In the engine fire detection system, how many loops must be exposed to fire to trigger an alarm?

In the engine fire detection system for the C-17, the requirement is that both loops must be exposed to fire to trigger an alarm. This dual-loop system provides redundancy and enhances reliability in detecting an actual fire condition. The reason both loops need to detect a fire is to minimize the chance of a false alarm. If only one loop were required, it could lead to scenarios where an erroneous alarm might be triggered by issues unrelated to actual fire conditions. By requiring both loops to be activated, it ensures that the fire detection system operates correctly and safely, enhancing aircraft safety during critical operations.

Other options do not accurately reflect the operational standards of the engine fire detection system. For example, relying on just one loop could compromise safety, and requiring specific heating of the control unit would not properly address immediate fire detection needs. Similarly, exposing the entire length of the sensing element to flame is not practical for determining the presence of a fire, as it is the detection of the conditions sensed by the loops that matter most.