A Clear Outlook

Inspecting the windows

AMTs should be familiar with the characteristics of acrylic materials and the composition of aircraft windows. Windows are a primary component of the aircraft structure and must be carefully maintained. There are some types of damage that an AMT should watch for each time they are around the aircraft.

To inspect properly, Cupery recommends at least a 500 candlepower light. Shine it at the window from every direction; up, down, sideways, forward, and upside down. You want the cracks or damage to reflect back to you. Ultrasonic equipment is also a necessary part of the inspection. You should make an ultrasound inspection of the window before making any repairs. This is to verify that there is enough material to make the repair. Then, inspect again after repairs to ensure windows are above minimum thickness. Cupery suggests that all aircraft windows are subjected to stresses that affect the weak point in the windows. Mounting bolt holes is an example.

Bolt hole cracks

Bolt holes are problematic. Cracks can occur from aircraft stresses and overtorquing mounting hardware. To adequately inspect bolt holes on the edge of the windshield, use a prism. The prism bends the light and allows you to view any damage on the edge of the windshield or around bolt holes. Clean the window well before using the prism or dirt particles will scratch your prism and window. Hold the prism tightly against the outside of the windshield and use glycol as a coupling agent.

Shelling/scaling

Shelling is the in-plane cracking of the acrylic panel that reflects back at you like the inside of an oyster shell when you shine light directly onto it. Typically this damage will be evident around bolt holes and in the outer radius areas of the windows. It may be caused by razor cuts, stress risers on the radius, or overtorquing bolts during installation and is referred to as scaling. It can cause structural failure and the window is usually removed from service.

Debonding

Debonding is a separation of two or more surfaces that have been bonded together through the use of specific bonding materials. This problem is noticed when moist air bypasses the desiccant system and fogging occurs. Once debonding begins, it quickly gets worse. Moisture builds in the bottom of the window and between the panels. As the aircraft goes to altitude, this moisture freezes and spreads the panes even further apart causing additional debonding. This is not a structural problem and is repairable; however, moisture can cause corrosion.

Delamination

Delamination is the separation of the acrylic or glass layer from the polyvinyl sheeting. Causes are usually stresses from normal airframe torquing, age, and UV degradation. Not considered a structural problem, it is actually a form of stress relief; however, as this problem progresses it may obstruct vision. Delamination can be distinguished from shelling because it doesn’t reflect back at you when you shine a light on it. Instead it just appears as clear bubbles, dull flat, or white discolored areas. Manufacturers have limits for how large an area is allowed.

Sealant damage

Window damage can occur by using the wrong types of sealants. Don’t ever let anyone talk you into using anything other than sealants that are approved by the manufacturer. Some quick curing sealants have accelerators that can attack the acrylic.

Others are alkaline and can also cause crazing. Cupery has found several cases of damage caused by someone using the wrong type of sealant.

A word of caution, if one window exhibits this type of damage, then most likely the others will be damaged as well. In fact, when you find damage on one always expect to find the same types of damage on the other windows.

Erosion

Don’t forget to inspect navigation and landing light lenses for erosion. These can be polished and hard coated. The motto is to see and be seen in our crowded skies.