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)."
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."
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."
"As a mechanic, you need to look at the windows, not through them," he stresses. "To inspect properly, we recommend at least a 500 candle power light and shine it at the window at every direction; up, down, sideways, forward, upside-down, etc., to try to get any cracks or damage to reflect back at you."
Ultrasonic equipment is also a necessary part of the inspection of any window. You should ultrasound the window before making any repairs to verify that there is enough material to make the repair. Then, inspect after any repairs to make sure the windows are above minimum thickness.
One word of caution related to ultrasonics, however. The density of the material used in the window will affect the reading you get on the ultrasonic equipment. So, you have to know what you're doing and make sure that your equipment is calibrated according to data from the manufacturer.
According to Cupery, "You have to compensate for the type of material you're working on to interpret what actual thickness you have. There are standards available to check your equipment and you have to use the correct standards or you can get bad readings. So, you not only need an ultrasound machine, you need to have data and the correct standard for the material you're working on. If you don't, you may be .030 below minimums and think that you are within limits. Or, you may unnecessarily reject an entire set of windows because you think windows are beyond limits, when they are actually still serviceable.
Types of damage
Probably the most common reason for removing windows from service is crazing. Crazing is "micro-cracking" that occurs at or just beneath the surface of the acrylic. Typically, crazing can be seen but not felt. Stress, UV degradation, airborne volcanic acids, or chemical damage may cause it. Crazing, if not addressed, can not only lead to window failure; it can also lead to cloudiness or deterioration of clarity. Suffice it to say that it is not just a cosmetic problem.
And, it should not only be a concern on outer window panels. According to Cupery, "Although many recognize crazing of outer panels as being dangerous, they don't take crazing or damage of the inner panel passenger windows seriously. Yet, we have seen time and time again where the outer panel failed and the inner panel prevented total failure of the window. Although the inner panel may not seem significant to many, it is critical that it be well-maintained as it is a crucial backup in the event of an outer panel failure."