Guest

Cockpit Windows

Tue Oct 03, 2000 8:11 am

O.K. guys, this is something I've never understood... Why on earth don't cockpit windows fog up?
I was once in the cockpit of a Lufthansa 747-400 on a flight from Mexico to Frankfurt. At one point over Gander I saw on the cabin screens that outside temperature was -74 degrees celcius yet in the cockpit it was nice and warm??? My car's winshield fogs up with the slightest temperature change, so why don't cockpit windows???
 
Ralgha
Posts: 1589
Joined: Tue Nov 09, 1999 6:20 pm

RE: Cockpit Windows

Tue Oct 03, 2000 9:29 am

Cockpit windows are heated  
09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
 
Guest

RE: Cockpit Windows

Tue Oct 03, 2000 9:40 am

Well, yes they are. but wouldn't they have to be super heated to go against -74 degrees celcius? I was thinking after I put up this post, that maybe there is extremely little humidity at that altitude. But then what happens when ascending and descending to cruising altitude?
 
Ralgha
Posts: 1589
Joined: Tue Nov 09, 1999 6:20 pm

RE: Cockpit Windows

Tue Oct 03, 2000 9:56 am

It's not the moisture outside that forms the fog, if there were any outside, the window's no colder than the air is even if it wasn't heated so it wouldn't condense on it. Since the window is heated, and it's multiple panes, the inside of the window isn't cold enough to condense the water in the air inside. Touch your cabin window, it's not real cold either   (I love this symbol)  
09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
 
Guest

RE: Cockpit Windows

Tue Oct 03, 2000 10:30 am

Thank You for you answer (the complete one)    
 
VC-10
Posts: 3552
Joined: Tue Oct 26, 1999 11:34 am

RE: Cockpit Windows

Wed Oct 04, 2000 8:32 am

On the 747 the windows are heated by a gold film heating element of 3000 watts located between the outer ply & the first inner layer of the 3 ply window. The temperature is controlled to approx 90 deg F (actual temp depends on the screen manufacturer) with an overheat setting og approx 105 deg F.

By this method the exterior ply is de-iced but additionally by having the screen heated it reduces the brittleness of the window, thus making it more resilient to damage e.g. birdstrikes.

The interior side if the window is defogged by

1. The Heating &

2. By there being a "curtain" of warm air being blown across it.


 
pmk
Posts: 609
Joined: Wed May 26, 1999 10:07 am

RE: Cockpit Windows

Wed Oct 04, 2000 8:43 am

Certain EXEC-Jets use Bleed air to keep the windshields clear!!


Peter
 
Guest

RE: Cockpit Windows

Wed Oct 04, 2000 8:56 am

There we go!!! Thanks a million for that answer VC-10.
By the way, your user profile and your answer to this and other threads won my respect!!!
 
VC-10
Posts: 3552
Joined: Tue Oct 26, 1999 11:34 am

RE: Cockpit Windows

Wed Oct 04, 2000 9:36 am

Thank you !

Just got to improve those typing skills now !
 
VC-10
Posts: 3552
Joined: Tue Oct 26, 1999 11:34 am

RE: Cockpit Windows

Fri Oct 06, 2000 5:21 am

Further to my statement that window heat is used to improve the resilience of the the windscreens. The MEL states that you are allowed to dispatch with one windscreen heat inop (not into known iceing conditions) but if it is a Triplex windscreen you are restricted to 330 Kts below 8000 Ft (max service ceiling for most feathered aviators).

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