Airplane Fire detection systems

Fire detection systems are available to monitor the temperature in engine and APU bay. If temperature is exceeded the pilot is alarmed. 
The folllowing types of sensors can be distinguished to detect fire:

1. Continuous sensors (fire wires)
2. Discrete sensors:
Bimetallic strips
Pneumatic detectors
Optical detectors

Of course, the design of fire detectors must meet stringent temperature requirements.
Fire suppression:
Shut down engine.
Isolate fuel system at firewall
Discharge extinguisher fluid (HALON 1301 (CF3Br)) in engine bay

Also, the fuel vapors inside the fuel tanks pose a potential explosion risk. For civil aircraft this can be a risk following an emergency landing. Possible risk reducers include:
1. Reticulating foam filling
2. Inert gas pressurization