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A.  The smoke detector tubing is aluminum alloy tubing with medium pressure flareless couplings. The main cabin tubes are routed under the floor of the main cabin and connected to a coupling in the crease beam at each station. A short piece of tubing connected to the coupling is then either looped into an air grille or connected to an opening in a dado panel. The tubing in the electronic equipment compartment is mounted under the main cabin floor. Two sensing ports are connected to each tubing run and to a smoke detector. A total of four separate tubing runs, each with two sensing ports are installed. The outlet tubes of the forward smoke detectors are connected together; a single tube then runs forward under the floor and connects to the equipment-cooling duct. The aft smoke detectors and outlets are connected in a similar arrangement.
B.  Six drain fittings are installed at low points on the smoke detector tubing (Fig. 1). All drain fittings are easily accessible except the aft drain fitting. Access to the aft drain fitting (station 839) is gained by removing the fifth light assembly in the aft cargo compartment ceiling. The drain fitting can then be reached through the opening.
 Test Switch
A.  The test switch on fire protection module P8-1 has three positions: OVHT INOP-SMOKE A, off, FIRE-SMOKE B. The switch is spring-loaded to the center off position. The test switch is used by the engine and APU fire detection systems as well as the smoke detection system; therefore, when testing the smoke detection system, only the master warning and control, and cargo smoke circuit breakers should be energized to prevent false indications. Holding the switch to either one of the test positions tests the associated smoke detectors and causes the warning devices to operate. The warning devices stop operating when the switch is allowed to return to the center position.
7.  Operation
A.  Normal Operation
(1)  
Smoke detection is automatic when power is applied to the airplane and system circuit breakers are closed. When the airplane is on the ground, the equipment-cooling blower draws air through the smoke detection tubing and detectors and exhausts it overboard. When the airplane is airborne, air is forced through the system by pressurization from the air conditioning system.

(2)  
Air is directed through each of the smoke detectors. When smoke is present in the air being sampled, by the A or B smoke detector, the smoke particles reflect light from the beacon light onto the photoelectric cell (Fig. 3). When the photoelectric cell resistance decreases enough, Q1 and Q2 in the associated control amplifier turn on and energize the relay.

(3)  
When the control amplifier relay is energized, voltage is applied to the FWD CARGO SMOKE or AFT CARGO SMOKE light on P8-1. Bias voltage is also applied to the base of transistor Q1 and the anode of SCR1 in P8-1. Transistor Q1 conducts and a ground is provided for the master fire warning lights and the alarm bell.

(4)  
Bell cutout and master fire warning light extinguishing is accomplished by application of voltage to the gate of SCR1 when the bell cutout switch is pressed or a master fire warning light is pressed. When SCR1 is triggered by the gate voltage, control voltage to Q1 is grounded and Q1 is biased off, thereby removing the ground for the bell and master fire warning lights. The cargo smoke lights remain illuminated until the smoke condition and alarm signal have cleared.

(5)  
If the FWD CARGO SMOKE light illuminates, smoke is present in either the electronic equipment compartment or in the main cabin area forward of approximately station 650. If the AFT CARGO SMOKE light illuminates, smoke is present in the main cabin area aft of station 650.

511 
May 15/69  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  26-16-0 Page 7 

B.  Test Operation
(1)  
The smoke detection system is tested by moving the test switch on P8-1 to the left or to the right. If the engine, APU, and wheel well fire protection circuit breakers are not pulled, those systems will also be tested. (Refer to Engine Fire Detection System, 26-11-0; APU Fire Detection System, 26-15-0; Wheel Well Overheat Detection System, 26-17-0.) When the test switch is moved to OVHT INOP-SMOKE A, both A smoke detectors are tested. When the switch is moved to FIRE-SMOKE B, both B smoke detectors are tested. Both the FWD CARGO SMOKE and AFT CARGO SMOKE lights illuminate when either A or B smoke detectors are tested.

(2)  
When the test switch is operated, dc voltage is supplied to either test relay A or test relay

B. The energized relay then completes voltage to the test light in both A or both B smoke detectors. If the smoke detection system is operable, the illuminated test lamps simulate a smoke condition and cause the photoelectric cells to apply a signal to the forward and aft control amplifiers, thereby causing the alarm devices to operate. All alarm conditions are removed when the test switch is released.
　
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