In which scenario is it acceptable to weld over an existing joint?

The correct choice regarding the acceptance of welding over an existing joint hinges on the understanding of safety, integrity, and industry standards in aviation repairs. Welding over an existing joint is typically discouraged because it can lead to structural weaknesses, especially if the original joint is already compromised.

When welding is considered in aircraft maintenance, critical evaluation of the joint’s history, the metallurgy involved, and specific guidance from governing bodies or repair manuals is essential. In many cases, such practices could invalidate aircraft warranties or, worse, compromise safety.

This emphasizes the importance of adhering strictly to the guidelines outlined in the aircraft's maintenance manual or the manufacturer's specifications. The repair manual often provides clear instructions on whether such actions are permissible and under what specific conditions, which illustrates why welding over an existing joint is generally treated with caution and often avoided.

Overall, the recommendation is not to weld over an existing joint unless explicitly stated and supported by the necessary documentation, therefore supporting the understanding that it is typically never deemed acceptable without stringent criteria being met.