Sunday, July 25, 2010

More Audio Panels Demystified

About two years ago I wrote an article on the KMA24 audio panel. You can find it on my blog at

Audio Panels Demystified

I thought that it would be a good idea to update that article, with a discussion of the newer Garmin 340 audio panel. On a macro level it works pretty much the same way as the KMA24, but there are enough differences to warrant an update. First a picture of the Garmin 340 audio panel is needed to serve as our roadmap.

Let’s break the panel down into groups and sub-groups.

Marker Beacons

1 Marker beacon lamps

2 Marker beacon audio select/mute button

3 Marker beacon intensity indicator LEDs

4 Marker beacon intensity selector button

Intercom System

5 Pilot volume control

6 Pilot voice-activated intercom squelch level

7 Co-pilot volume and passenger control (pull knob out for passenger volume)

8 Co-pilot and passenger intercom squelch level

9 Crew isolate mode button

10 Pilot isolate mode button

11 Passenger address (PA) function

12 Speaker function button


13 Audio select buttons (COM 1, COM 2, COM 3)

14 Transmitter (audio/mic) buttons (COM1 MIC, COM2 MIC, COM3 MIC)

15 Split com button (a really useful function)

16 Aircraft radio audio selection buttons (NAV 1, NAV 2, ADF, DME)

17 Annunciator test button

18 Locking screw access

19 Photocell (automatic annunciator dimming)

Failsafe mode

The power to the unit may be manually turned off by rotating the pilot’s volume control button (5) counter-clockwise until it goes past the detent. Normally power is controlled by the avionics master switch. In case of power failure, the pilot’s headphone and mic is connected directly to COM 1.

Com Functions

Selection of COM1, COM 2, or COM 3 for both mic and audio is done by pressing com mic function buttons (14). Selection of either COM1 MIC, COM2 MIC, or COM3 MIC activates both the mic and headset function for the selected COM radio. Obviously only one of the three may be selected at any given time. The selected COM audio automatically goes to headset. Additionally any additional audio may be heard by pressing one of the remaining COM buttons (13). When transmitting, the active COM MIC LED display blinks approximately once/second to indicate transmission is occurring.

Split Com

The split com button (15) is undoubtedly one of the most useful features in the audio panel. When it is pressed, the pilot hears and transmits on COM 1 frequency. The co-pilot hears and transmits on COM 2 frequency. The split com function is terminated by pressing the split com button a second time.

While in split com mode, it is possible for the co-pilot to make announcements to the passengers by pressing the PA button (11). Pressing the PA button a second time terminates the announcement mode and returns operation to normal split com mode.

Aircraft Radios and Navigation

The navigation radios audio function is controlled by the buttons of group 16. Pressing any of those buttons brings up the nav audio source. A second push terminates the nav audio source. Volume is controlled by the volume control on the radio unit itself.

PA Function

Passenger announcements may be made by either pilot or co-pilot by pressing the PA button (11). Pressing the PA button a second time terminates announcement mode.

Speaker Function

The speaker button (12) directs all aircraft radio output to the aircraft speakers. The speaker output is muted when a COM microphone is activated.

Isolate Functions

The CREW isolate button (9) isolate communication between pilot and co-pilot from the passengers. The passengers can communicate with each other but cannot hear either the pilot or co-pilot or hear the aircraft radios.

The PILOT isolate button (10) isolates pilot communication from both passengers and the co-pilot. The co-pilot and passengers can communicate with each other but cannot hear the pilot or any aircraft radios.

Test Button

The test button (17) illuminates all annunciators in the audio panel.

Marker Beacons

The marker beacon group (1 – 4) control operation of the marker beacons. Pressing the MKR/mute button (2) activates audio for the marker beacons. If no signal is received, pressing it a second time deactivates the function. If the MKR/mute button is pressed a second time while a signal is being received, the audio is muted by the MKR function is not deactivated. The marker beacon lamps (1) operate independently of any audio selection and cannot be turned off.

The sensitivity button (4) controls either low or high sensitivity of the marker beacons as indicated by the HI or LO LED indicators (3). LO sensitivity is used during an ILS approach. HI sensitivity may be used to get an earlier indication of the aircraft approaching a marker beacon or for identification of a marker beacon along an airway.

Marker beacons are activated by passing over a 75 MHz marker beacon transmitter. Whether the associated marker beacon tone is heard or not depends on whether the MKR/mute button (2) is selected.

The following table shows the operation of the marker beacons.

All in all, the Garmin 340 audio panel is a real 21st generation replacement for the trusty KMA24 audio panel that still ably serves in a lot of airplanes. Once you understand how the Garmin 340 audio panel works, I think you will be impressed by its capability and ease of operation.