Loss of communication generally comes in one of three varieties: inability to transmit, loss of reception, and--rarely--both. A pilot should have a systematic method to determine what has failed and how to fix it. Often the problem can be corrected in the air.
Most lost communications are pilot-induced. The pilot forgets to turn up the volume or improperly sets the audio panel. If you transmit and do not hear an answer, don't assume that you have not transmitted (see "Learning Experiences: Lost Communications," p. 51). First increase the volume of the radio. You can always tune to a busy frequency; 123.0 and 122.8 MHz are good choices to check reception. Another good choice is an automated terminal information service (ATIS) or automated surface observation system (ASOS) frequency at a nearby airport. Most pilots check the radio on the ground before takeoff. Communicating with ground control at a towered airport confirms radio function. At a nontowered airport you may ask for a radio check, and get an answer from unicom or another pilot on the frequency.
The next most common pilot-induced fault is incorrectly set audio panel switches. While audio panel switches vary, two important audio panel functions should be clearly understood. To effectively communicate, a microphone and a speaker or headset should be channeled to the same radio. (There are rare situations when you may wish to transmit on one frequency and listen on another--calling flight service when you must listen to their response over a VOR frequency is an example--but in most cases the mode is simplex, which means one frequency for two-way communications.) If the pilot has turned the wrong switch and inadvertently connected the microphone to one radio and the headphone/speaker to another, listening or transmitting will be in the blind.
Now try the second communications radio, assuming the aircraft that you're flying has two. If communications are restored, you have isolated the problem to the first radio. If communications are still not restored, unplug the headphone and microphone and then plug them in again; if the contacts are oxidized or corroded, this may solve the problem.
If adjusting the volume, checking the audio panel switches, trying the second radio, and resetting the plugs do not restore communication, further troubleshooting requires additional understanding of aircraft radio systems.
Receiving and transmitting each require four separate pieces of aircraft equipment. To transmit you need a microphone, an audio panel, a radio transmitter, and an antenna. To receive transmissions, an antenna, a radio receiver, an audio panel, and a speaker or headphones are needed. It is helpful to know how your aircraft is wired. For example, many light aircraft have two navigation/communication radios and two communication antennas. However, it is common to have only one navigation antenna with an antenna splitter to serve both navigation radios. There is usually little a pilot can do about antenna malfunction or a wiring problem between the antenna and the radio.
The next piece of equipment in line is the audio panel. If it is suspect, the entire panel can be bypassed so that communication passes directly from the radio to a headphone. Audio panels contain a speaker amplifier, so you will lose speaker function with the isolation, but your headset will operate. Audio panel bypass or failsafe is typically accomplished by turning off the audio panel. However, you should know how to enable the bypass function on the aircraft you fly. The bypass allows the use of only one radio, because you no longer have the option of switching. You should know which radio it is so that you can use the correct one in the event of a failure.
The most common electronic cause of failed communication is a microphone or headset wiring fault. Cockpit leads spanning mics and headphone-to-panel plugs are flexed and twisted many time each flight. As frayed wires and loose connections will defeat communications, it is advisable to have two complete headsets, or a combination of a back-up microphone and headset or cockpit speaker. Isolating a bad mic or problem headset is easy if you have a substitute for each. A common point of failure is the push-to-talk switch, which could prevent you from transmitting--or stick in the transmit position (look for a continuous transmit indication on your comm radio). If a yoke-mounted push-to-talk switch fails in the off mode and the airplane has a separate handheld mic, you should be able to use it to transmit; failure in the transmit mode is difficult to remedy in flight, and the best solution may be to turn off the radio.
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The most rarely used radio control is the squelch. It can extend communication capabilities and help to diagnose receiver problems. Comm radios are usually used with squelch enabled. Some radios allow an adjustment of squelch level, but others only permit squelch to be on or off. When squelch is active, an incoming transmission must be of a certain amplitude before it can pass to a speaker or headset. That is why the radio is quiet between transmissions. When an incoming transmission is strong enough, it "breaks squelch," and is heard.
If you are trying to listen to a weak transmission--perhaps a distant flight service station--amplitude may not be sufficient to break squelch. You cannot improve the problem by increasing receiver volume, but the remedy is to turn off the squelch so that a much weaker signal can be heard. However, a side effect is a hissing noise. While normally annoying and a reason to keep squelch enabled, receiver noise is helpful in diagnosis. If you are not sure your receiver is working, turn the squelch off. If you then hear a hissing noise, the radio and headset/speaker are probably working.
Transmission failure can be more subtle to detect. There may not be any failure if you are transmitting on a frequency where there is no one to reply. Perhaps your airplane is too distant from the intended receiver, or too low for the line-of-sight transmission required by the frequency band that aviation radios use. Perhaps you are tuned to the wrong frequency.
How do you know you are transmitting? The best way is to listen for a sidetone. When you transmit, the radio transmitter returns an audio echo to the headphones, so you can hear yourself talk. We unconsciously raise or lower our voice by listening to how loudly we hear ourselves speaking. This is why people with partial hearing impairment inadvertently talk loudly. However, this does not apply to the cabin speaker, which has no sidetone. This is another reason to have at least one headset aboard.
To demonstrate sidetone, simply listen carefully while transmitting. You should hear sidetone. Then pull the headphone plug (not the mic plug), and again transmit. The difference will be evident, as you cannot hear yourself talking. So, if you cannot hear sidetone, you are probably not transmitting--assuming that the headset is functioning normally. If the headphone is inoperative, or there are audio panel problems, the fault is on the receive side, and sidetone cannot be heard even if the transmitter is producing it.
On many radios a transmit annunciator lights when the push-to-talk switch is depressed. This is another partial test of transmission function.
Most modern aircraft radios have automatic microphone gain, the amount an amplifier increases signal amplitude. When a new microphone is used, gain is automatically adjusted. However, on older radios microphone function may be poor until an avionics technician adjusts gain to match the new microphone.
Modern aircraft radios also have automatic gain and volume control circuits. These adjust the level of audio output to a consistent level, independent of incoming signal strength. No matter how far you are from a transmitter the volume is kept constant. The controller in the tower does not sound louder the closer you approach. However, when signal strength decreases beyond a certain point, the signal is blocked (squelched). This may cause intermittent signal reception. Continuous reception of a weak signal can be accomplished by defeating squelch. At some point, however, even without squelch transmissions will fade into background static.
Many pilots use an intercom, either portable or panel-mounted. This introduces additional areas where there can be faults. However, there is always a way to bypass the intercom, which should help you to determine if the intercom is the source of the problem.
If these troubleshooting tips don't solve a communication problem, you will need a professional repair. However, your tests and observations will assist the avionics technician. If you pay for your own repairs, providing the results of these simple tests will help to minimize the bill.
The best way to assure communications is to carry a back-up handheld comm radio (also known as a transceiver) and extra batteries for it. Even with complete electrical failure, you have a way to communicate. A handheld will be more effective using an auxiliary external antenna and dedicated antenna plug. An outside antenna substantially increases the transmission and reception range over a "rubber ducky" antenna. You may have to be closer to a ground station to communicate with a handheld that you would if the aircraft radio was working.
A pilot prepared to troubleshoot and with reasonable back-up equipment should rarely lose communications while aloft.
Dr. Ian Blair Fries is a CFI, senior aviation medical examiner, and ATP, and holds a Lear 35 type rating. He serves on the AOPA Air Safety Foundation Board of Visitors and is cochairman of the AOPA Board of Medical Advisors.
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Links to additional resources about the topics discussed in this article are available at AOPA Flight Training Online.
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