A clear view of window maintenance requirements
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, right? Well, almost. One thing that many technicians tend to forget is that the window is a critical part of the aircraft "pressure vessel," and as such, they require at least as much attention as the surrounding structure. Additionally, a critical portion of the window is actually hidden from view by mounting flanges and fasteners — ignoring these areas is asking for trouble.
Bob Cupery, founder of Aircraft Window Repairs, with offices in California and Florida, says that a small crack or crazing can lead to in-flight failure of windows, which has proven to be catastrophic. In fact, several deaths have been attributed to window failures as a result of pilots or passengers getting pulled through openings.
Due to the relatively large amount of pressure placed upon windows at altitude, cracks and/or damage must be addressed on a regular schedule. Cupery explains the critical nature of pressurized windows and the need to replace any windows that exhibit cracking or damage.
"If you have a crack, there are pressures from the inside of the window, as well as from all directions from the mounting holes. All of these stresses mean that a window can quickly fail. To give you an example of what kind of damage a window can take prior to failing, consider the Gulfstream window. You start with a window that is 20 by 26 inches, which is 520 square inches, and with 7 psi of cabin pressure, that's 3,640 lbs. of force pushing on the window. If there is a scratch on the window, it becomes a stress riser and all stresses congregate in it. Allowable damage to a Gulfstream window is only .003 inches deep before the window must be repaired or replaced. Some manufacturers allow scratches up to 0.010 inches deep. The maximum scratch or crazing allowed in the Hawker cabin is limited to 1/4 inch."
Unfortunately, windows are not becoming more carefree as time goes on. It's just the opposite — windows actually require more attention today. Cupery explains, "Aircraft are being built to fly at higher altitudes, which means more pressurization to keep the aircraft cabin at comfortable altitudes. Additionally, aircraft windows are milled to finished thicknesses and can be made to much tighter tolerances than they used to be, which helps reduce the weight of the windows. The result is the windows are thinner than ever before. Windows of the past were built with up to one hundred thousandths of additional material for cushion — not so today."
The advent of stretched acrylic has reduced the incidents of window blowout significantly. In the 1950s, it was more common for windows to actually blow out because the windows were not stretched acrylic.
Today's pressurized windows are made only of stretched acrylic. This is acrylic that is heated and stretched to form a clear window with a long molecular structure. The directional grain structure adds rigidity and strength to the windows.
"Typical manufacturers start out with a billet of acrylic which is Mil-P-8184 cast acrylic," Cupery explains. "The billet is inserted into an oven and heated to 350 degrees F (until it is malleable), and is brought directly to the stretching machine before it cools. The stretching process is stopped immediately if there are any flaws, tears, or defects noted. Any stretched panels with defects are discarded immediately and never reused, which is one of the contributing factors to the high cost of stretched acrylic. In its stretched form, the acrylic becomes Mil-P-25690 (stretched acrylic)."