Microphones
Hand microphones and boom microphones may be plugged into the related jacks at the flight deck crew stations. Each oxygen mask also has an integral microphone.
Each hand microphone has a PTT switch to key the selected audio system. The PTT switches on the control wheel or ASP are used to key the oxygen mask or boom microphone, as selected by the MASK–BOOM switch. The MASK–BOOM switch does not affect the operation of the hand microphone.
Normal Audio System Operation
The Captain, First Officer, and Observer audio systems are located in a common remote electronics unit in the E & E compartment. They function independently and have separate circuit breakers. The audio systems are normally controlled by the related ASPs through digital or computerized control circuits.
Degraded Audio System Operation Airplanes with Digital Audio Selector Panels
If the remote electronics unit or ASP malfunctions, the ASP cannot control the remote electronics unit. Audio system operation can be switched to a degraded mode by placing the ALT–NORM switch to ALT. In this mode, the ASP at that station is inoperative and the crewmember can only communicate on one radio.
The ASP transmitter selectors are not functional, and any transmission from that station is on the radio shown on the chart below. The transmitter selector for the usable radio illuminates when a station is operating in the degraded mode. The receiver switches are not functional, and only the usable radio is heard at a preset volume, through the headset. The speaker and speaker switch are not functional at that station. In addition, the flight interphone and service interphone cannot be used. The control wheel PTT switch INT position and the ASP PTT switch I/C position are not functional, since the flight interphone is not functional.
The mask and boom microphones can be used for transmission on the usable radio. The MASK–BOOM switch works normally in the degraded mode. The mask and boom microphones can be keyed with the control wheel PTT switch MIC position or the ASP PTT switch R/T position. The hand microphone is not usable in the degraded mode of operation.
An audio system operating in the degraded mode cannot access the passenger address system through the audio control panel. The crewmember can still use the service interphone handset and PA microphone if they are installed on the control stand.
Copyright . The Boeing Company. See title page for details.
5.20.2 August 1, 2003
Communications -System Description 737 Operations Manual
Audio warnings for altitude alert, GPWS, and windshear are not heard on an audio system operating in the degraded mode.
CREW STATION AUDIO SYSTEM IN DEGRADED MODE RADIO AVAILABLE FOR TRANSMISSION AND RECEPTION AT DEGRADED STATION
CAPTAIN VHF–1
FIRST OFFICER VHF–2
OBSERVER VHF–1
Flight Interphone System
The flight interphone system is an independent communication network. Its primary purpose is to provide private communication between flight deck crewmembers without intrusion from the service interphone system. The ground crew may also the use flight interphone through a jack at the external power receptacle.
The pilots can transmit directly over the flight interphone by using the control wheel PTT switch. Alternatively, any crewmember with an ASP can transmit/receive over the flight interphone by using their related ASP and normal PTT switches. Any standard microphone may be used with the flight interphone system.
Service (Attendant) Interphone System
The service interphone system provides intercommunication between the flight deck, Flight Attendants, and ground personnel. Flight deck crewmembers communicate using either a separate handset (if installed) or their related ASP and any standard microphone.
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