Aircraft Windows

April 1, 2001

Aircraft Windows

Don't let their simplicity fool you

By Joe Escobar

April 2001

Aircraft windows — such simple looking aircraft components. Because of their apparent simplicity, their importance and criticality can sometimes be taken for granted. But if you end up having to replace a windshield due to negligence or damage, you will probably never take them for granted again. With some aircraft model windshields easily costing 25,000 dollars, it can be a very painful lesson to learn.

Oh, the pressure!
In terms of area, some windshields are the largest components on the aircraft pressure vessel that do not have any other structure supporting them like ribs or stringers. The cabin pressures that they must endure make even small defects critical. Any flaw can quickly lead to disaster if not attended to.

Tough operating conditions
Windshields and windows endure some harsh operating conditions. They are subject to temperature extremes that can occur within minutes. At high altitudes, they are subject to higher amounts of damaging UV radiation. Some scientists even state that minute particles of volcanic ash from major eruptions such as Mt. St. Helens linger in the upper atmosphere and are causing accelerated crazing of aircraft windows. They are even subject to damage on our part during routine maintenance. Using paper towels, shop rags, or unapproved cleaning products can also cause unwanted damage when cleaning the windows.

Preventing Damage
Now that we are aware of the cost of replacement windows, and the severity of defects, how can we prevent damage to windows? Well, one of the first steps is through employee awareness. If new employees are not aware of proper window maintenance techniques, then they are more likely to cause damage. Even non-maintenance personnel like the aircraft cleaning crew need to be made aware of proper window cleaning techniques.
"There are plenty of products that you can use that are not very favorable to windows," says Greg Piland of Wichita, Kansas-based Lee Aerospace, a repair station that specializes in aircraft window inspection and repair. Piland claims that much of the damage that windows sustain is from routine cleaning.
"People need to be aware of just how scratch sensitive windows are. Bug parts and dirt can cause an enormous amount of damage if they are rubbed all over the window while cleaning it,"adds Piland. Also, a fresh water rinse is very beneficial prior to wiping windows.
When cleaning the windows, use only approved products. In many cases, that is just a mild soap and water solution. Some commercial window cleaning products that work great on your bay window at home can cause an extreme amount of damage to acrylic aircraft windows. Always refer to the manufacturer’s maintenance manual.
As far as the cleaning material, you need a very soft cloth. Piland says that there are quite a few products available commercially that work extremely well. You want it to be soft, absorbent, and non-abrasive. Sheila Wadden agrees. She is a technician with Micro-Surface Finishing Products, Inc., a Wilton, Iowa company that manufactures Micro-Mesh products and also offers aircraft window repair services.
"Something that I try to tell all of the technicians I talk to is not to use paper towels," explains Wadden. "It is a wood pulp product, and is going to give you light hazy scratches if you use it."

Window inspection
Proper window inspections are essential in any maintenance program. With thorough inspections, defects can be detected before they have a chance to become a safety of flight issue.
The first step in any inspection is a thorough cleaning. A dirty window can mask defects. You are wasting your time if you try inspecting a window that has bug guts, dirt, and other debris all over it.
A good light source is another essential inspection tool. Dollar store flashlights will not work in this instance. It needs to be a fairly high-intensity light in order to provide adequate illumination during the inspection process.
Wadden shares some inspection tips. "You need to look at the window from all angles and viewing directions. Inspecting windows from the outside and inside is necessary." She also adds that having the aircraft outside is the best possible thing when looking for crazing as sunlight tends to highlight it.
Lee Aerospace’s Piland stresses the importance of referring to the maintenance manual. "They usually include limits for damage like scratches and crazing," he says. "Some even include optical requirements — giving limits on window distortion."
"The technicians should spend quite a bit of time looking around the profile of the window," offers Piland. "We’ve seen over the years that a lot of damage is right around the edges — either from where it’s been painted previously, a possible chemical burn, stripper burn, or razor blade cuts from where it has been masked and then unmasked."
He adds that the background can affect being able to detect flaws. "If you’re looking at damage from the inside of the window, and you’re up against a big white wall, it can be difficult to see some of the scratches and damage. It can actually be better in that case to stand outside the airplane and look in, identifying and circling where any defects are. You can then go inside and verify where they are and then measure them to see if they exceed published criteria."

What to do if damage is found
If you find damage during your inspection process, what do you do? Once again, you go back to your maintenance manual. It will usually give you damage criteria. There are basically three situations involved if you have defects on your window. If you are lucky, the damage will be within allowable limits and nothing will need to be done. In other cases, the damage exceeds limits for leaving as is, but is still within repairable limits. In the worse case scenario, the damage exceeds all limits, and will need to be replaced.

Getting your measurements
Once you know what your limits are, you need to get measurements in order to determine what needs to be done. For crazing, this usually entails obtaining a surface measurement. If the maintenance manual allows crazing, it often gives limits in terms of area.
For scratches, cracks, and impact damage, the process is a little more involved. First of all, the window thickness needs to be determined. An ultrasonic thickness gauge is usually used. Never rely on published thicknesses due to variances in manufacturing tolerances and the possibility that rework has already been done.
"A lot of people are not aware that many aircraft windows are laminated," warns Piland. "They may have a minimum thickness for the individual plies as well as an overall thickness minimum."
If you are measuring using an ultrasonic thickness gauge, it will measure through one ply. You need to take measurements from both the outside and inside at the same areas and then add them together for total thickness.
Once the thickness of the window is known, the depth of the defect needs to be determined. This is usually accomplished by using an optical micrometer. Other than in cases of severe impact damage, it will allow you to get a depth measurement of the defect.

Window repair
Your measurements will provide the diagnosis for the window. If the defect is above limits for leaving as is, but below the limits that require replacement — you have the option to repair the window.
A word of caution: Experience is essential to repairing windows, especially windshields. Just because you have removed a scratch from a cabin window on a King Air, don’t take it for granted that you can go and work on a damaged Learjet windshield. There are many techniques concerning damage removal that only experience can teach. Everyone I talked to agrees on one very important point — If you don’t have the experience or the equipment to repair the window then don’t attempt doing yourself. The facilities that specialize in window repair say that time and time again they see windows come in that someone attempted to repair at the site. In almost every case, the repair ends up costing much more than it would have if they would have just sent it in to begin with. Many times, the damage done is beyond repair.

Damage removal
As far as damage removal goes, there are several products out there to aid in the process. One of the most widely recognized ones is Micro-Mesh. Basically, the process involves starting with an abrasive around 1500 grit to remove the defects, and working your way down to a very fine polishing mesh until a polished surface is obtained.
There are several things to remember concerning damage removal. First of all, do not concentrate your effort in one small area. Doing this results in distortion that will be very hard to correct. Any repair needs to take place over a large area with gradual blending in order to avoid this effect. Also, removing the damage first with the coarsest material is essential. Spending too much time with a finer grit trying to remove a defect will cause more problems than if a courser grit was used to begin with.
Another thing to be aware of is the effect that window angle has on imperfections. The slope angle of the window magnifies any optical imperfections in the window. A slight distortion in a cabin window that is straight up and down may not be noticeable. The same type imperfection in a highly slanted windshield will be magnified. When inspecting windshields after repair, be sure to tilt them when inspecting for optical clarity. Some manufacturers even require this in their maintenance manual.

Pre-buy inspection
With more and more people realizing the cost of windows, many are choosing to have an experienced technician inspect their windows prior to a purchase. Lee Aerospace offers this service. They will go out on request and perform a pre-buy inspection for window condition. This allows any problem areas to be addressed prior to the purchase — possibly saving thousands of dollars.
Greg Piland states, "Just like you wouldn’t want me performing a pre-buy inspection on an aircraft engine, many technicians in the field are not able to perform as thorough a job as we can when it comes to a pre-buy inspection. There are sometimes issues that involve airworthiness that may not be easily evident."

Be careful!
A closing thought has to be "Be Careful!" This pertains to performing routine maintenance on windows, inspecting them, and attempting repairs. With care and attention, these critical aircraft components can give you many years of trouble-free use. Window Crazing

Crazing is a form of heat damage that occurs in a transparent thermoplastic material.
Crazing appears as a series of tiny, hair-like cracks in the surface of the plastic. If a material is heated non-uniformly, one side will expand more that the other, and the stresses caused by the uneven expansion produces the crazing.
Crazing destroys the strength of the material and makes it difficult to see through.

Dictionary of Aeronautical Terms, Second Edition

The Source

Additional resources....
AC 43.13-1B

Lee Aerospace, Inc.
9323 E. 34th Street North
Wichita, KS 67226-2621
(316) 636-9200

Micro-Surface Finishing Products, Inc.
P.O. Box 70
Wilton, IA 52778
Toll Free (800) 225-3006

Aircraft Window Repairs
2207 Border Ave.
Torrance, CA 90501
(310) 212-7124

Perkins Aircraft Services
2300 W. 6th St.
Fort Worth, TX 76107
(800) 880-1966