A crack is the first sign of impending failure
|Crack in aircraft fuselage|
We all know what a crack is when we see one. So what do we do about it? This guide attempts to establish a framework of knowledge about how cracks are viewed in the aircraft industry.
|Cylinder barrel crack -aircraft piston cylinder|
1. The Definition
A crack is a type of discontinuity brought about by tensile stress the result being that things are no longer held together. This definition establishes that a crack is a sign of impending failure that prompts a course of action when found. You have to do something about it.
|Stress ahead of visible end of crack|
2. What to do about it?
All detected cracks are a concern, regardless of their size. Leaving a crack in situ is rarely an option.
Many aircraft fly with known cracks if engineering demonstrates that the residual strength exceeds ultimate load; including crack growth until the next inspection. What to do about it is the decision of engineering presented through the aircraft's maintenance instructions. Most modern aircraft use a SEM (Structural Repair Manual).
|Crack from poorly drilled hole|
3. Who's in Control?
This is the key item of importance to the mechanic--the mechanic must defer to engineering or appropriate maintenance instructions as to the proper course of action when a crack is discovered. It is not in the mechanic's (nor aircraft owner's) area of expertise to access residual strength and crack growth rates.
|Stop drilled and crack with patch behind|
4. What is NOT a repair
Stop drilling is not a repair; it's a temporary intervention. A stop drill hole does not properly restore the load-carrying capability of the structure and the underlying structural problem is still not properly addressed.
5. When Maintenance Makes the Decision
Shortly before the right wing broke off of a Turbo-Mallard (N2969) killing all 20 people aboard, at a meeting with pilots the following comment was made: "the maintenance supervisor told them that the skin-cracks in the airplanes were superficial." NTSB accident report NTSB/AAR-07/04.