Airframe Technology: A Clear Outlook

Sept. 5, 2014
Repair tips from Aircraft Window Repairs' Bob Cupery

My first aircraft window job was to resurface several cabin windows on a Boeing 707.  Repairing and polishing aircraft is a delicate, multi-step, monotonous job. We started the process with soft rubber sanding block and 2000 grit paper, with strict instructions from the crew chief to make vertical sanding strokes only.

For those that have done this job you know 2000 grit paper is about as abrasive as cigarette smoke and takes ages to make much progress. After about three hours of sanding my teammate decided that an orbital pneumatic sander with a felt pad and some buffing compound would be a better choice for this job. He applied the sander to the window and after a few quick revolutions; the window turned a white opaque color and glazed rock hard. The end result of that maneuver was the window had to be replaced, my teammate incurring the wrath of the old crew chief and a quick trip to the bottom of the B list.

If you surveyed the OEMs, the back shops for the airlines, and the window repair companies, you would find that servicing, repairing and replacing aircraft windows is big business. There is a lot of tribal knowledge and expert craftsmanship associated with repairing aircraft windows.  Bob Cupery, founder and current CFO and QA manager for Aircraft Windows Repairs of Torrance, CA, is the guru of repairing aircraft windows.

Cupery is one of us and a great American success story. He was an AMT for Northwest and DOM for Northrop, and listed in Who’s Who in America Industry and Finance. Bob started business in his garage and it became the first FAA certified repair station solely dedicated to the repair of acrylic windows and lenses for pressurized aircraft. Cupery and his wife Kathi have been in the business for about 32 years. I asked him to pass along some of his aircraft window repair expertise with a focus on window maintenance and the different types of damage that AMTs should look for. He quickly agreed and provided an abundance of valuable information. 

Windows surround us; they literally are part of almost every structure built to move or shelter people, cars, boats, buildings, and aircraft. We rarely notice them until we can’t see through them. In fact, windows are so much a part of our lives we often think they are all alike. However, when we as AMTs start treating all windows the same we’re asking for trouble. For one thing, not all windows are made of glass. With few exceptions, aircraft windows are formed from stretched acrylic which provides a relative inexpensive, tough, shatter proof transparency ideal for use on airplanes. What’s more (and something many people forget); aircraft windows are primary structures carrying the same loads as the metal fuselage.

Cleaning and care

Acrylic aircraft transparencies require careful treatment and protection. They also discolor with age. Even with a conscientious maintenance program, you may still notice some discoloration – yellowing or milk-colored. Such color cannot be removed without weakening the acrylic and eventually you’ll have to retire those old panels. Nevertheless, you can extend the useful life of aircraft windows by following a few suggestions.

First, never use an ammonia-based glass cleaner on acrylic windows. Ammonia may be great for glass, but it’s hard on plastic. Even approved plastic cleaners in aerosol applicators should be avoided. While the cleaning agent is safe, the majority of aerosol chemicals can damage acrylic.

To clean a pane properly, apply an approved plastic cleaning agent to the windows. Ensure that you fold the towel over each time you wipe across the window presenting a clean side of the towel to the surface on each pass; this will remove the abrasives and safely buff the window. Sure, you’ll use a few more paper towels and a bit more cleaner but that costs a lot less than having to refinish your windows or purchasing new ones.

When it come to inspections Cupery recommends that an AMT be very diligent and always make a careful, detailed inspection of all windows. Always, remember certain window conditions and minor damage can accelerate to a condition that requires an expensive replacement.  

Inspecting the windows

AMTs should be familiar with the characteristic 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 500,000 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 of any window. 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, it is necessary to 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 that you otherwise could not have seen. 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 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. This is a condition that can cause structural failure and the window is usually removed from service.


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 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 here, 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 window always expect to find the same types of damage on the other windows. 


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

I asked Cupery to pass along a bit of his experience to new AMTs and someone wanting to become a window repair specialist. “For the AMTs, when you are working around windows, please think before you act.” He has seen many windows in his shop that someone polished and distorted. These windows cannot be corrected and all you can do is chase the distortion around the window.

Aircraft windows are strong but can be easily damaged by a careless act: dropping a tool, dragging an air hose or electrical cord across a window, or wiping it with a dirty shop towel. If you are measuring window damage, measure twice. Even new windows can be at OEM limits. For the person considering becoming a window repairman, this is craftsman work and a labor of love. Repairing and polishing windows is intense, meticulous, unforgiving work. According to Cupery, it is a worthwhile effort “because a clean and polished window in an expensive corporate jet is a safe and beautiful thing.”

Charles Chandler was assisted  by Bob Cupery in writing this article. For more information on Aircraft Window Repairs, visit or call (310) 212-7173.