What is the key indicator of a functioning fire extinguisher during preflight?

The key indicator of a functioning fire extinguisher during preflight checks is that the gauge shows green. A green gauge indicates that the extinguisher is fully charged and ready for use. This is crucial for flight attendants, as they must ensure that all safety equipment is operational before taking off. If the gauge is in the green zone, it gives assurance that the fire extinguisher can effectively combat a fire if needed.

In contrast, a broken pin, red in the gauge, or a missing seal would suggest that the extinguisher is either tampered with or unserviceable. A broken pin may imply that the extinguisher has been used or is unsafe to operate, while a red gauge indicates a low pressure, meaning it may not function correctly in an emergency. A missing seal often indicates that the extinguisher may be compromised or has been accessed. Therefore, the green gauge is crucial as it verifies the extinguisher's readiness for safety purposes.