You're in the midst of carefully preflighting an airplane that you haven't flown before, and upon examining the horizontal stabilizer, you discover a two-inch-long crack in the composite cap on the outboard end of the left stabilizer. At each end of the crack there is a small hole drilled into the composite material. By the time you complete your preflight you've found three other such cracks in various aluminum skins and fiberglass fairings. Each one ends in a small drilled hole.
Small cracks in the surfaces of airplanes are an unavoidable consequence of the harsh environment and working conditions to which airplanes are subject. Cracks form because of stresses, such as vibration, slipstream forces, and temperature extremes, that are applied to the surface over hundreds or thousands of hours of flight. Few airplane owners or operators can afford to replace every skin or fiberglass piece that develops a crack, so mechanics use a technique called stop-drilling to limit the length of a crack once it is detected. While this technique is most often used on fairings, caps, and other non-structural components, on some aircraft, it may be used on wing spars and windows.
The technique involves drilling a small hole at each end of the crack and then smoothing off any burrs using a larger-diameter drill bit. The circular shape serves to disperse the stresses that caused the linear cracking. Usually, stop-drilling is effective at limiting the amount of cracking to what had occurred before the crack was treated.
Mechanics use their judgment in deciding whether a crack can be stop-drilled, or whether the skin or fairing should be patched or replaced. Stop-drilling also may be a temporary fix until the affected area can be permanently repaired by patching or replacing. If you are concerned about any cracks you see when preflighting, even those that have been stop-drilled, ask for the mechanic's opinion.
