Making new windows for a 172

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Chris M
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Making new windows for a 172

Post by Chris M »

The club 172 is due for some new side windows. These aren't overly expensive for airplane parts but will still be in the $700 range for all 4 by the time we've got them here (waiting on quotes). Which got me thinking: The side windows are just flat stock plexiglass/acrylic cut to shape. That isn’t a hard thing to make, and plexiglass sheet is relatively inexpensive.

In regulatory terms, would it be permissible to cut our own windows and have them installed? What would I need to do to make sure it’s all good to go with the powers that be? I’m not looking to cut corners on something, just seeing is it’s a feasible option to save some money.
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PilotDAR
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Re: Making new windows for a 172

Post by PilotDAR »

First, review Airworthiness manual 571.06:

https://www.tc.gc.ca/eng/civilaviation/ ... s-1827.htm

And, appendix A of that standard:

https://www.tc.gc.ca/eng/civilaviation/ ... a-1893.htm

Read the intent of both of those carefully, the words are specifically chosen. With those in mind, then review AC43.13-1B, Chapter 3:

https://www.faa.gov/documentLibrary/med ... .13-1b.pdf

Particularly 3-24, 3-27, and 3-40.

Then, confer with the person who will be asked to sign out the repair, will they entertain your approach?

That should get you started in your decision...
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ahramin
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Re: Making new windows for a 172

Post by ahramin »

An excellent post PilotDAR. I've been planning on making a new front windscreen and here is the paragraph from 43.13 I found relevant:

REPLACEMENT PANELS. Use material equivalent to that originally used by the manufacturer of the aircraft for replacement panels. There are many types of transparent plastics on the market. Their properties vary greatly, particularly in regard to expansion characteristics, brittleness under low temperatures, resistance to discoloration when exposed to sunlight, surface checking, etc. Information on these properties is in MIL-HDBK-17A, Plastics for Flight Vehicles, Part IITransparent Glazing Materials, available from the Government Printing Office (GPO). These properties are considered by aircraft manufacturers in selecting materials to be used in their designs and the use of substitutes having different characteristics may result in subsequent difficulties.
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Heliian
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Re: Making new windows for a 172

Post by Heliian »

Buy the prefab windows.

I would charge you more than $700 to hand make the windows anyways. 2 sheets of the transparent material is several hundred dollars(that is for the standard mil spec, it would seem that the material needs to have a mil spec as opposed to consumer grade, also needs traceability). and then the labour @ $100/hr, just to fab the windows and then installation fees.

Like all aircraft parts, you're paying for the certifications and not just the basic material.
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edmanster
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Re: Making new windows for a 172

Post by edmanster »

The certification will be at least the price of the new pre-cut parts.

I would also like to mention a substitute material LEXAN.
We used to use lexan all the time in those 'simple days'. Lexan is a superior product because if i remember correctly -
1/ it does not craze. 2/ When bent - it does not explode in shards - it just bends .. 3/ better fire burn characteristics. 4/ (usually an issue with windshields) does not crack - or have a crack run across until stop-drilled. 5/(not sure) - but will not melt or 'go milky' when wiped with solvents (especially MEK).
.. none of these issues too important for single-engine Cessna side windows. But one important consideration - we used to avoid lexan on float planes (side windows)
because it was extremely difficult to 'kick out' when the aircraft was submerged.
Plexi would break into shards when kicked - cut ur leg or arm to bits - but would eventually allow access to outside.
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Chris M
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Re: Making new windows for a 172

Post by Chris M »

Thanks for the input guys. Like I said, not looking to skimp on things, I was just wondering if it was a feasible alternative. I'll still speak with our mechanic about it but suspect we'll just order the new windows from the usual suspects.

Cheers!
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PilotDAR
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Re: Making new windows for a 172

Post by PilotDAR »

cut ur leg or arm to bits
Is a "ur" a "your" which is missing half the word?

Quoting in part from AWM 571.06 (5) (c):

" .......the design data used for the making of the part may be established by inspecting and testing to determine the correct:

(i) materials; ......"

Lexan is not a correct substitute for plexiglas. It's a different product, superior in some ways, and inferior in others.

Interesting thinking about kicking out a side window, but for my experience on Cessna singles, most side windows big enough to escape through usually either open, or are in a door which does. These windows are not intended to be kicked out for emergency exit, I'd focus exit effort on getting a door open as the design intends. But I still would not use Lexan for those windows either.
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edmanster
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Re: Making new windows for a 172

Post by edmanster »

*ur* .. it the Millienum version of 'your'
- pretty hard to open the side door in a 172 / 185 when the guys sitting in the fwd seats not moving & you are sitting in the aft seat. Also note the big fixed flat side windows adjacent to you. ... at least on the Cessnas i used to deal with (& also since these windows are not to be used as an escape route .. I will just sit here & wait until help comes instead) .. .. and the part about "correct materials" - this is what u call a limited STC / substitution of superior materials. Gee .. I would prefer something that does not craze, crack or melt when the paint shop does a poor job taping the windows ... Some day when u have to drill a hole in some transparency (OAT gauge on some older windshields for example) - you will wish the material u drilled through did not crack so abruptly.
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Heliian
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Re: Making new windows for a 172

Post by Heliian »

edmanster wrote: Mon Feb 05, 2018 8:16 am *ur* .. it the Millienum version of 'your'
- pretty hard to open the side door in a 172 / 185 when the guys sitting in the fwd seats not moving & you are sitting in the aft seat. Also note the big fixed flat side windows adjacent to you. ... at least on the Cessnas i used to deal with (& also since these windows are not to be used as an escape route .. I will just sit here & wait until help comes instead) .. .. and the part about "correct materials" - this is what u call a limited STC / substitution of superior materials. Gee .. I would prefer something that does not craze, crack or melt when the paint shop does a poor job taping the windows ... Some day when u have to drill a hole in some transparency (OAT gauge on some older windshields for example) - you will wish the material u drilled through did not crack so abruptly.
Lexan is not a proper substitute and unless you got it certified, it is definitely NOT an LSTC. Again, you need a MIL spec reference in the first place.

As far as drilling plastic, it's easy, use the correct tools. I have drilled thousands of holes in "plexiglas" and have yet to crack one.

You need to find a new paint shop if they keep messing up your windows. The only time I saw a window destroyed was by a young guy like yourself, who knew it all and could do everything faster. It's not a race.
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robertw
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Re: Making new windows for a 172

Post by robertw »

To "make" a part per CAR 571.06 is actually quite difficult. It states:

(5) The person certifying a repair or the incorporation of a modification which includes the making and installing of a part must ensure that the following standards of airworthiness have been complied with:

(a) except as provided in paragraphs (b) and (c), all repair parts must conform with the applicable type design data. Where documents such as Maintenance Manuals, Structural Repair Manuals, or other service information do not provide all of the information required to fully describe the attributes of the part, it is necessary to obtain a copy of the manufacturer’s drawings and all associated specifications;

(b) no repair part made under these provisions shall be marked with the part number specified in the type design;

(c) subject to subsection (6), where parts are no longer in production by the manufacturer of the part, or an authorised representative, and the type design data for the part is not available, the design data used for the making of the part may be established by inspecting and testing to determine the correct:

(i) materials;

(ii) dimensions;

(iii) hardness and temper;

(iv) surface finish; and

(v) protective coatings.


In this case, it appears that you can still get windows from Cessna, so you would need Cessna's drawings to make the said window, and I doubt they would be willing to fork them over. Making windows (that last) is not a simple process. I know. The organization that I work for makes them. As other posters have noted, there are several Mil specs out there (I am familiar with MIL-P-8184F and MIL-P-5425E) and each has it's strengths. The process to cut, drill and form them is not simple. Cutting the window profile out and ensuring that the edges have been properly sanded / polished to ensure that cracks do not form is required. Drilling holes with incorrect drill bits or technique will cause chipping around the holes, which lead to cracks very soon down the road. If there is even a 1 degree deflection in the mounting surface, and the window has not been thermally formed and stress relieved properly, the window will fail pre-maturely (likely on installation).

I would suggest that in the effort to save a couple of hundred bucks you will end up spending several thousand and end up with a product that does not meet regulatory requirements, and also is substandard that will fail prematurely. Just pay for the ones already certified.
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PilotDAR
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Re: Making new windows for a 172

Post by PilotDAR »

this is what u call a limited STC / substitution of superior materials. Gee .. I would prefer something that does not craze, crack or melt when the paint shop does a poor job taping the windows ...
I am extremely acquainted with serialized STC's which have replaced limited STC's. It could be possible to have a serialized STC issued for a material change. The cost to document this and demonstrate design compliance would likely exceed the cost of buying the Cessna parts.

If I'm worried about a paint shop damaging a window, I'll change paint shops, not windows.
Some day when u have to drill a hole in some transparency (OAT gauge on some older windshields for example) - you will wish the material u drilled through did not crack so abruptly.
If I need to drill a hole, I'll use the correct drill and technique, and not worry about the window cracking.
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