Communications

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Contents

Contents

• Audio Control Panel
• VHF Radio
• Nav Radio
• Marker Beacons
• SELCAL
• Cockpit Voice Recorder
• Flight Interphone
• Service Interphone
• Transponder
• ELT
• Antennae
• Limitations

All of the information, photographs & schematics from this website and much more is now available in a 374 page printed book or in electronic format.

*** Updated 14 Nov 2021 ***

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Audio Control Panel

This type of ACP has cylindrical button volume controls, others have sliders.

Radio/Int works in the same way as the rocker switch on the control column. ie in the INT position bypasses the mic selector to transmit on the flt interphone.

The filter switch, Voice-Both-Range, allows better reception of either voice or morse identifiers on NAV & ADF radios. Check that this switch has not been left in the V position if you can't get an ident.

Mask/Boom simply selects either mask or boom mic. Check this if nobody can hear you transmit - especially after your oxy mask mic check!

Alt/Norm in the ALT position puts the ACP into degraded mode. If the Capts ACP is in degraded mode, he can only transmit on VHF1 through mask or boom and can only receive VHF1 at a preset level. The F/O's ACP in degraded mode is the same but uses VHF2. Note aural warnings will still be heard over the speaker.

-200 ACP

VHF Radio

Most 737's have three VHF radios and at least one HF radio. This unit allows for selection of any of those at this station. The TEST button is a squelch and is used to hear faint stations that do not have the strength to breakthrough. If you switch the panel OFF at the same time as TEST is applied this holds the test condition thereby allowing you to hear faint stations without having to hold down the test button - very useful for copying distant weather!

Photo Niklas Andreae

Nav Radio

There are very many different Nav radio boxes in the worlds 737's the second shown here is the new Multi-mode nav panel which is used to tune VORs, ILS, & GLS. More details about their use in the Navigation section.

Marker Beacons

The markers are pre-tuned to their 75MHz frequency and illuminate when overflown. The marker tone can also be heard if selected on the ACP.

Selective Calling, SELCAL

The SELCAL light will illuminate and a two-tone chime sounds if the aircraft is being selcal’d on either HF or VHF.

This particular panel is a very old unit and most operators have had to improvise the method of radio connections to it. Typically, in the past, diodes would be used to "OR" the VHFs together to illuminate one of the lights.  Over the last 15 years, the vast majority of the SELCAL panels have a light for each of the radios (VHF-1, VHF-2, VHF-3, HF-1, HF-2) and in some cases, include the Attendant call, and SATCOM call.

Cockpit Voice Recorder

The CVR records the headset and microphone of all 3 ASP's and the ambient cockpit sounds all on separate channels. The recordings start with the first rise in engine oil pressure and go onto a 120 or 30min (as fitted) continuous loop tape until 5mins after last engine shutdown. In the event of an incident crews are advised to pull the CVR c/b after final stop to avoid automatic erasure. It is illegal to stop the CVR in flight. The CVR is located in the aft cargo hold.

FDRs and CVRs have an underwater location device that, when activated by water, emit an ultrasonic pulse at 37.5 kHz every second for a duration of at least 30 days.

The CVR is located in the aft cargo hold.

For information on the Flight Data Recorder (FDR) click here

This video covers the CVR, FDR, ELT, LF-ULB, DFDAU and QAR on the 737. It also discusses the use of ACMS data within an airline and how hard landings are measured.

The External Power Hatch is located beneath the F/O's DV window. It is used by groundcrew to connect the Ground Power Unit and headset for pushback communications with Flight Interphone.

The service interphone is used by engineers to communicate with the service interphone stations inside the aircraft. Note that the Service Interphone switch on the aft overhead panel must be switched ON for this use.

There is also a pilot call button and a nose-wheelwell light switch to assist the groundcrew to insert the steering bypass pin.

Service Interphone

The switch on the aft overhead panel activates the external jacks to the service interphone system. Normal internal service interphone operation is unaffected by switch position.

Transponder with integrated TCAS.

Note that with the mode switch in the STBY position, no transmissions will occur. When selected, modes A and C will only transmit when the aircraft is not on the ground, but mode S will operate regardless of air/ground status.

Number pad transponder

Note: Mode S transponders will be mandatory in Europe after March 2007.

ELT

The blue CALL light on the overhead panel illuminates and a single-tone chime sounds when either the cabin crew (service interphone) or ground crew (flight or service interphone) are calling the flight deck.   

ACARS