Stop Drilling Fatigue Cracks in Aircraft
Before you begin understand that stop drilling a crack is not a repair but a temporary intervention. Think before you stop drill. Is this appropriate?
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.
Drill a .25 inch diameter hole .1 inch beyond the visible crack tip. Then inspect for cracks using a HFEC (High Frequency Eddy Current), or if not available, dye penetrant inspection. If no crack is found, enlarge the hole to .31 inch. Deburr the hole very carefully. Use a reamer to clean the hole.
The stop drill hole must be large enough and placed properly to not only capture the crack tip but also remove any strain hardened zone at the tip of the crack. Drill using a SHARP (read this as new) drill bit.
In the picture above the end of the visible crack in the paint is not the end of the crack in the metal. Remove the paint and dye penetrant inspect to find the end of the crack. If you don't remove the crack tip then stop drilling is useless. Also, the crack may be extend further on the underside of the sheet. If any part of the crack extends beyond your stop drill, then you have failed and your repair is a waste of time.
All open holes should be plugged if possible. Every open hole distorts under loading. Cyclic distortion is what causes metal fatigue failure. Some methods to minimize hole deformation and cracks is by plugging the hole with a tight fitting fastener, by installing a rivet into the hole, "idle rivet", or by reinforcing with a patch. An example is a "dime/dollar" patch illustrated in AC43.13B on page 4-32. Its advantage is it will take stress equally from all directions, unlike other repairs.
An example of what can happen to unplugged stop drill holes is contained in the NTSB accident report where 20 people died when the wing broke off of a Turbo Mallard (N2969). "The skin crack under the doublers at right WS 34 (see figure 4) intersected three unfilled machined holes in the skin that exhibited features consistent with stop drill holes."
Three stop drilled holes in a primary structure! Why were there were any stop drill holes in the primary structure?
A stop drill hole is made at the tip of a fatigue crack to temporarily retard the propagation of a fatigue crack until a more permanent repair can be made. 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.