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:
- 2 VHF voice communications radios.
- 2 VHF navigation radio receivers for receiving VOR and LOC signals. The LOC frequencies are paired with UHF frequencies for receiving glideslope signals.
- 1 ADF (automatic direction finding) receiver.
- 1 HF (high frequency) voice communication radio.
- 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.
- 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.
- 1 weather radar, and, yes, radar is a radio (radio detection and ranging) employing both a transmitter and a receiver.
- 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.
- 1 GPWS (ground proximity warning system). I'm not sure whether it and the radar altimeter shared a box/circuitry/antenaes.
- 1 GPS receiver, which we got only in the last few months before I retired in 1999
- 1 ELT (emergency locator transmitter)
- life raft locator beacons. I can't remember how many life rafts there were.
- 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.
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):
- Glide Slope - for ILS guidance
- Localizer - for ILS guidance
- Weather Radar
- Radio Altimeter
- ADF 1 - Navigation
- ADF 2
- Marker Beacon - for approach guidance
- DME - Navigation
- VOR - Navigation
- TCAS x2
- ATC 1 - Maybe transponder, ACARS
- ATC 2
- VHF 1 - Regular voice communication
- VHF 2
- HF - For long range voice communication, such as oceanic crossings
- 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 Radar Altimeter (4300 MHz) 400mW TCAS 400W Transponder (1090 MHz) 631W WX Radar (9.3 Ghz) 120W
For instance, it's interesting how the HF radio uses much more power than the VHF radio, and that the transponder uses the most power on the list.
This depends on the plane - some planes don't have radios at all!
"Squawking" in this sense refers to punching a number into your transponder: http://en.wikipedia.org/wiki/Transponder_(aeronautics)
There are some squawk codes on that page if you want to read more!