Turbine engines are powerful apparatuses, allowing large, heavier-than-air aircraft to traverse high altitudes with ease. To maintain compliance with FAA mandates and to uphold the safety and integrity of the aircraft, the turbine engine should be inspected and maintained on a regular basis. Typically, turbine engines will follow a maintenance schedule that is determined by flight hours and engine cycles. While flight hours are fairly straightforward in measuring the elapsed time between liftoff and touchdown, engine cycles are a little more difficult to define.
Generally, a complete engine cycle will consist of three stages, those of which are the engine start, takeoff, landing, and engine shutdown. As a cycle cannot end until the engine has been fully shut off, an aircraft can technically conduct multiple takeoffs and landings without completing a single complete engine cycle. Generally, the amount of takeoff and landing procedures that are carried out by the aircraft are not considered when determining engine cycles, instead focusing on each engine start and shutoff.
With such information, one may then conclude that engine cycles are less important than flight hours, but there are specific reasons for why such measurements are conducted. During each engine cycle, the various rotating components and parts of the turbine engine will be exposed to centrifugal force and thermodynamic stress, both of which result in wear and tear to the assembly. To guarantee that proper servicing is conducted for an engine, the pilot should always maintain and update an accurate engine logbook, complete with all flight hours and engine cycles that occur between inspections.
With a measurement such as flight hours, engineers may determine the margin in which various engine components can continue to operate with optimal performance before requiring repair or replacement. With engine cycles, on the other hand, engineers will determine the Low Cycle Fatigue (LCF) and how many cycles the engine can undertake before LCF failure is a concern. With the LCF, engineers establish what is known as the Service Life Limit of a particular turbine engine. When an engineer is calculating the Service Life Limit of a given turbine engine, they will often take advantage of the particular manufacturer specifications of the aircraft in question.
In some instances, service bulletins may be established for specific engine components, allowing engineers to know what is life limited. When a component is life limited, it will often necessitate a Life Limited Part Log, that of which is used to write down flight hours, engine cycles, and all maintenance that is carried out for the engine. As some service bulletins may even dictate how engine cycles should be tracked and recorded, it is important to be aware of the requirements established for your turbine engine and engine components.
For some aircraft that conduct multiple takeoffs and landings during each engine cycle, the way in which recordings are carried out may differ. Sometimes, a service bulletin will require an alternate way of tracking cycles as multiple takeoffs and landings without an engine shutoff can lead to increased fatigue damage that is not properly tracked. If engineers are not fully aware of the condition of a particular aircraft and all assembly components, pilots run the risk of causing the ejection of metal fragments from the engine, posing a major safety risk to surrounding individuals, buildings, and more. As such, damage fractions should be properly recorded in logs as dictated by the applicable alert service bulletins for a particular engine.
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