# What radios are airliners / large aircraft equipped with?

I know that pilots have to talk to the tower over the radio, and apparently you have to tune the radio to the right tower frequency. I've also heard that in an emergency, you're supposed to "squawk 7700." Presumably you're not just making bird noises with your mouth, but some sort of radio code. Are these the same radio system? What radio communication systems are modern big jets equipped with?

Numbering the radios that I can think of offhand in the old 747-100 and -200 aircraft that I used to fly produces:

1. 2 VHF voice communications radios.
2. 2 VHF navigation radio receivers for receiving VOR and LOC signals. The LOC frequencies are paired with UHF frequencies for receiving glideslope signals.
4. 1 HF (high frequency) voice communication radio.
5. 2 transponders to send whatever the "squawk" is when interrogated. #1 was used on odd days of the month, #2 on even days. There are a number of standard codes, but ATC assigns a 4-digit code to each aircraft during flight under their control. There is never an 8 or a 9 in the code because it's an octal code.
6. 1 SELCAL (selective calling) receiver, which will sound an alert if it receives a signal sent by a ground station. It may not be a separate radio but just a box listening for a tone on one of the voice communication radios.
8. 1 radar altimeter for use in the final stages of the landing approach. Some were "talkers" in that starting around 50 feet, they would announce your altitude in increments of 10 feet.
9. 1 GPWS (ground proximity warning system). I'm not sure whether it and the radar altimeter shared a box/circuitry/antenaes.
10. 1 GPS receiver, which we got only in the last few months before I retired in 1999
11. 1 ELT (emergency locator transmitter)
12. life raft locator beacons. I can't remember how many life rafts there were.
13. life raft voice communication on 121.5 and maybe 243.0.

I may not have remembered them all.

Modern transport category aircraft would also have SATCOM. Military aircraft would have UHF voice communications and mission appropriate weaponry radios.

• Modern aircraft would also have data HF and SAT links used by ACARS. – Jan Hudec Sep 27 '14 at 9:04
• Thanks for the detailed answer! Could you clarify what the two navigation radio receivers were for? Are those the radios that look for the navigation beacons on the ground? – raptortech97 Sep 27 '14 at 11:30
• the black boxes' locator beacons are technically audio devices (they ping in the supersonic range underwater). – ratchet freak Sep 27 '14 at 15:11
• @ratchetfreak If the black boxes come to rest on land (not underwater), are these audio devices detectable? – Terry Sep 27 '14 at 20:05
• @DeltaLima I had completely forgotten about alternating usage of the 2 transponders until I saw your comment. Good catch! I edited my answer to conform. – Terry Sep 30 '14 at 16:58

This site has good information about the various radios on the 737 classic vs. NG, which will be similar to other modern airliners (though locations may differ somewhat). The graphics at the bottom show the external antennas, which include (for the NG):

1. Glide Slope - for ILS guidance
2. Localizer - for ILS guidance
4. GPS
8. Marker Beacon - for approach guidance
11. TCAS x2
12. ATC 1 - Maybe transponder, ACARS
13. ATC 2
14. VHF 1 - Regular voice communication
15. VHF 2
16. HF - For long range voice communication, such as oceanic crossings
17. ELT - Emergency Locator Transmittor

Please note that the above location diagrams are only a guide as the antenna fitted depends upon the customer avionics options. Eg some NG's do not have ADF but do have SATCOM or IFTS/Airphone.

More and more planes have satellite antennas for in-flight internet access, too. Military planes will also include more antennas for things like UHF and SATCOM.

Another site offers some details about certain radios on the 737 NG, including the power each one uses:

HF Voice Transceiver        400W
VHF Voice Transceiver       25W
DME (962 to 1150 MHz Xmit)  316W
TCAS                        400W
Transponder (1090 MHz)      631W