Definition
Small openable panels or vents built into the side windows of some aircraft cockpits, designed to be opened in flight to provide ventilation, clear smoke from the cabin, or improve visibility when the main windscreen is obscured.
Plain English
A small window-within-a-window in the cockpit that the pilot can slide or push open during flight to let air in or smoke out.
Context Anchor
Seen in cabin fire and smoke procedures, where the pilot may need to ventilate the airplane after or while handling an emergency.
Derivation
The name comes from their original purpose: giving pilots a way to see or get fresh air during 'foul weather' (rain, fog, smoke) when the main windscreen was unusable. Older aircraft often had hinged side panels for this reason. The term stuck even though their modern use is mostly for ventilation and smoke clearance.
Why Pilots Care
Allows controlled removal of smoke or fumes without opening larger windows that could create excessive wind or handling problems.
Intuition Check
Do not read this as a time period of bad weather. In this context, foul weather windows are physical windows on the airplane.
Example Sentence 1
After the electrical smoke filled the cockpit, the pilot cracked open the foul weather window to improve visibility while flying to the nearest airport.
Example Sentence 2
On the rainy flight the foul weather windows let in fresh air without soaking the cockpit.