What common feature do all aircraft fire extinguishing systems have?

All aircraft fire extinguishing systems incorporate a discharge control switch as a standard feature. This component is crucial because it allows the flight crew to activate or control the release of the extinguishing agent. In emergency situations, being able to quickly and effectively deploy fire suppression is essential to ensure passenger safety and protect the aircraft's systems. The control switch enables the crew to respond promptly to any signs of fire, whether in the engine compartment or other critical areas of the aircraft.

The other options either do not universally apply to all fire extinguishing systems or refer to features that are not inherent or standardized across different systems. For example, emergency landing procedures can vary significantly by aircraft and situation, standard fuel types are not relevant to fire extinguishing systems, and real-time monitoring systems are not commonly part of every fire extinguishing setup. Therefore, the discharge control switch is the fundamental feature that is consistently found across all aircraft fire extinguishing systems.