ATPL · Technical · 04 Aircraft General Knowledge 3 Powerplant lesson — Page 261, Lesson 351

I want to walk you through what happens to a turbine blade over its working life, and then the materials we use to build it. This is the heart of why an engine has a finite life and why we have strict temperature and rpm limits. Let's start with the metal of the blade itself. When a turbine blade is subjected to high temperature and high stress, it can permanently stretch — it grows longer and cannot reform back to its original length. That permanent stretching is called creep. Even if the materials are perfect and the crew has observed every temperature and rpm limit carefully, creep will still cause the blade length to increase over time and over engine operational cycles. So a blade has a finite life before failure occurs — it is not designed to last forever. Now, there are two distinct ways a blade can fail, and they are defined by how early they happen. Low cycle fatigue describes relatively early turbine failure due to high operational demands — think of severe power changes, hard accelerations, high stress events that happen few times but do a lot of damage. High cycle fatigue describes failure after a longer turbine life due to lesser operational demands — many smaller, gentler cycles that eventually wear the blade out. So the same blade can fail early from big demands or late from many small ones. Now let's look at the materials, because that is what determines how much temperature the blade can survive. The earliest gas turbine engines used high temperature steel for the blades. That material imposed a severe limit on the temperature at the rear of the engine. And here is the key point: the gas turbine engine is a heat engine, so if you limit the temperature, you limit the power output as a direct consequence. Hotter gas means more power, and steel could not take the heat. The next advance was nickel based alloys, and these were subsequently superseded by super alloys. A super alloy is a complex mixture of many different metals — chromium, cobalt, nickel, titanium, tungsten, carbon, and so on. These give a maximum temperature limit of approximately 1100°C, or if the blade is cooled internally, 1425°C. That internal cooling is a big deal — it buys you over 300 degrees of extra capability. Then we have two more modern manufacturing practices. The first is powder metallurgy, where powdered super alloys are hot pressed into a solid state. The second, used in the most advanced engines, is single crystal casting — and that is the search for even stronger materials, because traditional metal manufacturing has its limits. So the story is simple: creep stretches the blade over time, fatigue decides how soon it breaks, and the material decides how hot you can run the engine. Let's look at the figures that show this.