5. Discharge Line
A. The discharge line disperses the released extinguishing agent from the discharge plug to the APU shroud. The line is routed from the bottle installation aft into the APU compartment and into the APU shroud. A nozzle disperses the extinguishing agent around the APU at a proper rate for effective extinguishing.
B. A tee in the discharge line connects the discharge line to the system discharge indicator. The indicator is on the lower empennage near the extinguisher bottle. When the bottle is discharged, pressure in the discharge line is suddenly increased, causing a plunger in the indicator to knock out a yellow disc. The plunger then blocks the line preventing any extinguisher agent from being dumped overboard.
6. Indicators Discs
A. Two indicators, a red and a yellow, are installed in the airplane skin. (See figure 1.) The indicators are on the lower right side of the empennage section at approximately station 1064. The red thermal relief indicator disc is blown out in event of an extinguisher bottle overheat. The yellow system discharge indicator disc is knocked out by a plunger in the event of a normal fire extinguisher discharge.
7. Operation
A. The APU fire extinguisher system may be controlled from two locations on the airplane. The two methods of control are normal operation and remote operation. Normal operation is control of an APU compartment fire using the APU fire switch on the fire protection system module. Remote operation is the control of an APU compartment fire using the APU remote fire switch and extinguisher switch on the remote APU fire control panel.
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B. Normal Operation
(1) When a fire warning indication is received, the APU fire switch handle in the control cabin will be unlocked. When it is determined a fire condition exists and corrective action must be taken, pull out on the fire switch handle. When the handle is pulled out, the fire extinguisher system is armed. Besides arming the fire extinguishing system, the APU fire switch performs the following functions:
(a)
Close the APU fuel valve and close the inlet door.
(b)
Disconnect the APU generator by tripping the generator control relay. The relay de-energizes the generator field and disconnects the generator from the ac bus within five to ten seconds.
NOTE: APU fire warning circuit normally automatically shuts down APU. For additional information on APU shutdown, refer to Chapter 49, Airborne Auxiliary Power.
(2) When the system is armed the APU fire switch handle may be rotated either to the left or right to discharge the APU bottle. When the handle is rotated in either direction (for at least one second), the squib is fired, rupturing the sealing diaphragm and releasing the extinguishing agent. The extinguishing agent flows directly from the bottle to the APU shroud. The pressure of extinguishing agent in the discharge line causes the yellow disc in the discharge indicator to be knocked out, showing the system has been actuated. When the bottle pressure approaches approximately 250 psi, the APU bottle discharge light illuminates, indicating the APU bottle is discharged and inactive.
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C. Remote Operation
(1) When the APU is being operated for maintenance purposes and no one is in the control cabin, the APU can be controlled for shutdown and fire extinguishing purposes from outside the airplane. The APU is controlled from the remote APU fire control panel (P28) located on the aft bulkhead on the right side of the main wheel well. When it is determined a fire condition exists and corrective action must be taken, pull down on the remote fire switch handle. When the handle is pulled the fire extinguishing system is armed. Besides arming the system, the remote APU fire switch performs the following function;
(a)
Close the APU fuel valve and close the inlet door.
(b)
Disconnect the APU generator.
NOTE: For additional information on APU shutdown, refer to Chapter 49 APU Engine Controls.
(2) When the system is armed the APU remote fire extinguisher switch is positioned to DISCHARGE for at least one second. The squib is fired, rupturing the sealing diaphragm and releasing the extinguishing agent. The extinguishing agent flows directly from the bottle to the APU shroud. The pressure of extinguishing agent in the discharge line causes the yellow disc in the discharge indicator to be knocked out, showing the system has been actuated.
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