following happens: the engine stops because the fuel control  shuts off, the engine is isolated from the aircraft systems, and  the fire extinguishing system is activated. Some aircraft use  fire switches that need to be pulled and turned to activate the  system, while others use a push-type switch with a guard.  To prevent accidental activation of the fire switch, a lock is  installed that releases the fire switch only when a fire has been  detected. This lock can be manually released by the flight  crew if the fire detection system malfunctions.  [Figure 17-17] Transport aircraft need to have the following provisions for  each cargo or baggage compartment: A Class A cargo or baggage compartment is one in which  the presence of a fire would be easily discovered by a  crewmember while at his or her station and each part of the  compartment is easily accessible in flight. A Class B cargo, or baggage compartment, is one in which  there is sufficient access in flight to enable a crewmember  to effectively reach any part of the compartment with  the contents of a hand fire extinguisher. When the access  provisions are being used, no hazardous quantity of smoke,  flames, or extinguishing agent enters any compartment  occupied by the crew or passengers. There is a separate  approved smoke detector or fire detector system to give  warning at the pilot or flight engineer station. A Class C cargo, or baggage compartment, is one not meeting  the requirements for either a Class A or B compartment but  in which:  The cargo compartment smoke detection system gives  warnings in the flight deck if there is smoke in a cargo  compartment.  [Figure 17-18]  Each compartment is equipped  1. There is a separate approved smoke detector or fire  detector system to give warning at the pilot or flight  engineer station.  2. There is an approved built-in fire extinguishing or  suppression system controllable from the cockpit.  4. The required crew emergency exits are accessible  under any cargo loading condition. 3. There are means to exclude hazardous quantities of  smoke, flames, or noxious gases from the flight crew  compartment.  2. The controls for shutting off the ventilating airflow  to, or within, the compartment are accessible to the  flight crew in the crew compartment.  1. There is a separate approved smoke or fire detector  system to give warning at the pilot or flight engineer  station.  Class E cargo compartment is one on airplanes used only for  the carriage of cargo and in which: 4. There are means to control ventilation and drafts  within the compartment so that the extinguishing  agent used can control any fire that may start within  the compartment. 3. There are means to exclude hazardous quantities  of smoke, flames, or extinguishing agent from any  compartment occupied by the crew or passengers.  3.  There must be means to allow the crew to check, in  flight, the functioning of each fire detector circuit. 2.  The system must be capable of detecting a fire at  a temperature significantly below that at which the  structural integrity of the airplane is substantially  decreased. 1.  The detection system must provide a visual indication  to the flight crew within 1 minute after the start of a  fire. Cargo Fire Detection Cargo Compartment Classification Class A Class B Class E Class C Cargo and Baggage Compartment Fire Detection  and Extinguisher System Figure 17-16.  Engine and APU fire switches on the cockpit center  overhead panel. 17-14