How many consecutive fasteners must meet flushness requirements before countersinking can begin?

The requirement for consecutive fasteners to meet flushness specifications before beginning countersinking is set at five. This standard ensures consistency and quality in the assembly process, as it allows for the verification of the installation's accuracy. Focusing on several fasteners rather than just one helps to minimize errors that might arise from isolated discrepancies.

Using five consecutive fasteners creates a reliable baseline to confirm that the overall installation is within acceptable limits before proceeding to countersink. This is crucial because countersinking is a permanent modification that can affect both the structural integrity and aesthetic quality of the assembly. Having a standard number, like five, ensures engineers and mechanics maintain a high quality of workmanship throughout the assembly process.