B737 Window 4

[LEM]

Anybody know why the number 4 window has got three panes instead of two, like all others?

[DDG]

#4 window

Hi LEM.
#4 window on the B737 Classic & NG`s has an extra glass layer for impact protection,MOSTLY FOR BIRDSTRIKES.

[LEM]

Window 1 and 2 (the more exposed to birds) with two panes, and window 4 with three???

[Seat1APlease]

Does that one have electric heating? just a thought

[Brenoch]

All the windows are two glasspanes laminated to a vinyl-core.
The No4 window has got an additional vinyl layer laminated to the inside surface of the glasspane.
They are all heated, the No1 window via the FWD window heat switch and 2,3,4 & 5 via the side one.

Regs

[LEM]

Hi Brenoch,

The No4 window has got an additional vinyl layer laminated to the inside surface of the glasspane.

D'you know exactly why?

Btw, window 3 is NOT electrically heated, and is constructed of two acrylic panes vented to the cabin.

[Brenoch]

I honestly don't know the exact reason why but I'll guess.

Don't know about the "classic" but certainly on the NG the no3 window is heated, however as opposed to the no1 & 2 windows the heating on 3,4 & 5 is via a conductive coating on the inner glasspane.
This is not to increase the elasticity of the window in the event of a birdstrike but more as a anti-fogging device. A birdstrike on the no3 or 5 window would probably be more accurately described as the bird having a planestrike. The no 4 window on the other hand is more forward-facing and because of that it may have been given the extra shatter-proofing vinyl layer.

I stress to add that I'm only speculating now.

Regs

[IFixPlanes]

The additional layers on Window # 4 prevent glass from scattering through the cockpit in the event of bird impact. The additional layers have no structural importance.

Ingo

[LEM]

Roger that, thanks.