Engine with major inspection intervals of 400 hours or more should perform major inspections if what percentage or less of the interval remains?

The correct answer is based on maintenance practices that aim to ensure the reliability and safety of engine operations. For engines with major inspection intervals of 400 hours or more, performing major inspections if 25% or less of the remaining interval is left is a best practice. This approach allows for timely detection and correction of potential issues before they develop into serious problems.

To elaborate, if an engine operates within defined intervals—like 400 hours—monitoring the percentage of that interval which remains is crucial. Once only 25% or less of that time remains, it indicates that the engine has been in operation for a significant amount of its expected lifespan. At this stage, it's prudent to conduct a major inspection to assess wear and tear and to identify any components that may need servicing or replacement. This practice aligns with the proactive maintenance philosophy, which seeks to prevent failures before they occur, as well as ensuring compliance with safety and operational standards.

In contrast, letting the engine run past the 25% threshold before inspecting can increase the risk of unplanned failures and could lead to more extensive downtime and repair costs. By adhering to the 25% rule, operators can manage their maintenance schedules effectively while extending the engine's overall operational life and reliability.