Which system features a single-loop fire detection without fire protection?

The system that features a single-loop fire detection without fire protection is the main landing gear bay. This configuration allows for the detection of smoke or fire in the landing gear area using a single-loop system, which is designed to monitor for anomalies. It is important to note that landing gear bays typically do not have active fire suppression mechanisms because the nature of the risks associated with landing gear fires is different from those of engine compartments or APU (Auxiliary Power Unit) compartments, which have dual-loop detection and fire suppression systems. Therefore, while the main landing gear bay is equipped with fire detection functionality, it lacks the robust fire protection measures found in other critical areas of the aircraft, such as the engine and APU compartments, where immediate response to fire situations is essential.