When Silence Aint Golden

Troubleshooting avionics before calling the mechanic may save you time and money


Flight instructors spend hours discussing, drilling and training for handling in-flight emergencies. Things like engine failures, instrument malfunctions or electrical meltdown occupy an important part of the syllabus.

But what about the nagging little (and not so little) problems that crop up on the flight deck? Problems in the avionics often pose no dire threat to the safe conclusion of the flight, but represent a hazard based on increased workload and distraction. Then again, sometimes the little problems are just an appetizer for an entree of trouble that may be just around the corner.

There are many common problems with avionics that any pilot can troubleshoot and fix. Others may need a technician to fix, but a little investigation by the pilot can help determine if the problem is a safety concern or a mere inconvenience. Who knows, you may catch an indication your ILS receiver is about to die while youre on the ground, instead of learning it while descending toward minimums.

Common problems
When things go wrong – or even if you suspect theyre a bit not right – look around for obvious signs of problems. That means checking for popped circuit breakers, signs of electrical problems like low voltage, water or foreign objects, and loose connections.

Low voltage is a very important concept that many pilots dont consider. If you dont fly every day, or even every week, count on your battery losing charge just sitting there. Many times a problem like weak radios or an HSI that doesnt slave can be traced to a low battery condition.

Before you panic at the sight of a sick avionics stack, take the plane out of the hangar, start her up, and give the battery a chance to charge by spending some time on the run-up pad with some decent RPM. Check the water level in the battery, too. It cant hold a charge if its dry as a bone.

Consider also recent work that has been done. Coincidence is a powerful force, but nothing in the airplane is more reliable than Murphy. If you have the interior redone, and the intercom stops working, there may be a relationship. If you just had the airplane annualed and the VORs dont receive, maybe the antennas were disconnected and not hooked back up again.

Communicate any recent work, no matter how trivial, to the avionics shop after a malfunction. This fact may help an avionics technician cut down time and cost of troubleshooting.

There are two very good reasons to learn some basic avionics troubleshooting. The skill may enable you to remedy what seems like an in-flight failure and continue a safe flight. You can also save money by either avoiding the avionics shop or giving your avionics technician a very good description of the problem. If you can localize the difficulty, you can avoid the expense and headaches of wild goose troubleshooting.

The most fundamental cockpit troubleshooting tool is a working knowledge of how your avionics operate normally. That means having a clear picture of the nominal position of switches and knobs. But with the increasing complexity of displays and navigation systems, that also means how to easily obtain a default, uncluttered picture of what is going on.


Communications is one of the most important parts of flying. That makes your comm radios importance behind only the wings, power plant and propeller. Most pilots have experienced that awful feeling of isolation when you inexplicably get cut off from the voices on the ground. Fortunately, I have the voices in my head to keep me company, but most pilots dont.

When things get too quiet or you are otherwise falling into the category of failing to communicate, you need to identify which one of three elements is missing. Can you hear anything? Can anybody hear your voice? Is the radio being keyed (put in transmit mode)?

As always, look for the obvious. Switches set in the wrong place, a volume control bumped to the wrong position. In 20 years, I have solved comm problems in 150s to Falcon 50s by flipping a single switch.

So, set all volume controls to 12 oclock, set the mic select for auto, or comm 1, and flip the toggle switches to up. Check your microphone or headset. Never fly with only one microphone or headset, and never trust the cabin speaker. They are junk. Yoke mounted PTT switches are cheesy, too, so carry a hand mic.

If you can key the transmitter but not transmit voice – that is, if youre told youre broadcasting carrier only – the PTT is okay. If there is not even a carrier, suspect the PTT.

Remember, too, that older radios often have flaws in the potentiometers that control the volume. Dirt can cause a dead spot where there is no sound, but working the volume knob or even just moving it a couple of degrees may restore normal operation.

Intercoms should have a fail-safe mode, so if you are having trouble talking outside the airplane, turn off the intercom to eliminate it from the circuit. You do know how to do that, right?

If you still havent found the problem, open the squelch on your coms. Usually that is done by pulling out the volume knob, but on older radios – and there are lots out there – look for a manual control that needs to be twisted in one direction.

If you can get white noise into the headset or speaker, youll know that the receive audio path is present and accounted for, and that the headset/speaker is working.


GPS works so well that you probably dont expect to see problems. In fact, you probably harbor a false sense of security created by the accuracy and dependability of the signal. When it goes south, you wont believe it.

But they do occasionally exhibit anomalous behavior. Your trusted navigator can go stupid when you least expect it.

The first thing to check when this happens is that your current flight plan, next intended waypoint and destination are what you expect. Be sure that the system hasnt stepped to the next waypoint before you expected it.

Many systems have automatic sequencing that will step to the next waypoint at the closest-point of approach. Unfortunately, they cant always anticipate where ATC will send you, so the GPS may change destinations before you are ready.

Next, check out the satellite status page. If you learn no other GPS function than Direct-to navigation, you at least should know how to check satellite health. Find the page that shows an overview of the signal condition sometime when everything is working, and make a habit of looking at it once in a while.

Key things to look at on the satellite page are signal strength, elevation and azimuth.

GPS signals in space are line of sight and cover over 10,000 miles. They are easily blocked. If a satellites elevation is close to the horizon, the signals from it can be blocked by the aircraft and not be available when you need them.

Loran has the same sort of considerations. Look for the signal page and become familiar with the normal signals. When things go haywire, look at this page. Loran is affected dramatically by low-frequency signals emanating from anywhere. That includes sunspots or even your trim servo.


If the DME doesnt show any distance, then, like the VOR/LOC, youll have to listen to the Morse code identifier. The DME audio is a very unpleasant sound. What you are hearing is the audio hiss of the pulses being received. Every minute the noise will sort itself out into a recognizable group of dits and dahs, allowing positive identification of the station.

The DME and transponder both operate at nearly the same frequencies. They are so close, in fact, that often one of the boxes will suppress the other so they dont interfere with each other. Sort of an electronic, after you, friend, transmit control.

If there is any sort of problem with the DME signal, put the transponder in standby (and vice versa) to see if there is an interference situation.

DMEs of the transistor age are accurate for distance, but groundspeed and time to station indication is always predicated on the rate of change in distance.

They arent as smoothed as the GPS calculations, so the only time the GS and TTS are correct is when you fly directly toward or away from the station. If the numbers dont agree between your handheld GPS and your DME, odds are that the GPS is correct and the DME is working as well as it can.


Most pilots still depend on the ILS, but with the advent of GPS for en route navigation and non-precision approaches, the VOR gets less and less exercise. The VOR and localizer usually share an antenna, a receiver, and an indicator.

In the olden days, if you tried your VOR en route and it didnt work, you knew the chances were pretty good that the localizer would be unreliable too. Navigating with GPS, you dont know. It is not unusual to arrive at the destination, ready to make an ILS approach to 26L and find the needle dead.

What now? The initial tasks are the same regardless of whether it is VOR or localizer that is dead. First, verify that you hear the Morse code identifier. This will, in one move, determine that you are on the right frequency, that the transmitter on the ground is working and that your receiver is operating. GPS installations that share an indicator should automatically revert to the localizer when a localizer frequency is selected, but if you are on a VOR approach, you will probably need to manually switch from GPS to NAV to use the old wiggly needle.

Look for signs of life. The nav indicator has two or three meter movements. Sometimes either the meter or a wire will fail. That means the left/right meter might be dead while the to/from/off flag continues to function. That wont do you any good navigationally, but at least you know what the problem is.

Here is a quick check of VOR health. The course width is supposed to be about 20 degrees, stop to stop. When you check the accuracy at the VOR checkpoint, move the OBS 10 degrees to either side of the radial, and the needle should just kiss the last dot.

If the needle swings too far or not far enough, then the VOR converter is not working correctly. Your VOR flying may be sloppy or ragged and the autopilot intercepts will be erratic.

In many airplanes, there are two navs installed but only one antenna. If both navs behave the same way, you either have read the frequency wrong or you have an antenna problem.

One more cockpit troubleshooting tidbit, this time for the localizer. Have you ever experienced a needle wig-wagging unmercifully, much faster than you can chase it? Try varying the RPM slightly. Propeller modulation will occasionally crop up.

This happens when the arc of your propeller slashes through the localizer beam at 90 to 150 times a second. With a 3-bladed prop that is 1,800 RPM, or 2,700 with a 2-blade. As the blade interrupts the signal, the resulting modulation interferes with the localizer and gives you a windshield wiper effect on the indicator. The good news is there is nothing wrong with your set, and a few RPM either way will eliminate the condition. Its called, in highly esoteric technical terms, prop mod.


The other item of vital importance is the ATCRBS transponder. This little bit of gear has less direct value to the pilot than it does for the convenience of controllers. However, if it malfunctions, your life is miserable.

The key to the transponder is its reply light. Assuming that you havent left it in standby mode – which is what the controller implies when he asks you to reset your transponder – the reply light should blink whenever the unit receives an interrogation and then sends a reply.

In todays airspace, the lamp is almost always glowing. So if you are over LA and its not glowing, your transponder is probably not receiving a signal. Most transponders will blink when they try to transmit, even if that transmission is not successful.

If the reply lamp is active and ATC doesnt get your response, there are several things to consider in troubleshooting.

Aircraft position is one. The airplane may be blocking the transponder antenna. Its the tiny stub or blade antenna, usually on the bottom of the airplane. Sometimes, a gear door will block the signal. Transponder receivers are omni-directional, but the ground antenna is highly directional. Make note of the aircraft location, orientation and configuration (gear or flaps down, etc.) when youre told by ATC you may have transponder problems.

This is also a good place to apply one rule of thumb. Dont go to the avionics shop with a problem the first time you see it (unless it is a total failure). Wait until an intermittent problem occurs three times, and note the conditions each time.

If ATC reports a problem, turn off the DME and see if it clears up. Both systems operate on similar frequencies and have suppression capability to prevent interference and cross-talk. If the suppression fails or some other malady occurs in the DME, it might be affecting the transponder.

If ATC doesnt receive the correct squawk code, it could be a sign of a dirty switch, so wiggle the knobs, or turn them through a few dozen times.

Intimately attached to the transponder system is the encoding altimeter or encoder. This box simply converts the barometric pressure to a data format compatible with the transponder for transmission to the ground. Developed in the 1940s, the system is really a slick data transmission system, using the 4096 bits to give 50-foot resolution to 100,000 feet.

Because reliable transmission of the encoder data depends on 10 wires, each with discreet data bits, losing one part of the data can make a huge difference in the transmitted altitude. It is part of the fundamental design that something as simple as a broken wire will introduce such a gross error that it will be detected.

Fortunately, modern solid state encoders are fairly reliable and relatively cheap. Their low cost means that when it does fail, its usually not too big a deal to replace the encoder, matching it to your standard altimeter.

The matching is called correspondence, and should be checked at least every 24 months, even if you dont fly IFR. ATC and TCAS systems depend on reliable Mode C data.

If ATC complains about your Cessna 177 flying at FL420, and you arent, make a note of the altitude you are actually and what the Mode C is displaying on the ATC radar. It is quite possible that the avionics shop can tell by looking at this information which wire or bit is not right.


Pity the poor unloved ADF.

Nobody likes flying the ADF, they arent very accurate, the audio stinks, and they take up room and useful load. Still, if you want to fly to the Bahamas, Canada or in other undeveloped countries, the ADF still has value. The ADF uses two antennas, athough they are often in the same housing. Theres a long sense antenna and a direction-finding loop antenna.The first thing to do when experiencing an ADF abnormality is to switch to ANT mode. This eliminates the loop antenna, and allows clear station identification.

Naturally, the needle wont point in this mode, and some systems (like the Bendix/King KR 87) will stow at 90 degrees. This will show you that the needle drive is working, too.

Beware, also, that an ADF unused is soon an ADF unusable. The tuning mechanism can get corroded if the unit is not used from time to time.

-by Gary Picou

Gary Picou is an avionics expert who works for PS Engineering.


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