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Thursday, December 10 2015 21:16

Troubleshooting Your Navcom

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Try these troubleshooting tips before you visit your favorite avionics shop for service.

We all know it takes fuel to fly our aircraft from point A to point B, but we sometimes take for granted that communicating and navigating along the way is just as important to knowing how to manage your engine and fuel reserves.

After more than 20 years running an avionics shop, I have seen, heard and experienced many different types of navcom problems. Some are really complicated, but many times the issues can be very simple.

The techniques outlined in this article will help you determine if you truly have a problem with your aircraft’s navcom system, or if it’s just a “switchology” problem.

Transmit or receive? Antenna or Audio?
Communications transceivers (i.e., com radios) have been around for aviation use for almost 100 years and were a huge step ahead of the prior method of hand signals and flags.

Problems with com radios can be broken down into two major categories: transmit and receive. Sometimes a pilot may experience problems with both transmit and receive, but usually it is one or the other.

Com receiver problems may also be broken down into two major categories: problems with the antenna, and problems with the audio system. The majority of the problems I see are associated with the audio system. (Antenna problems do occur, but are less frequent.)   

Most of the time, antenna problems are associated with a broken or deteriorated antenna—and this can be picked up on by the owner-operator during a walkaround. As long as the antenna is not physically broken and the white fiberglass resin isn’t flaked off, it’s probably okay.

Troubleshooting the audio panel, headsets and speakers
Receiver audio problems are most of the time associated with the pilot’s headset not being fully plugged in. Or sometimes, the pilot’s leg rests against the jack and causes it to short out. A short can cause intermittent operation or even static.

Always, always, check that your headset is fully plugged into your jack(s) if you can’t hear the receiver or intercom. Then check to see if your passenger jacks are fully plugged in and that no one has their leg resting on the headset cables.

Keep your headset cables in good working order and inspect the cables regularly because many people shut them in doors, slide seats over them or even worse.

Do not leave headsets plugged in all the time; they will corrode. Regular plugging in and removal is good for the jacks and the headset plugs—it cleans off any tarnish that might be on them.

headset cable 1web

If you’ve determined that your headset is plugged in properly and no one has their knee laying on the jacks, double-check your audio panel and make sure that you have selected the desired COM 1, 2, or 3 to listen and/or monitor.   

Next, make sure that the volume on the radio is set to an acceptable level. The easiest way to do this is to enable the squelch function on the radio and listen to the white noise that the radio produces in order to adjust the receiver to your hearing.    

Next, check the headsets themselves. I always recommend that pilots turn their headset volume fully up, and to make sure everyone’s in the aircraft is fully up. Then adjust the receiver volume with the squelch turned on in order to suit the PIC’s hearing.    

Once that has been done, adjust any other receiver the same way. If the intercom or receiver audio is too loud for a passenger, instruct them to lower the volume on their individual headset.   

Finally, does anyone remember what we used before every aircraft we sat in had headsets and intercoms? That’s right, the overhead speaker. Test your overhead speaker regularly and replace it when it is no longer audible.

These days, so many people take for granted that they don’t need the speaker, but it might keep you from having to remember those dreaded light gun symbols. You just never know when you might find yourself in IMC and a nervous passenger has laid his knee against the headset jacks.

Now you have silence just when you know you should be getting your next vector to turn final call—and you’re flying right through the final approach course. You quickly reach up and engage the squelch on your radio and there’s still silence.

But when you hit the speaker button, you can hear ATC repeating your call sign and urging you to acknowledge. That cheap $15 speaker just might have saved your life!

Troubleshooting a com transmitter
Com transmitter problems are much like receiver problems and can have many of the same issues that plague the receive side.

The biggest complaint that I get from owner-operators is that they have “carrier and no voice.” What this means is that ATC is able to determine that a COM is transmitting on that frequency but there is no “intelligence,” or modulation, present.

Most of the time, this is due to a problem with the headset itself, or the connection between the headset and the COM (such as the jacks). Make sure that the intercom microphone is working and that you can hear that first.

If the intercom mic is present, do you hear a change in your voice (side tone) when you transmit? If you do hear the side tone of yourself talking, it usually means that the audio from your voice is making it to the COM. 

Next, did you check to make sure that the frequency is properly selected on the com? Sometimes you will load the next frequency into the standby on the com, but when you hit it to flip-flop, the radio does a double-take and puts you back on the old frequency. This is common with some older radios.  

If ATC still isn’t hearing you, reach down in the middle console and grab your trusty old hand mic and try that. (Hopefully it isn’t buried in your glove box or in the seat pocket behind you and out of reach!) Make sure the hand mic is fully plugged in before you try and use it. Regular inspection of the hand mic cable to make sure it isn’t frayed will also save you from transmitter headaches. 

I can’t stress this hand mic point enough: I see aircraft come into my shop regularly with transmit problems and often it boils down to the hand mic being just a little bit unplugged. Make sure it stays plugged in all the way and that the cable isn’t frayed.

Troubleshooting navigation
According to our friends at the FAA, airborne navigation was first conceived by the U.S. Postal Service in the 1920s, with VHF/UHF navigation developed during the 1940s.    

Troubleshooting airborne navigation receivers is a relatively simple process and can be either antenna- or indicator- related. (This article focuses on VHF NAV (VOR/ILS) and GPS receivers, not other types of airborne navigation. —Ed.)   

Navigation antennas are often mounted on the top of the tail of the aircraft and resemble what most people call “cat whiskers” or “towel racks.” When trying to isolate a problem with range in your VHF NAV, it is usually a good idea to look at the condition of the NAV antenna. 

Does the antenna appear bent? Are both radials present on both sides of the vertical stabilizer? Here again, make sure the radials are covered in white fiberglass and that the fiberglass isn’t beginning to flake off, a sign of a deteriorating antenna.    

Many owners of older aircraft need to consider updating their VHF NAV antenna. More and more, these antennas are causing weak reception issues due to the age of the resistance matching network in line with the antenna called the balun.  

Many of these antennas and antenna baluns were installed at the time of the aircraft’s manufacture and are approaching 50 years of age. If your VOR can no longer indicate more than about 20 miles from a station, then this is probably what you are experiencing.1 

Navigation indicators come in many shapes and sizes with many different types of capabilities—but even if they are digital on an EFIS-type display, they should be tested periodically at a known VOR test point.  

Even if you don’t fly IFR, periodic testing is in order to make sure the indicator is showing as it should.

More tips about nav radios
 Familiarize yourself with your navigation system each time you get in the aircraft. Make sure you know where each switch is set and what it does. Many times I’ve had folks taxi back to my shop telling me they can’t get the nav to work when it was just a switch that we set differently than the owner was used to and he was unaware of that mode altogether.    

Regularly exercise your nav systems and make sure you know what each mode does. Select that nav radio on the audio panel and make sure that you can identify the station. A simple action like this can help save you money and keep you at the top of your game with your nav system.

Troubleshooting GPS
Most pilots are using GPS navigation these days, and one important thing aircraft owners and operators need to know about GPS is that the signal strength is very low by the time it makes it from space to earth.   

Most of the problems we see in the avionics shop with GPS are associated with bad receivers or bad antennas. The GPS antenna is always on top of the aircraft and is usually shaped like a teardrop. GPS units are usually sophisticated enough to tell the user that they are having problems through a continuous internal self-test, but there are a couple of things to watch.  

Look at the signal strength page on the GPS and make sure that you have at least six to nine satellites showing at all times with plenty of strength. Most receivers today are at least 12-channel, and you should see at least that many.  

Should the signal strength indicate low, the easiest thing to do to determine why is to systematically start turning electrical sources off to see if the strength returns.   

The very first thing I always tell my customers is turn off your cell phone. The tiny transmitter inside the phone in your pocket is usually no more than three to four feet away from the GPS receiver or the antenna, and it will knock a GPS offline. It doesn’t happen that often, but it does happen.   

Always, if you are having GPS problems, first try turning off your cell phone. Then start shutting off other portable items one by one to see if the strength of the GPS signal goes up. If not, begin turning off whatever you can on the panel until you isolate the source of the interference.  

This action won’t always find the source of your GPS receiver problems, but it might. Then you can feel accomplished walking into the avionics shop already knowing the source of the problem.

The answer can be simple
Sometimes the most obvious reasons for failure of a navcom system are overlooked. These troubleshooting tips can save you time and money, and keep your aircraft flying in top shape. As I said at the beginning of this article, you need fuel to fly an aircraft—but if you don’t have radios, you’re unable to go anywhere.

1 According to an article in the Winter 2012 issue of SatNav News published by the FAA, the agency will “transition to a minimum operating network (MON) of roughly half of the 967 VORs by 2020 to serve as a backup navigation system in the event of a GPS outage,” and will later remove all, or nearly all of the VORs.

Jason K. Moorefield operates the avionics shop currently owned by Freedom Aviation at Lynchburg Regional Airport (KLYH) in Virginia. He is an electrical engineer, A&P-IA and a private pilot that has spent more than 20 years working on privately owned aircraft and avionics. Send questions or comments to This email address is being protected from spambots. You need JavaScript enabled to view it..

Contact Freedom Aviation: www.flyfreedomaviation.com

Phone: 434-237-8434

 

Last modified on Wednesday, February 17 2016 23:01

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