FAA Aviation Mechanic General (AMG)Materials, Hardware and ProcessesEasy
A technician discovers a small crack in a non-critical sheet metal skin panel during inspection. Before applying a doubler repair, the technician drills a small hole at the end of the crack. What is the purpose of this hole?
- AAllow trapped moisture to drain from the repair area
- BRelieve stress concentration to stop the crack from propagating
- CReduce the overall weight of the repair
- DProvide a rivet hole location for the doubler patch
Show answer & explanationAnswer & explanation
Correct answer: B. Relieve stress concentration to stop the crack from propagating
Stop-drilling places a small round hole at the very tip of a crack, which distributes the stress over a larger area and eliminates the sharp point that concentrates stress, preventing the crack from continuing to grow before a permanent repair is made.
Why the other options are wrong
- A. Moisture drainage is unrelated to the purpose of stop-drilling a crack.
- C. Weight reduction is not the reason for stop-drilling; it is a structural repair technique.
- D. Rivet hole locations are determined separately by the doubler pattern, not by the stop-drill location.
Stop-Drilling
A repair technique where a small hole is drilled at the tip of a crack to relieve stress concentration and prevent further crack propagation before permanent repair.
- Hole must be centered exactly at the crack tip
- Removes the sharp point where stress concentrates
- Typically a temporary measure until a doubler or patch is installed
Memory trick: 'A round hole rounds off the sharp stress' - drilling stops crack growth