What is the purpose of the guarded EMER DEPRESS PBA?

The guarded EMER DEPRESS PBA is designed specifically to manage situations where there is a need for fresh air circulation, particularly in scenarios where fumes or smoke may have entered the cockpit. By activating this switch, it allows the pilots to open the outflow valve manually, facilitating the purging of the contaminated cabin air and bringing in a fresh supply of air. This is a critical safety feature in emergency situations, as it helps maintain a clear environment for flight operations and improves visibility and overall conditions in the cockpit.

The other options do not correctly describe the specific role of the EMER DEPRESS PBA. Controlling cabin pressure is typically managed through other systems in the aircraft rather than this switch. Similarly, emergency lighting is activated through separate mechanisms that do not involve the operation of the EMER DEPRESS PBA. Adjusting cockpit temperature is also controlled through different environmental control systems that focus on climate management rather than air quality in the event of a hazardous situation.