What Purpose Do The Two Outer Panes Of A Passenger Aircraft Cabin Window Serve And What Are They Made Of?

Airline windows typically comprise three or more layers of glass (or acrylic) to provide insulation from the very cold atmosphere at altitude.

The tiny silvery cylinder is really the edge of a small hole drilled in the middle layer to allow the pressure to equalise between the layers while minimising convection.

The condensation around the hold is due to the inner airspace cooling. Ice often forms here.

The position of the hole is chosen to maintain the best clear viewing area when condensation forms, to minimise the likelihood of a crack forming between the hole and the edge of the window, and to avoid excessive condensation pooling over the hole, which could freeze and block it.

Published in The Hindu on June 20, 2002.