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Cracks and Stops

Every plane has at least one -- a crack in a formed part, or more than likely, a composite part or fairing of some sort on the airframe.Every plane has at least one -- a crack in a formed part, or more than likely, a composite part or fairing of some sort on the airframe. Most of these cracks are benign, apart from looking bad to the naked eye. However, all cracks need immediate attention to keep them from growing.

FACT: Once a crack has started, whether it is on a wing tip fairing, an elevator fairing, or part of the skin, that crack will continue to grow or propagate UNTIL it is stop drilled. The stresses that caused the crack will continue to cause it to grow unless your A&P stop drills the end of the crack properly.

STRESS DRIVES CRACKS. Nothing makes a crack grow more than stress. If you have an active crack (i.e., not stop drilled) on your plane, several factors will help it continue to grow:

  • Thermal cycles. The sun rising and setting thermally cycles the skin of your airplane. As the metal and composites expand at slightly different rates, the crack will grow incrementally with each cycle.
  • Vibration. One byproduct of running your engine, or flying through the air is airframe vibration. The vibration adds stress to the crack, which will cause it to grow.
  • Impact. Hitting a cracked part, even with a glancing blow, can cause a crack to open up quickly.
STOP DRILLING is a technique where your mechanic uses a small drill bit to relieve the stress at the end of the crack. By drilling a small hole directly over the end of the crack, all the way through the part, most cracks can be stopped cold in their tracks. However, if your mechanic is unable to get all the way through the crack, it may continue on past the hole. In such cases, a second hole has to be drilled to stop the crack.

USE GOOD JUDGEMENT. If your mechanic says he has to remove the part to properly stop-drill the crack, listen to him. Having the part removed to be properly stop-drilled will usually be more successful than trying to do it while it is still mounted on your airframe.

NON-STRUCTURAL REPAIRS may be made to fairings, in accordance with the associated regulations for aircraft maintenance. However, if a part becomes cracked to the point where it is no longer in one piece, or has several cracks, the time has usually arrived to replace the part. Remember, fairings are in place to allow the air to slide over your fuselage and controls, reducing drag. They also help to keep moisture out of the inside of your airframe. Thus, making repairs with RTV cement, while inexpensive, can prove to be more costly in the long run.

INSIDE INFORMATION – Not all cracks are alike. Some aircraft, such as Beech / Raytheon products, use lightweight materials such as magnesium in some of their control surfaces. Cracks in such surfaces are generally not capable of being stop-drilled, and are usually grounds for reskinning. This is due in large part to the need to maintain the control’s weight in balance. Also, if your mechanic stop drills such controls, it can allow moisture intrusion, which can eat the metal away from the inside out!

BOTTOM LINE: Make sure your mechanic abides by your aircraft manufacturer’s instructions, and your cracks can all be brought under control or corrected. While a stop-drilled crack isn’t very attractive, it is far better than having a fairing crack off and bang against your aircraft in flight. The next time you walk around your plane, take a moment to look for these cracks. If you find any cracks, take action to get them corrected as soon as possible. Your efforts will be rewarded with your parts, as they stay flying -- with your airplane.

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