Pygmalion From United States of America, joined Jun 2006, 956 posts, RR: 38 Reply 4, posted (7 years 1 month 4 weeks 10 hours ago) and read 19953 times:
The hole is there to equalize pressure between the inside of the cabin and the actual window which is the outer pane. The inner pane is just to keep you crazy pax from scratching the outer one which could make it crack.
The crazing in the outer window is from acid in the upper atmosphere. Boeing now uses glass laminates instead of just acrylic windows to help prevent crazing. The acid in the upper atmosphere comes mostly from volcano eruptions (really!). The particles from eruption react with moisture and create acid (much like acid rain) It plays hell with the windows and the paint gloss for about 4-5 years after an eruption. The effect is encountered across the globe as the upper level winds circulate the acids. Boeing has done studies since Pinatubo and St Helens and you can correlate the crazing and paint degradation peaks with the eruptions.
Crownvic From United States of America, joined Oct 2004, 1728 posts, RR: 6 Reply 5, posted (7 years 1 month 4 weeks 9 hours ago) and read 19894 times:
Pygmalion...While I do agree with you that crazing is caused by this, the biggest offender of this problem is sloppy maintenance...I have been on many aircraft where you could not see out of the windows, not because of crazing, but becuase of deep scratches and gouges that occurred during maintenance. Sometimes during polishing and cleaning, the workers carelessly hit the windows with no regard for the damage. To prove this, you can see that some windows are fine where a group of windows is bad. If it were a crazing problem, ALL of the windows on the a/c would have fairly equal damage.
I recently flew on a Mesa CRJ-900 that was fairly new, yet the windows were the worst I have ever seen on an airliner.
JetMech From Australia, joined Mar 2006, 2617 posts, RR: 53 Reply 6, posted (7 years 1 month 4 weeks 8 hours ago) and read 19810 times:
Quoting Pygmalion (Reply 5): The hole is there to equalize pressure between the inside of the cabin and the actual window which is the outer pane. The inner pane is just to keep you crazy pax from scratching the outer one which could make it crack.
IIRC from my major maintenance days, the Boeing 747 windows were designed as such. Two panes of acrylic were assembled together with a plastic or rubber seal. The seal went around the entire circumference of the two panes and separated them by about 0.5".
This entire assembly was placed into the window cutout forging and was held in with several spring clips that were screwed into the circumference of the cutout forging. The seal was designed so that there was no direct contact between the acrylic panes and the cutout forging.
The small hole was located on the lower part of the inner or secondary pane. The purpose of the hole was to transfer pressurisation loads to the outer or primary pane. If the outer primary pane failed, the pressurisation loads were then automatically transferred to the inner or secondary pane. I seem to remember that the 747 had a third pane that was part of the plastic reveal located on the cabin sidewall panels. This inner-most pane was known as the "scratch pane", and it was this one that prevented scratches and damaged to the two pressure panes.
Interestingly, the first 10 or 11 windows on the nose of the 747 were firmly bolted into the cutout forgings. This prevented the panes being smashed in if struck by a bird. When viewing a 747 from the front, the first 10 or 11 rows of windows have a slight amount of "frontal area" due to the tapering of the nose.
Greasespot From Canada, joined Apr 2004, 3066 posts, RR: 22 Reply 8, posted (7 years 1 month 4 weeks 4 hours ago) and read 19520 times:
Quoting Crownvic (Reply 5): To prove this, you can see that some windows are fine where a group of windows is bad. If it were a crazing problem, ALL of the windows on the a/c would have fairly equal damage.
Not all windows in the airplane are the same age. Some get replaced for damage. Crazing can and does affect groups of windows.
Sometimes all you can do is look them in the eye and ask " how much did your mom drink when she was pregnant with you?"
TWAL1011727 From United States of America, joined Mar 2006, 610 posts, RR: 0 Reply 11, posted (7 years 1 month 3 weeks 6 days 15 hours ago) and read 19267 times:
Quoting JetMech (Reply 6): Interestingly, the first 10 or 11 windows on the nose of the 747 were firmly bolted into the cutout forgings. This prevented the panes being smashed in if struck by a bird. When viewing a 747 from the front, the first 10 or 11 rows of windows have a slight amount of "frontal area" due to the tapering of the nose.
Under the heading: LONG TERM DAMAGE, it explains a year after the Pinatubo eruption there was a marked increase aircraft acrylic window crazing, attributed to the increase of sulfuric acid droplets in the atmosphere.
Buzz From United States of America, joined Nov 1999, 697 posts, RR: 23 Reply 18, posted (7 years 1 month 3 weeks 5 days 5 hours ago) and read 18854 times:
Hi BoeingOnFinal, Buzz here. Crazing is a microscopic surface cracking, usually only about .005 deep. On some older plastic side windows we were allowed to polish it out... takes more than a few hours to grind the plastic down and polish it. In the last 10 years we've replaced most of our passenger windows with a different version, has a tougher coating on the outside.
I'll have to disagree about the acid effect, or UV light causing a lot of the crazing. It has an effect, and Lexan tends to suffer rapidly from UV light.
Volcanic ash causes a lot of other things, but the side windows aren't rapidly eroded by it.
The fastest way to craze a plastic window seems to be regularly hose it with hot glycol (deice the airplane). The temperature differential between the surface and the rest of the plastic causes some crazing. We're supposed to spray the area above the windows and let the hot glycol wash down.
The forward windshields usually have a glass layer for abrasion resistance. I haven't seen a crazing problem there. Other problems... but they don't craze.
Does that answer the question? It's hard to see through a crazed window, the sunlight reflects off of all the micro-surface-cracks and of course your eye looks at the brightest things first.
HAWK21M From India, joined Jan 2001, 31457 posts, RR: 57 Reply 20, posted (7 years 1 month 3 weeks 5 days 4 hours ago) and read 18840 times:
Quoting BoeingOnFinal (Reply 19): How many layers of windows are there in the cabin, and in the flightdeck?
On a B737 there are Three.The Middle & outer pane carry the loads.The Acrylic Inner pane is decorative.As for the Flight deck #1,2,4&5 L&R Windows are Two Glass layered centred with PVC & Heating Element. #3 L&R is unheated & of Acrylic.
Pygmalion From United States of America, joined Jun 2006, 956 posts, RR: 38 Reply 24, posted (7 years 1 month 3 weeks 3 days 21 hours ago) and read 18582 times:
Quoting EMBQA (Reply 9): Quoting Pygmalion (Reply 4):
The crazing in the outer window is from acid in the upper atmosphere
Crazing is caused by UV light.
This question came up earlier this year and I actually had to get AMM referances to prove these points.
If you have access to the AMM, then look up the Service Letter for Chapter 56 (Windows) There is an all model service letter on accelerated crazing from upper atmosphere acids from 1996 (still current and effective). THough I cant quote it here, it has a good discussion on where crazing comes from and why it gets accelerated.
25 EMBQA: First, I know my ATA Chapters by heart.. pretty sad. Pax Windows are 56-20. I just searched ALL of the Embraer Manuals...the only ones I can check fr
26 HAWK21M: Any link possible.Understandably would be tough A reference Chapter-Subject-Section would do. regds MEL
27 DC8FriendShip: That why my streamlight lenses craze, or is it skydrol exposure (or both)?