Fire detection and protection systems 

The fire detection system of the Saab 340 provides for detection of engine fire, exhaust duct overtemperature, and smoke in various areas of the fuselage. It consists of separate detection circuits for each engine, three circuits for exhaust duct overheat, and three circuits for fuselage smoke detection. Visual and aural warnings are provided for all three detection systems. The fire-extinguishing system consists of three fire bottles (two for the engines and one for the cargo compartment) that are manually activated from the cockpit. The engine fire bottles are interconnected so that both bottles may be used for either engine. 

The engine fire detection system consists of sensor loops, a detection control unit, ENG fire pull handles, and warning and annunciator lights. In addition, a fire warning bell provides aural warning. Circuit integrity is monitored by amber L/R FIRE DET FAIL L/R annunciators. The system requires 28-VDC power. The exhaust duct over temperature detection system consists of overheat sensors installed around each engine tailpipe, sharing the detection control unit with the engine fire detection system. Warning and annunciator lights and triple chimes provide visual and aural warning of overtemperature conditions. The system is tested with the FIRE test switch.

The fuselage smoke detection system includes smoke detector units in the cargo compartment, lavatory, and avionics area of the control cabin. Warning and annunciator lights and triple chimes provide warning of smoke concentrations in the designated areas. Power is 28 VDC from the battery bus. The engine fire-extinguishing system consists of two fire bottles charged with Halon 1301 extinguishing agent, pressurized with nitrogen, and
discharged by electrically activated squibs. The bottles are armed and activated electrically from the cockpit. In addition, the bottles arc guarded against overpressure. The cargo compartment fire-extinguishing system consists of one fire bottle, also charged with Halon 1301 extinguishing agent and pressurized with nitrogen. The bottle is discharged with a switch on the overhead panel. A white bottle low-pressure light (CARGO EXTG) is also located on the overhead panel. 

Engine Fire Detection Components

Sensor Loop
A sensor loop routed along the floor and eductor shroud of each engine fire zone, is a tube of heavy-wall inconel containing two conductive elements insulated by a ceramic material. One element is welded to the center contacts of the end connectors; the other element is welded to the connector shells to provide a ground potential. As the loop is heated, the resistance of the ceramic insulator drops until an equivalent resistance of 350 +25 ohms across the entire loop is reached. This enables the control unit to send out a fire warning signal. A control unit with electronic circuits monitors the resistance of the sensor loop in each engine. If a preset resistance value that indicates an engine fire is reached, the unit activates the fire warning system. However, if the resistance of the loop changes too rapidly, the loop has shorted out, and a failure indication will be presented.

ENG Fire Handles
The ENG fire handle illuminates red when an engine fire occurs, remaining on until the fire is extinguished. When the handle is pulled, the following occurs:
• The fuel supply to the engine is shut off.
• Bleed air from the engine is shut off.
• The fire warning bell is silenced.
• The MASTER WARNING light resets.
• The ENG FIRE light changes from flashing to steady.
• Both engine fire bottle circuits (main and reserve) are armed.
• Hydraulic pressure to right propeller brakes is shut off (R ENG handle).
• The field relay on the starter-generator is tripped. 


System Operation

When the sensor loop resistance decreases to 350 ±25 ohms, the control unit is activated to illuminate the red ENG fire handle The red MASTER WARNING light and the red ENG FIRE lights also come on, both flashing. In addition, the fire bell sounds continuously. The MASTER WARNING light can be extinguished by depressing the light or by pulling the ENG fire handle (which also changes the flashing ENG FIRE light to steady). The ENG fire handle light remains on as long as the fire condition exists. A short circuit in the engine fire detection system is indicated by flashing of the amber MASTER CAUTION and FIRE DET FAIL lights and a single chime. Pressing the MASTER CAUTION light will turn it off and change the flashing FIRE DET FAIL light to steady. The fail light stays on as long as a short circuit exists. Engine Fire Detection Test Two switches on the TEST 1 panel permit testing of the engine fire detection system for proper operation and for short circuits. Holding the FIRE switch in either operating position (L or R) checks wiring integrity of the fire detection system by illuminating the MASTER WARNING, ENG FIRE, and ENG fire handle lights. The fire bell also sounds. The switch also tests the exhaust duct overtemperature detection circuit. To test the system for shorts, actuate the FIRE SHORT switch. This simulates a short circuit in both engine fire sensor loops, illuminating both MASTER CAUTION lights and the L
and R FIRE DET FAIL lights and producing a single chime.

The engine exhaust duct over temperature detection system provides an indication of nacelle overheat in the area of the engine tailpipe. The system consists primarily of three overheat detectors located radially around the tailpipe
The detectors are preset, nonadjustable, bimetallic switches mounted in heat-expandable cases. At 235° C expansion of the case closes the switch to complete a warning circuit. The detectors are wired in parallel; actuation of any of the three switches provides the warning. A temperature increase to 235 ° C in the tailpipe area causes one or more of the three overheat detectors to actuate. This completes circuitry to provide repetitive chimes and causes the red MASTER WARNING and the applicable red (L or R) TAILP HOT light to flash. Pressing the MASTER WARNING light will extinguish it and silence the triple chime. The TAILP HOT light will cease flashing but will remain on as long as the overheat condition exists. Testing Holding the FIRE switch on the TEST 1 panel in either operation position (L or R) checks the exhaust duct over temperature detection system by illuminating the MASTER WARNING and the applicable TAILP HOT lights. Repetitive chimes are heard. Repeat the test with the TEST switch in the opposite position. illuminated, to change from flashing to steady and remain on until the smoke is eliminated.



Two 125-cubic inch spherical fire bottles, each with a pressure gage, pressure relief/fill port, and two outlets, are charged with Halon 1301 extinguishing agent pressurized to 600 psi with nitrogen. Bottle discharge is initiated by application of electrical current to either of two cartridges on the bottle. The detonated cartridge drives a slug through a closure disc, releasing the agent into distribution lines. Each cartridge contains two separate explosive charges to assure detonation, either charge alone applying sufficient force to rupture the closure disc. However, the initiation of one charge detonates the other charge. Overpressure protection is provided by a burst disc with a pressure range of 1,400 to 1,800 psi, allowing discharge into the nacelle equipment bay. Thermal protection of the bottle is provided by an insulation blanket surrounding the bottle. The bottle pressure gage is checked through an inspection window in the right equipment bay door. Operation Either bottle can be discharged into the fire zone of the nacelle where it is installed, or it can be routed to the nacelle on the other wing to serve as a reserve for its fire bottle. The released agent is discharged into the engine area through two nozzles.

If a fire occurs in the left engine area, the L ENG fire handle illuminates, the L ENG FIRE annunciator flashes, and the fire bell sounds continuously. Pulling the illuminated L ENG fire handle silences the fire bell and arms both FIRE EXTG switches Moving the left FIRE EXTG switch to ON completes circuitry to fire the left main squib, detonating a cartridge on the left fire bottle. The contents of the bottle are discharged through a two-way check valve and spray nozzles into the engine fire area. If additional extinguishing agent is required to the left engine, moving the right
FIRE EXTG switch to ON fires the left reserve squib on the right fire bottle. Extinguishing agent from the right bottle flows through the right crossover valve, through the crossover line and crossover valve, and through a two-way check
valve into the left engine spray nozzle. Procedures for right engine fire extinguishing are essentially the same, with the R ENG fire pull handle illuminating to serve warning. In that event, the R ENG handle should be pulled and the right FIRE EXTG switch moved to ON to fire the right main squib on the right bottle, releasing extinguishing agent into the right engine fire area.

Cargo Fire System

The cargo compartment fire-extinguishing system consists of one or two Halon 1301 fire bottles equipped with an  explosive cartridge located behind the rear bulkhead in the cargo compartment. The distribution tube is routed close to the interior walls, terminating at the discharge nozzle. The nozzle provides an even distribution of extinguishing agent in the compartment. Smoke in the cargo compartment is signaled by illumination of the MASTER WARNING and CARGO SMOKE lights and repetitive chimes.

The fire extinguishing system is activated by moving the CARGO FIRE EXTG switch to the ON position. This electrically fires the explosive cartridge on the compartment fire bottle, releasing extinguishing agent in two stages. The first stage rapidly floods the compartment with agent in order to extinguish the fire. The second stage releases a regulated flow of the agent that maintains, for 40 minutes, concentration levels high enough to prevent reignition. The white CARGO EXTG light on the overhead panel is illuminated when the normal bottle pressure of 360 psi decreases to a level of 310 to 260 psi due to leakage or activation. An optional two-extinguisher system is available. It provides two Halon 1301 bottles, each having its respective discharge switch and monitor light. This option provides an increase in the time that sufficient Halon concentration can be maintained in the cargo compartment from 40 minutes to 70 minutes, provided the second bottle is activated 35 minutes after the first bottle.

Bottles are not to both be discharged within two-minutes of each other. Doing so will result in sufficient pressure
buildup in the cargo compartment to cause blowout panels to open, resulting in the loss of Halon. 


Lavatory Fire Detection & Extinguisher

On aircraft with Mod 1379 incorporated, an automatic fire extinguisher system is installed to provide fire protection for the lavatory waste container. One Halon-filled fire extinguisher is installed with a discharge pipe extending to the trash container. A fusible plug in the terminal end of the discharge pipe will melt if a fire occurs, allowing Halon to be discharged into the waste container.