The average aircraft is exposed to extreme pressure, heat, and cold all during normal operation. With all these stresses attacking the various inner and outer structures of the aircraft, it's not surprising that cracks can develop in practically any material. Drilling a hole at the tip of the crack is a process known as stop drilling. It's a well-tested method of keeping cracks from growing, but there are some facts you need to know about these maintenance methods before breaking out your aircraft drill.
While stop drilling is an acceptable and safe way to stop a crack from spreading that develops in an aircraft's structure or skin, it doesn't actually fix the underlying problem. Most cracks form due to vibrations, and drilling a stop hole does nothing to restore the lost strength of the material or prevent vibrations from causing more cracks to form in the same area. Thorough testing and inspection are necessary to determine if a crack you've stop drilled needs more repair or if the material is still strong enough to function safely with the fracture in place.
Even when you determine an aircraft crack needs more than just stop drilling, you can combine this technique with other repairs for the best chances of success. For example, covering a cracked area with a patch of sheet metal is one of the most common ways of reinforcing a weakened area. It's perfectly acceptable to drill the crack before applying the patch to ensure it doesn't continue to spread and split under the patch. If the crack doesn't call for a reinforcement patch but still needs more than just a stop hole, inserting pins into the stop holes at each end of the crack has shown a greater resistance to secondary fracturing than just leaving the holes empty.
Understanding why stop drilling works to control cracks is essential to knowing when and where to use them. Regardless of the material or placement, a crack always has the most stress concentrated at the sharp ends of the crack. A simple single line crack has two high-stress points, while a split fracture may have many more ends that need drilling. It's essential to add a stop hole at every end of a crack to diffuse the stress. By ending a crack in a smooth and rounded hole, you're distributing the stress that originally caused it across a much wider and more reinforced area so it can't continue to grow.
The size of the hole you add at each end of a crack determines how well the hole works at controlling stress. When too small of a hole is used in an attempt to avoid weakening the material you're drilling, the crack continues to grow because the radius of the hole can't handle the stress. Larger holes control cracks for longer, up to a point of failure determined by the specific material you're drilling. Checking up-to-date Federal Aviation Administration (FAA) repair standards will help you determine the perfect stop drill hole size.
Finally, drilling one to two extra holes that aren't connected to the crack may improve the total stress resistance if they're placed properly. These holes must be located near the original stop holes but far enough away that they don't compromise the general strength of the material you're repairing. Since this is an uncommon procedure that is still under development, you'll need to check for machining and aircraft repair research before attempting it on an aircraft. This kind of complex stop hole drilling is also best combined with other repairs since the presence of extra openings increases the effects of future tensile and vibration stress.
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