Final answer:
A mechanic should listen for abnormal sounds such as grinding or banging during engine coast down, which could indicate mechanical issues. The absence of such noise in engines like the GE90-110B1 suggests good condition, but any concerns should prompt further inspection.
Step-by-step explanation:
During the countdown of a turbofan or turbojet engine after shutdown, a mechanic should listen for any abnormal noises that may suggest issues. These could include grinding, banging, or whining sounds indicative of loose parts, bearing failures, or foreign object damage.
As a powerful turbofan like the GE90-110B1 engine on a Boeing 777 winds down, the lack of any unusual noise would imply the engine components, such as the fan blades, are potentially in good working order. However, an in-depth inspection is necessary if any abnormal sounds are present to prevent future failures and maintain the safety and reliability of the engine. It is also important to remember that the increase in angular velocity during the engine startup should be smooth, as should the decrease during coast-down, reflecting the quality of the engine's design and mechanical integrity.