Window Repair Basics: A clear view of window maintenance requirements

Window Repair Basics A clear view of window maintenance requirements July 1999 Aircraft windows are typically a low maintenance item, as long as they remain clear and scratch-free. There's little that needs to be done to keep them airworthy...


After the acrylic is stretched, it is taken to a table where the edges are cut off and any milling processes are performed. The acrylic panels are then polished on a large machine to the correct Mil-Spec thickness. After this process, the large panels are then cut and formed, and radiused into the shape of the window as required.

Cupery adds, "It's interesting to note how stretched acrylic cracks. Remember that when you stretch acrylic, you pull the molecules into long, flat molecules, so, that's the way cracks progress also. If a crack starts in one area, it may run across the window and possibly large chunks of window will leave the aircraft at once. It's not unusual, for the broken pieces to be ingested into the engine inlet and cause engine damage or failure. Fortunately, windows are typically made of an inner and outer panel. Often, if one of the panels cracks, the other will hold."

Causes of failure
Like any other type of material under stress, it doesn't take much damage to cause a complete failure of the component. One hairline scratch or micro-crack on a window with several thousand pounds of force against it can quickly turn into a major crack. For this reason, it's important to identify stress risers and, if they exceed the maintenance manual specifications, remove them from the window prior to further flight.

According to Cupery, "Stress risers come in the form of scratches, crazing (or micro-cracking), small cracks, razor cuts, etc. Normal expansion and retraction occurring during pressurization cycles will cause the stress risers to grow in length and depth. Left unrepaired, the damage will progress to a point beyond specified repair limits (which is the minimum thickness allowed before the structural integrity of the panel is violated), and may eventually cause window failure. It's your job and responsibility as a mechanic to point out stress risers to aircraft owners and explain the importance of having the risers removed as soon as possible."

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An example of how significant even minor scratches (stress risers) are, came in a form of a side cockpit window that was placed into operation after some of the vent holes were blocked during a paint operation. This window is normally vented with small holes that prevent the pane from fogging and allow pressure to equalize between the dead airspace in the window and the cabin during climb and decent. With the vent holes blocked, the window was pressurized from the inside and the result was the window blew out. Although the window blew as a result of the vent holes being blocked, it was interesting to note where and how the window actually cracked. The cracking occurred in a circular fashion that followed some minor cleaning scratches that were on the surface of the window. Normally, these scratches would have been insignificant, but in this case, they provided a perfect path for the window to crack.

Cracking from tool marks or bolt hole damage should also be another cause for concern. The mounting flange of the window itself should be inspected on a regular basis for any sign of cracking or damage. This can best be accomplished by using a prism to see beneath the window frame. Cracks in the outer edges or around bolt holes really should be detected well before entering the viewing area of the window. Since there are up to two inches of material under the frame of the window, a crack that has entered the viewing area is already up to two inches long — too late for comfort.

"In order to adequately inspect bolt holes on the edge of the windshield," says Cupery, "it is necessary to use a prism. The prism is held tightly against the outside of the windshield and glycol is used as a coupling agent. The prism bends the light and allows you to view any damage on the edge of the windshield or around boltholes that you otherwise could not have seen. One thing that you want to remember when using a prism is to clean the window well before using the prism. If you don't, you will have dirt particles that will scratch your prism and your window as you slide it around the edges."

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