gh123
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Why The Little Hole At The Bottom Of The Window?

Tue Oct 17, 2006 10:19 am

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.
 
N231YE
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RE: Why The Little Hole At The Bottom Of The Window?

Tue Oct 17, 2006 10:23 am

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

Anyways, I believe it is for either moisture control or pressure equalization (or both).
 
dz09
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RE: Why The Little Hole At The Bottom Of The Window?

Tue Oct 17, 2006 10:24 am

to get rid of condensation???
 
malaysia
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RE: Why The Little Hole At The Bottom Of The Window?

Tue Oct 17, 2006 12:44 pm

I hate crazing..... getting a window seat and seeing so much crazing on the window, but thats the actual one, not the plastic cover.
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pygmalion
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RE: Why The Little Hole At The Bottom Of The Window?

Tue Oct 17, 2006 2:23 pm

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
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RE: Why The Little Hole At The Bottom Of The Window?

Tue Oct 17, 2006 2:35 pm

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.
 
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jetmech
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RE: Why The Little Hole At The Bottom Of The Window?

Tue Oct 17, 2006 3:39 pm

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.

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[Edited 2006-10-17 08:42:14]
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RE: Why The Little Hole At The Bottom Of The Window?

Tue Oct 17, 2006 4:25 pm

Thanks. I've always wondered what the small holes were for
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greasespot
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RE: Why The Little Hole At The Bottom Of The Window?

Tue Oct 17, 2006 7:47 pm

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
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EMBQA
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RE: Why The Little Hole At The Bottom Of The Window?

Tue Oct 17, 2006 8:24 pm

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.
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David L
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RE: Why The Little Hole At The Bottom Of The Windo

Tue Oct 17, 2006 10:40 pm

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
 
twal1011727
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RE: Why The Little Hole At The Bottom Of The Window?

Wed Oct 18, 2006 8:31 am

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.


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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
 
MissedApproach
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RE: Why The Little Hole At The Bottom Of The Window?

Wed Oct 18, 2006 9:34 am

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?
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474218
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RE: Why The Little Hole At The Bottom Of The Window?

Wed Oct 18, 2006 9:57 am

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

Volcanic ash (which is very caustic) is also a major cause of window crazing.
 
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HAWK21M
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RE: Why The Little Hole At The Bottom Of The Window?

Wed Oct 18, 2006 11:41 pm

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
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EMBQA
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RE: Why The Little Hole At The Bottom Of The Window?

Thu Oct 19, 2006 6:33 am

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.
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474218
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RE: Why The Little Hole At The Bottom Of The Window?

Thu Oct 19, 2006 10:14 am

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.
 
BoeingOnFinal
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RE: Why The Little Hole At The Bottom Of The Window?

Thu Oct 19, 2006 6:17 pm

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..
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Buzz
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RE: Why The Little Hole At The Bottom Of The Window?

Thu Oct 19, 2006 6:40 pm

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
 
BoeingOnFinal
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RE: Why The Little Hole At The Bottom Of The Window?

Thu Oct 19, 2006 6:47 pm

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
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HAWK21M
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RE: Why The Little Hole At The Bottom Of The Window?

Thu Oct 19, 2006 7:49 pm

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
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Starlionblue
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RE: Why The Little Hole At The Bottom Of The Window?

Thu Oct 19, 2006 8:28 pm

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

Doesn't it also protect the pax from cold middle panes?
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HAWK21M
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RE: Why The Little Hole At The Bottom Of The Window?

Fri Oct 20, 2006 7:50 pm

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
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474218
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RE: Why The Little Hole At The Bottom Of The Window?

Fri Oct 20, 2006 11:40 pm

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.
 
pygmalion
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RE: Why The Little Hole At The Bottom Of The Window?

Sat Oct 21, 2006 3:26 am

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.
 
EMBQA
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RE: Why The Little Hole At The Bottom Of The Window?

Sat Oct 21, 2006 6:42 am

Quoting Pygmalion (Reply 24):
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).

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 from Home....and found no SNL for what you are talking about. I lost as to why you are so hooked up on this volcanic ash deal. I'm not doubting you, I've just never read about it in an AMM
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HAWK21M
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RE: Why The Little Hole At The Bottom Of The Window?

Sat Oct 21, 2006 8:02 pm

Quoting Pygmalion (Reply 24):
If you have access to the AMM, then look up the Service Letter for Chapter 56 (Windows)

Any link possible.Understandably would be tough  Smile
A reference Chapter-Subject-Section would do.
regds
MEL
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DC8FriendShip
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RE: Why The Little Hole At The Bottom Of The Window?

Sat Oct 21, 2006 11:46 pm

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

That why my streamlight lenses craze, or is it skydrol exposure (or both)?
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HAWK21M
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RE: Why The Little Hole At The Bottom Of The Window?

Sun Oct 22, 2006 12:29 pm

Quoting DC8FriendShip (Reply 27):
That why my streamlight lenses craze

Isn't it glass.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
EMBQA
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RE: Why The Little Hole At The Bottom Of The Window?

Mon Oct 23, 2006 4:46 am

Quoting HAWK21M (Reply 28):
Isn't it glass

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 around SkyDrol dosn't help.
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