What must be done if a spark plug is dropped when removed from an aircraft engine?

When a spark plug is dropped during removal from an aircraft engine, it can sustain unseen damage that may affect its performance. The best practice in this scenario is to use a replacement plug. This ensures that any potential damage that could have occurred to the spark plug, such as fractures in the insulator or damage to the threads, does not lead to engine performance issues or failures.

Relying on a spark plug that has been dropped, regardless of how it appears visually, could lead to compromised ignition performance during operation. A spark plug's reliable function is critical for maintaining engine performance and safety, and using a replacement guarantees that the component will meet the required performance specifications without hidden defects.

While inspection of the dropped spark plug is a normal practice in maintenance, it may not be sufficient to establish whether the plug is still safe and effective to use. Therefore, in aviation, where safety is paramount, the decisive action is to install a new spark plug rather than risk a potential malfunction from a dropped unit.