Form And Function

How Aircraft Radios Work

We tend to take radio communications for granted. It's considered the norm to talk and listen to someone or something virtually all the time we're flying. Indeed, for those learning in areas where flight traffic is thicker than striped jerseys at a soccer match, learning the ways of the radio can be as tough as the basic task of mastering control of the airplane.

And there's yet another distinction - talking on the frequency and actually understanding how the radio works. By radio communications standards, our black boxes are quite simple. What you'll usually find is a combination box called a nav/ com. It houses, as the name suggests, both navigation and communication radios. It used to be that these were in separate boxes in the airplane, and parts common to both, such as power supplies and speaker amplifiers, were needlessly duplicated. In the modern world, however, the nav/com is king.

Items left to the pilot's control in these radios are few. You get to select the transmit and receive frequency, set the volume, and usually determine the squelch threshold. This merely adjusts the lowest level of the receiver's sensitivity. Because there is always static on aircraft frequencies, setting the level at which the receiver mutes saves ears and tempers. Squelch control also can lead to some confusion. If you ever find that your transmissions aren't being answered, check the squelch. Rotate the squelch control until background noise, which sounds like eggs frying in a pan, comes through. Then rotate the control just until the sound stops. In radios without a separate squelch control, pull on the volume knob; in almost all cases, this overrides the automatic squelch. This way you know whether your receiver is working. If there is no noise, then it's possible that the volume is turned down too low, the audio panel may be set up incorrectly, you may be using a bad headset, or the radio is simply broken.

Some pilots have asked why we still select the actual frequency that we want to use - wouldn't channel numbers be easier? ("Mooney Three-Four-November, contact approach on channel 17.") The current practice of using actual frequencies dates back to the days before crystal and microprocessor frequency control. Just as with an old 1940s home radio, pilots selected frequencies by hand, often matching a crystal transmit frequency with a variable receiver.

Not only do we have computers controlling tuning - with quite good accuracy and durability - we are used to digital displays and some neat tricks associated with them. The majority of modern radios have what are called flip-flop displays. The active frequency is stored in one position and a standby frequency is held in another; this allows you to pretune for the next frequency or keep the one that you were just using handy in case nobody answers at the next controller's position.

In the event that you are given a contact frequency with 0.25-kHz spacing, such as 132.075 MHz, you should try pulling the small tuning knob before you think that the radio doesn't have 2s or 7s in the last digit. Other radio types have these frequencies in the normal spinning of the dial; you just won't see the trailing 5 on the display.

No matter what type of radio you are using, there is one cardinal rule of radio communications: Listen before you speak. Unlike telephones, our communication channels are simplex, or one at a time. Two pilots transmitting simultaneously will create a squealing noise for everyone else on the frequency.

Pilots sometimes call in on the wrong frequency. Chances are that they just punched in the wrong numbers, but occasionally there's some miscue concerning multiple radios. For the sake of redundancy and convenience, some airplanes have dual nav/coms. You can only transmit on one at a time, and the audio panel - usually at the top of the stack - controls which radio is in use. In some models, it is possible to listen to both radios at the same time or even to inhibit the receive on the transmitting radio. Check the configuration if you're in doubt.

Remember, too, that aircraft radios work in the VHF band. (That stands for very high frequency, a term coined before the advent of microwave technology. These frequencies actually reside in the lower-middle frequency neighborhood these days.) The signal is line-of-sight, meaning it can only be received if there's nothing to get in the way between the transmitter and the receiver. So don't expect to hear the ATIS if there's a mountain between you and the airport.

Radios don't fail all that often - certainly they malfunction far less frequently than just a few years ago - but it does happen. When you think the com is dead, first check the squelch to see if you can receive. Assuming that you can, it's possible that you aren't transmitting. Try another radio.

Now, for the sake of argument, let's assume that you have just the one radio. If, for example, you're coming in to land at a towered airport, try reaching the controller on the ground frequency. Sometimes radios keel over on just one or a handful of frequencies.

If that's still a no-joy proposition, bone up on your light signals (You do keep a copy of these handy, don't you?) or reach for your handheld transceiver. Some flight instructors scoff at the idea of buying a handheld radio early in your flying career, but the purchase makes sense for a couple of reasons. First, it's there when you really need it, whether on your first solo cross-country - and don't think it's never happened - or during an instrument flight down the road. Besides, you can always tune the receiver to local tower or approach facilities and practice lingo and learn any local customs. You'd be surprised how effective a few hours of eavesdropping can be.