(a)  
Communications should be loud and clear between stations.

(b)  
Varying the volume control on each audio selector panel should increase or decrease the associated headphone audio level.

(c)  
Varying the volume control on the captain's or first officer's audio selector panel should increase or decrease the associated loudspeaker audio level.

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(d)  
Varying the volume control on a loudspeaker should increase or decrease the loudspeaker audio level.

(e)  
Both loudspeakers should be muted whenever a PTT switch is pressed at the captain's, first officer's or observer's station.

(f)  
Ensure the loudspeakers can be turned off and on.

NOTE:  The second observer has a headphone jack paralleled to the first observer's audio selector panel. If necessary, adjust the flight interphone amplifier gain potentiometer on the front of the audio accessory unit to obtain a comfortable listening level.
(3)  
Select OXY on each audio selector panel and establish communications between the captain's, first officer's and first observer's stations using their oronasal mask mikes. Tap the side of the mike to produce audio signals and press the audio selector panel PTT button to operate the mike.

(a)  
Communications should be loud and clear between stations.

(b)  
Both loudspeakers should be muted whenever a PTT switch is pressed.

(c)  
The PTT switch should be spring loaded to an off position when released.

(4)  
Repeat step C.(3) between the captain's and first officer's stations using their oronasal mask mikes and their control wheel PTT switches held to the MIC position. Results shall be similar.

(5)  
Position the mike selector switch on all audio selector panels to OFF.

(a)  The INT light should extinguish.

(6)  
Select BOOM with the BOOM-OXY switch and push up the INT toggle switch on all audio selector panels.

(7)  
Establish communications between the captain's and first officer's stations using boom mikes and the INT side of the control wheel PTT switches.

(a)  
Communications should be monitored at all stations regardless of the position of the audio selector panel mike selector switches.

(b)  
Both loudspeakers should be muted whenever the PTT switch is pressed.

(8)  
Position the PTT switches to the off position.

(9)  
Using a handset plugged into the FLIGHT jack on external power receptacle panel P19, establish communications with each audio selector panel.

(a) Handset audio should be heard loud and clear with headphones at each station.

(10)
Turn off the interphone loudspeakers, and push down the INT toggle switch on each audio selector panel.

(11)
If no longer required, remove electrical power from the airplane.

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FLIGHT INTERPHONE SYSTEM - DESCRIPTION AND OPERATION
EFFECTIVITY
Airplanes with Digital Flight Interphone
1.  General (Fig. 1)
A.  The flight interphone system provides facilities for interphone communication among flight compartment crewmembers and ground personnel at the nose gear interphone station, and provides the means to key, transmit, and receive on airplane radio systems and receive on airplane navigation receivers.
B.  In addition the low range radio altimeter (LRRA) system audio will be heard through the flight crew headsets.
C.  The flight interphone system consists of audio selector panels, a remote electronics unit, jack panels, push-to-talk switches (PTT), handheld microphones, boom microphone/headsets, oxygen mask microphones, loudspeakers, and associated equipment.
D.  The captain and first officer each have an audio selector panel, a cockpit speaker, a control wheel PTT switch, a boom microphone headset and oxygen mask microphone, plus jacks for a headset and a handheld microphone.
E.  The first observer has an audio selector panel, an oxygen mask microphone, a handheld microphone and a headphone. The second observer has a headphone jack. The audio selected by the first observer is heard at both observer stations.
F.  The flight interphone system receives power from the dual power source of 28v dc battery bus and 28v dc bus No. 2, through circuit breakers and diodes to allow either bus to power all circuits from load control center P6.
　
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