26 Fire protection

 

TTP	B1-L3	ATA 26
Beech 90 Series	B2-L3	Fire Protection

INTRODUCTION

 

The fire-protection system provides detection, warning, and fire-extinguishing capability in either engine nacelle. The optional fire-detection system identifies and provides cockpit warning of a fire. If the optional fire-extinguisher system is installed, it smothers flames using a chemical agent when activated.

GENERAL

 

Photoconductive cells in each engine nacelle sense the presence of flames. Fire indications are displayed on the annunciator panel in the cockpit. Fire-extinguisher cylinders in the nacelles discharge to extinguish the fire when the fire- extinguisher switches in the cockpit are pushed.

FIRE DETECTION

 

DESCRIPTION

 

The fire-detection system identifies the existence of a fire in either engine nacelle. Once a fire is detected, a fire warning displays in the cockpit.

 COMPONENTS

 

The fire-detection system is designed to provide warning in the event of an engine compartment fire. The system includes:

•  Set of three photoconductive cells

 

•  Control amplifier on a panel on the aft side of the forward pressure bulkhead

•  Annunciator warning light for each engine

•  Test switch on the inboard copilot subpanel

•  Circuit breakers on the right CB panel

 

 OPERATION

The photoconductive cells are sensitive to infra- red rays and are positioned to receive direct and reflected rays to provide coverage for the entire engine compartment. The cell emits an electrical signal proportional to the infrared intensity and ratio of the radiation striking the cell. Heat level and rate-of-heat increase are not contributing factors in the activation of the cells. To prevent stray light rays from signaling a false alarm, a relay in the control amplifier closes only when the signal strength reaches a preset alarm level. When the relay closes, the appropriate annunciator illuminates. When the fire has been extinguished, the cell output voltage drops below the alarm level and  the  control  amplifier automatically  resets. No manual resetting is required to reactuate the detection system.

DIAGNOSTICS Testing

The test switch on the subpanel has four positions: OFF–1–2–3. (CAA modified systems have an additional test position.) When the switch is rotated from OFF to any one of the three test positions, the output voltage of the corresponding flame detector in each engine compartment is increased to a sufficient value to signal the amplifier that a fire is present. The annunciator warning lights illuminate as the selector is rotated through each of the three positions.

 

Failure of an annunciator warning light to illuminate indicates a malfunction in that detector circuit.

 The fire-detection system should be tested in all three positions prior to each flight to verify the circuitry for all six fire detectors is functioning properly. The system may be tested either on the ground or in the air at any time.

 If the fire-detection annunciators fail to illuminate when the test switch is in any of the three test positions, a malfunction is indicated in one or both detector circuits (one in each engine compartment zone) being tested by that position of the test switch. If the fire-extinguishing system is installed, the fire lights on the fire-extinguisher switches also illuminate when the system is tested.

 

The test switch tests the fire circuitry, but does not check the detector itself. Testing the detector (and the circuitry) can be accomplished by shining a light onto the detector. On earlier high-threshold logic (HTL) detectors, shining a red-lensed flashlight directly  at  the  detector  activates  the system. This procedure does not work on the later Walter Kidde units. To test these detectors, use an “AA” mini-maglite with the reflector and lens unscrewed (with good batteries). Move the light slowly (once or twice a second) across the face of the detector to simulate a fire.

FIRE-EXTINGUISHING SYSTEM

DESCRIPTION

The optional engine fire-extinguishing system consists of a supply cylinder, on brackets behind the main spar in each wheel well, and plumbing that carries the extinguishing agent to spray nozzles in each of the engine compartment zones. Each supply cylinder is charged with two and one half pounds of halon 1211/1301 and pressurized with dry nitrogen to 450 psi at 22°C (72°F). Four spray nozzles are positioned under the engine exhaust area, with another pair mounted in the accessory area.

OPERATION

These strategically positioned nozzles discharge the entire supply of fire-extinguishing agent into the engine compartment within approximately one-half second. Each fire extinguisher is actuated by its respective control switch on the upper instrument panel. When the switch is actuated, a cartridge fires releasing the extinguishing agent into the plumbing and out the nozzles. The switches are powered from the hot battery bus. These switches contain three indicator lights colored and marked as follows:

 

•  Red  light—Placarded  L–R  ENG  FIRE EXT PUSH, warns of the presence of fire in the engine compartment.

 

•  Amber light—Placarded D, indicates the system has been discharged and the car- tridge is empty.

 

• Green light—Placarded OK, is provided only for the test function.

 To  actuate  the  system,  raise  the  safety-wired clear-plastic switch cover and press the face of the lens. When the system has fired, the amber or yellow D light illuminates and remains illuminated, regardless of the battery switch position, until the depleted extinguisher cartridge has been replaced.The pressure gauge on each extinguisher supply cylinder can  be checked during inspections to assure each cylinder is fully charged.

FIRE-PROTECTION SYSTEM

 

CONTROLS AND INDICATIONS

FIRE DET and FIRE EXT Test Switch 

Rotating the test switch knob to the following positions tests the indicated circuit and illuminates the associated annunciator light:

 

•  OFF—Removes power from the fire-detection and extinguishing test system

•  1—Tests the first circuit

•  2—Tests the second circuit

•  3—Tests the third circuit

 

The L–R ENG FIRE EXT PUSH–D–OK portions of the fire-extinguishing pushbuttons illuminate during their respective phases of the system test. The D portion of the pushbuttons also illuminates when the associated fire bottle is discharged.

Fire-Extinguishing Pushbuttons

 

Pushing  the  illuminated  L  ENG  FIRE  EXT PUSH or R ENG FIRE EXT PUSH pushbuttons discharges the fire-extinguishing agent into the associated nacelle.

 Annunciator Panel

 

L ENG FIRE or R ENG FIRE message advises the  pilot  of  a  fire condition  in  the  associated nacelle.

 FIRE AND SMOKE DETECTION

 

Prior to LJ-114, the system includes two indicator lights on the instrument panel, a test switch on the left subpanel, and a circuit breaker on the floor-board circuit breaker panel. The circuit breaker is installed on the right subpanel on LJ-114 and after. The test switches are installed on the left instrument panel prior to LJ-302 and on the ped- estal on LJ-320 and after. The warning light is incorporated in the annunciator panel on LJ-114 and after.

 The flame detectors, sensitive to infrared rays, are positioned in the engine compartments to receive direct and reflected rays, thus viewing the entire compartment  with only three  cells.  Heat  level and rate of heat rise are not factors in the sensing method. The cell emits an electrical signal, whose potential is proportional to the infrared intensity and ratio in the radiation striking the cell. To prevent stray light rays from signaling a false alarm, a relay in the control amplifier closes only when the signal reaches a preset alarm level. When the relay closes, the appropriate warning light on the instrument panel glows steadily. After the fire is extinguished, the cell output voltage drops below the alarm level and the relay in the control amplifier opens. No manual  resetting  is required to reactivate the detection system.

 The smoke detector contains a constantly burn- ing light and a photoconductive cell enclosed in a perforated case. Smoke particles entering the case reflect infrared rays from the light into the cell, which transmits a signal to the left engine fire detector control amplifier.

 The potential of this signal is proportional to the density of the smoke. When the signal strength is sufficient to close the relay in the amplifier, the left engine fire light flashes to signal a fire in the nose equipment compartment on planes prior to LJ-137. On LJ-137 and after the smoke detector was disconnected from the LH fire warning light. An additional amplifier was installed in the fuselage adjacent to amplifier for the fire detectors. The new amplifier transmits the signal from the smoke detector to a warning light in the annunciator panel in the event of fire in the nose compartment.

 The test switch on the subpanel has positions to test each of the detectors. The system may be tested anytime on the ground or in flight by rotating the switch through the various positions. The three positions in the fire detector range activate both the right and left flame detectors. Failure of a light to glow in any one position indicates trouble in that particular detector  circuit rather than a trouble affecting all detectors.

ENGINE FIRE EXTINGUISHER (OPTIONAL)

 

A fire extinguisher supply cylinder is mounted on brackets at the aft side of the main spar in each wheel well of the main landing gear. Each cylinder is charged with 2-1/2 pounds of Bromotrifluoro-methane (CBrF3) pressurized to 450 psi at 70°F by dry nitrogen. The line from the cylinder runs along the inboard side of the nacelle and branches into a network of spray tubes about the engine to diffuse the CBrF3 in the event of a fire. Four of the spray tube nozzles are positioned to discharge beneath the engine exhaust case, another pair to discharge in the engine accessory area, and the remaining nozzle to discharge into the ram air inlet for the starter generator.

 

NOTE: Airplanes prior to LJ -318 had several nozzles discharging into the engine compressor inlet area, but these were relocated to the exhaust case area.

 The system is designed to simultaneously discharge a 15 percent concentration of fire extinguishing agent for a half second into each of the nacelle fire zones designated in the last sentence preceding the note above. Each fire extinguisher is actuated by its respective switch mounted in the pedestal on airplanes prior to LJ-138, except LJ-121 and LJ-128. On serial LJ-121, LJ-128, LJ-138 and after, these switches are illuminated and one is installed on each side of the instrument panel. The system may be activated by raising the transparent plastic cover over the press-type switch and depressing the red plastic face on the switch placarded FIRE EXT-PUSH TO EXT.

 Switch for the respective engines are located on the instrument panel just below the annunciator panel, and are wired in conjunction with the annunciator to provide an additional warning to ensure activation of the proper switch. Each extinguisher gives only one shot to its engine. Pushing the switch with cause an explosive squib to release the fire extinguishing agent. The switches are protected by a 5-ampere fuse at the battery bus. To assure operation of the system during cold weather, each supply cylinder is encircled by an electrothermal heater on aircraft prior to LJ-490.

 

NOTE: On LJ-490 and after the electrothermal heater,  thermostat  switch, and heater circuit breaker were removed from the fire extinguisher system, after a series of low temperature discharge tests had proved that the system would function effectively without them.

 Any time the temperature drops below 35°F, the heaters are actuated by thermostats mounted adjacent to the supply cylinder in each main landing gear wheel well. On aircraft prior to LJ-114, a 5-ampere circuit breaker at the floorboard circuit breaker panel provides protection for the heater circuit. This circuit breaker is rated at 10-amperes on LJ-114 through LJ-489 and is located on the RH subpanel.

PTP  Beech 90 Series	B1	LOC	FOT	SGH	R/I	MEL	TS
	B2	LOC	FOT	SGH	R/I	MEL	TS