Porthole in Canberra canopy

[HP42]

Simple question really, Canberras have small portholes set into the main canopy glass. Daft question but what is it for?

[Procopius]

Isn't it because the curvature of the canopy causes distortion? I always thought that was the "direct-vision panel".

[Cheshiretaurus]

Its called a direct view (DV) window and can be opened in flight for visibility to land if the visibility through the canopy has become obscured for any reason due ice/mist/damage/smoke etc. Also good on the ground for ventilation/speaking to ground personnel before start without having to open the hatch.

[Dave Wilson]

Just don't open it at 48,000'...

[Cheshiretaurus]

Just don't open it at 48,000'...

You would then have the cleanest cockpit ever.

I had the opportunity to sit in the Canberra nose at Elvington a couple of weeks ago and opened the DV window, it opens inwards and hinged rear wards and is a plug type window so when the cockpit is pressurised it would be held in place by the cockpit pressure. Its unlikely it could be opened whilst pressurised as even a small pressure differential between inside and outside of say 4psi would work out to about 200lbs pressing on the window (I guessed its about 8" in diameter so 50" sq area)

By the way the window next to you in a average airliner at 40,000 ft will have about half a tonne pressing on it.

Just a thought next time you fly

[HP42]

Really interesting answers, many thanks! The thought of trying to land with only that tiny window for reference would concentrate the mind somewhat -the yellow and black handle might seem a safer option!