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Why The Little Hole At The Bottom Of The Window?  
User currently offlineGh123 From , joined Dec 1969, posts, RR:
Posted (8 years 1 week 3 days 4 hours ago) and read 27522 times:

I have always wondered why there is a little hole at the bottom of the windows on planes. Is it for pressurisation purposes?

Thank you in advance.

29 replies: All unread, showing first 25:
 
User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 1, posted (8 years 1 week 3 days 4 hours ago) and read 27554 times:

Thread should technically be in the Tech/Ops forum...

Anyways, I believe it is for either moisture control or pressure equalization (or both).


User currently offlineDZ09 From United States of America, joined Sep 2006, 491 posts, RR: 1
Reply 2, posted (8 years 1 week 3 days 4 hours ago) and read 27518 times:

to get rid of condensation???

User currently offlineMalaysia From United States of America, joined Nov 1999, 3361 posts, RR: 0
Reply 3, posted (8 years 1 week 3 days 2 hours ago) and read 27133 times:

I hate crazing..... getting a window seat and seeing so much crazing on the window, but thats the actual one, not the plastic cover.


There Are Those Who Believe That There May Yet Be Other Airlines Who Even Now Fight To Survive Beyond The Heavens
User currently offlinePygmalion From United States of America, joined Jun 2006, 966 posts, RR: 38
Reply 4, posted (8 years 1 week 3 days ago) and read 26965 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.


User currently offlineCrownvic From United States of America, joined Oct 2004, 1935 posts, RR: 5
Reply 5, posted (8 years 1 week 3 days ago) and read 26880 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.


User currently offlineJetMech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 6, posted (8 years 1 week 2 days 23 hours ago) and read 26797 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.

View Large View Medium
Click here for bigger photo!

Photo © Rez Manzoori - FlightLineImages



[Edited 2006-10-17 08:42:14]


JetMech split the back of his pants. He can feel the wind in his hair.
User currently offline777ER From New Zealand, joined Dec 2003, 12218 posts, RR: 18
Reply 7, posted (8 years 1 week 2 days 22 hours ago) and read 26683 times:
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Thanks. I've always wondered what the small holes were for

User currently offlineGreasespot From Canada, joined Apr 2004, 3085 posts, RR: 20
Reply 8, posted (8 years 1 week 2 days 19 hours ago) and read 26506 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.

 no  Not all windows in the airplane are the same age. Some get replaced for damage. Crazing can and does affect groups of windows.

GS



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?"
User currently offlineEMBQA From United States of America, joined Oct 2003, 9364 posts, RR: 11
Reply 9, posted (8 years 1 week 2 days 18 hours ago) and read 26487 times:

Quoting Pygmalion (Reply 4):
The hole is there to equalize pressure between the inside of the cabin and the actual window which is the outer pane.

The hole is to prevent fogging and moisture from building up.

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.



"It's not the size of the dog in the fight, but the size of the fight in the dog"
User currently offlineDavid L From United Kingdom, joined May 1999, 9524 posts, RR: 42
Reply 10, posted (8 years 1 week 2 days 16 hours ago) and read 26440 times:

Quoting EMBQA (Reply 9):
This question came up earlier this year and I actually had to get AMM referances to prove these points.

It sure did:

http://www.airliners.net/discussions/tech_ops/read.main/146605


User currently offlineTWAL1011727 From United States of America, joined Mar 2006, 631 posts, RR: 0
Reply 11, posted (8 years 1 week 2 days 6 hours ago) and read 26253 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.


View Large View Medium

Photo © Rez Manzoori - FlightLineImages

Note the B747 picture and wonder...too bad the radome is there....that would be choice seats if there were windows on the nose.

KD


User currently offlineMissedApproach From Canada, joined Oct 2004, 713 posts, RR: 2
Reply 12, posted (8 years 1 week 2 days 5 hours ago) and read 26217 times:

Quoting Greasespot (Reply 8):
Some get replaced for damage.

Is there a specific criteria for replacement (ie. 40% obscured) or is it just done at the discretion of the mx personnel?



Can you hear me now?
User currently offline474218 From United States of America, joined Oct 2005, 6340 posts, RR: 9
Reply 13, posted (8 years 1 week 2 days 5 hours ago) and read 26204 times:

Quoting EMBQA (Reply 9):
Crazing is caused by UV light.

Volcanic ash (which is very caustic) is also a major cause of window crazing.


User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 14, posted (8 years 1 week 1 day 15 hours ago) and read 26086 times:

Quoting EMBQA (Reply 9):
Quoting Pygmalion (Reply 4):
The hole is there to equalize pressure between the inside of the cabin and the actual window which is the outer pane.

The hole is to prevent fogging and moisture from building up

A black mark on the side outboard of the hole indicates a leaking seal.Time for replacement.
regds
MEL



Think of the brighter side!
User currently offlineEMBQA From United States of America, joined Oct 2003, 9364 posts, RR: 11
Reply 15, posted (8 years 1 week 1 day 8 hours ago) and read 25982 times:

Quoting HAWK21M (Reply 14):
A black mark on the side outboard of the hole indicates a leaking seal.Time for replacement.

Not always. I've seen those marks created just by a high spot in the seal causing a disruption in airflow and dirt to build up.

Quoting 474218 (Reply 13):
Volcanic ash (which is very caustic) is also a major cause of window crazing.

I've never seen, heard or read that in any aircraft AMM. They all mention UV light.



"It's not the size of the dog in the fight, but the size of the fight in the dog"
User currently offline474218 From United States of America, joined Oct 2005, 6340 posts, RR: 9
Reply 16, posted (8 years 1 week 1 day 4 hours ago) and read 25916 times:

Quoting EMBQA (Reply 15):
I've never seen, heard or read that in any aircraft AMM. They all mention UV light.

A quick goggle search revealed numerous sites were the effects of volcanic ash on aircraft windows is discussed including:

http://pubs.usgs.gov/pinatubo/casa/index.html

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.


User currently offlineBoeingOnFinal From Norway, joined Apr 2006, 476 posts, RR: 0
Reply 17, posted (8 years 1 week 20 hours ago) and read 25851 times:

I did a search on wikipedia on carzing, but couldn't make much of it, except that it differs from a crack. Could someone explain to me the term cazing?

Thanks..



norwegianpilot.blogspot.com
User currently offlineBuzz From United States of America, joined Nov 1999, 697 posts, RR: 21
Reply 18, posted (8 years 1 week 20 hours ago) and read 25840 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.

g'day


User currently offlineBoeingOnFinal From Norway, joined Apr 2006, 476 posts, RR: 0
Reply 19, posted (8 years 1 week 20 hours ago) and read 25837 times:

Oh, I see, so all the "scratches" you see when looking through the window into the sun are crazes?

Also, as you explained above, some windoes are plastic. Is that the most common, or is that just the inner slide? How many layers of windows are there in the cabin, and in the flightdeck?

Sorry for hosing you down with questions, but I am quite interested in details like that  Smile



norwegianpilot.blogspot.com
User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 20, posted (8 years 1 week 19 hours ago) and read 25826 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.
regds
MEL



Think of the brighter side!
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17068 posts, RR: 66
Reply 21, posted (8 years 1 week 18 hours ago) and read 25819 times:

Quoting HAWK21M (Reply 20):
The Acrylic Inner pane is decorative

Doesn't it also protect the pax from cold middle panes?



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 22, posted (8 years 6 days 19 hours ago) and read 25647 times:

Quoting Starlionblue (Reply 21):
Doesn't it also protect the pax from cold middle panes

Thats not the Primary purpose.The Inner Pane is mounted with the Decorative liner Sidepanels.
regds
MEL



Think of the brighter side!
User currently offline474218 From United States of America, joined Oct 2005, 6340 posts, RR: 9
Reply 23, posted (8 years 6 days 15 hours ago) and read 25619 times:

Quoting HAWK21M (Reply 22):
Thats not the Primary purpose.The Inner Pane is mounted with the Decorative liner Sidepanels.

Mel, has the right answer, the inner pane is part of the decorative sidewall. The actual name for the inner pane is a "scratch pane" it is there to prevent scratches on the load carrying panes.


User currently offlinePygmalion From United States of America, joined Jun 2006, 966 posts, RR: 38
Reply 24, posted (8 years 6 days 11 hours ago) and read 25568 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 Post contains images 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)?
28 HAWK21M : Isn't it glass. regds MEL
29 EMBQA : Nope.. Polly-Carb Plastic. I have a Streamlite as well and know what you mean. My guess would be heat. That little light gets pretty hot and being ar
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