Descripcion boeing 727 ata 26 - fire protection

1. LINE AND BASE MAINTENACE TRAINING
FIRE PROTECTION
B727-200
AIRWAYS CONSULTORIA AERONÁUTICA LTDA

2. AIRWAYS TREINAMENTOS FIRE PROTECTION
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GENERAL
Fire protection is provided by detectors located in the engines, wheel wells and APU. The engines, APU,
and lavatories are equipped with fire extinguishers.
The engine and wheel well fire detection systems provide both visual and aural warning of a fire or
overheat condition. Dual sensing elements are utilized in the engine areas to increase system reliability.
The lavatory smoke detection system consists of a smoke detector installed in each lavatory. When
smoke is detected an aural warning is activated which can be readily detected by a flight attendant in the
passenger compartment.
There are three fire extinguishing systems which utilize Freon gas a the extinguishing agent. Two of
these systems, one for the engines and one for the APU, have fire warning and warning test capability. The
third system, which is installed in the lavatories, operates automatically and has no warning or test capability.
Portable fire extinguishers are provided in the cockpit and passenger cabin. The lavatory fire extinguisher
system may not be installed on early airplanes.

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ENGINE FIRE DETECTOR
The engine fire detection system provides fire detection on the engine and engine firewalls. Fire detection is
provided by dual shielded sensing elements, systems A and B, mounted on the engine and engine firewall
areas. The detection system is an electrical heat sensing system that responds to a general overheat condition
or localized fire condition by activating warning lights and an alarm bell.
The engine fire detection system consists of: engine fire detection assembly, firewall sensors, engine sensors,
bell reset switch, system A and system B test switches, three detector test switches, six amber detector fault
lights, red fire warning lights, and alarm bell. The engine fire detector control assembly, three detector test
switches, six amber detector fault lights, system A and system B test switches and bell reset switch are on the
pilots' overhead panel. Warning lights in the engine fire switch handles, are on pilots' overhead panel P5 with
two fire warning lights on pilots' lightshield P7. The alarm bell is in the aural warning devices box in the ceiling
panels over the third crewman's station (Fig. 1).
The engine and firewall sensing elements are enclosed within a shroud which provides impact protection. Two
elements, systems A and B, are within a single shroud, thus providing a redundant system. Each system
is connected to a detector circuit within the engine fire detection control assembly that monitors the electrical
resistance of the sensing element. The fire warning lights and alarm bell will operate if a fire condition exists.
The detector fault lights will illuminate if a short circuit should occur. The system A and system B test switch
circuits check continuity of the sensing elements and simulates a fire condition which causes the detector
circuit to operate the fire alarm lights and bell (Fig. 2).

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FIRE DETECTOR CONTROL ASSEMBLY
A. The fire detector control assembly is located on the pilot's overhead panel in the flight compartment. The
assembly contains six amber colored lights and three switches arranged in three groups and three control
units. Each group consists of two lights and a switch and represents a designated fire area. The amber lights
provide visual indication of sensing element loops circuit integrity during normal operation or test procedures.
The switches have three selective positions; AB (both), loop A, and loop B. The switches provide means for
selecting individual loops for integrity tests, or maintaining fire detection operation in an area where a fire
detector circuit has been analyzed as open by an integrity check. For normal system operation, the switch is
placed in the AB (both) position .
The fire detector control units are hermetically sealed electrical devices that contain the circuits for monitoring
the resistance values of the sensing elements. There are three control units, one for each fire area. There are
two monitoring circuits in each control unit that provide a separate monitoring circuit for each sensing element
loop in an area. Each monitoring circuit contains two relays for each system that are energized when a
resistance value, corresponding to a fire condition, is received from the sensing element loops. When the
relays in each circuit are energized , the respective fire warning components for the area will be activated. An
additional relay in each unit is used to cut off the fire warning bell ringing. The relay is energized by the fire
bell reset switch.

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ENGINE/WHEEL WELL FIRE DETECTION

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Warning of engine fires or overheat conditions is provided from either or both fire detector loops A and B
located in the engine compartment.
The wheel well areas contain fire detectors connected in a single loop.

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Wheel Well Fire Detection Circuit

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SENSING ELEMENTS
Dual sensing elements provide redundant fire detection for the engine and firewalls. The sensing elements are
twisted together and shrouded within a single case which is attached with quick-release clamps. Twisting the
sensors together prevents one sensor from shielding the other and provides uniform detection throughout its
length.
The sensor is a thermistor device which has a resistance inversely proportional to temperature; that is, as the
sensor temperature is raised by heating, its resistance lowers. The sensor is composed of two wires
embedded in a thermistor material, which is encased in a heavy wall inconel tube for high strength at elevated
temperatures. The electrical connector at each end of the sensor is completely insulated by ceramic insulators.
On all engines , the sensor is completely shrouded in a 1/2-inch diameter perforated stainless steel tube
(shroud) between the electrical connectors. The shroud protects the sensor from common hazards such as
breakage due to vibration, abrasion of sensor against airplane structure and damage from maintenance
activity in the area. The sensors are supported away from the sides of the shroud by teflon-impregnated
asbestos bushings.
The resistance of the sensor is a logarithmic average function and varies inversely with its length, the
increments of length being essentially resistances in parallel. For a given installation, the heating of less
than a full length on a sensor will require it to be heated to a higher temperature for the resistance to decrease
to the alarm point. This characteristic permits the system to integrate the varying temperatures throughout the
installation rather than sensing only the highest local temperature.

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FIRE EXTINGUISHING
Engine fire extinguishing is accomplished by discharging Freon gas around the top, bottom and sides of
the engines. The extinguishing agent is contained in two bottles on the right aft side of the aft pressure
bulkhead. Each bottle is discharged separately. The bottle to be discharged is selected by the engine fire
extinguisher bottle transfer switch. The discharge circuit for the selected bottle is armed when an engine fire
switch is pulled. Discharge of the engine fire extinguisher discharge switch associated with the pulled engine
fire switch.
DC power for discharging the extinguishing bottle is available whenever the battery switch is ON.

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SYSTEM INDICATOR DISCS
Two red and one yellow indicator discs installed on the aft fuselage below the Nº 3 engine are designed
to be blow out by gas pressure. Absence of a red disc indicates thermal discharge of a Freon bottle due to
high temperature. Absence of the yellow disc indicates the fire extinguishing system has been activated.

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