a different environment due to salt or pollution, can jeopardize the durability.

Engine durability

It’s clear from the above that engine technology is at the bleeding edge, as there is so much to gain in lower fuel consumption and, therefore, emissions by employing advancements in materials, coatings, and cooling. Any problem means the engines need to go off the aircraft and spend months in a repair shop.

Airframe design, on the other hand, has reached a maturity where the unknown unknowns are few and relatively straightforward to handle. This is what’s behind our ingress for this series. Engine development is today a larger challenge than airframe development.

We will spend the next Corners looking at some examples of bad spots in engine durability and argue why these are not easy to predict.