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Cessna's Snap, Crackle, Pop
In a recent class, attended by about 150 folks, I asked how many had strange noises, pops and cracks in their headsets and/or noise showing up on the weather detection equipment such as the Stormscope or Strikefinder. A whopping 60% admitted they had this problem. Some had noise that they could live with and others said the noise was really a nuisance and was like flying in a torture chamber. I know at Avionics West we often get noise problems but I had no idea the problem was so widespread.
Other pilots I know also have complained of headset noises and dots on the weather detection equipment. I decided to investigate the issue and write an article about it. The first question is; why in the last five years or so, are our Cessnas picking up so much noise? There are many reasons for this, the foremost being that our new equipment is very sensitive to noise, spikes caused by strobes, beacons, alternators and just about any other thing that can produce noise. The new intercoms are great but unless they are properly installed you will often hear noises in them either from the strobe, beacon, alternator, fuel pumps or a host of other noise varmints. In yesteryears we didn't have ultra sensitive equipment such as the nice intercoms or weather detection equipment we now have so we had few problems with noise.
Another reason we now pick up noise is because the airframes are getting old. We often change engines, paint the aircraft and do new interiors, but when was the last time you re-wired your aircraft? Old wiring, poor grounds and connections, non-shielded wiring all play havoc with modern avionics. Our old Cessna factory wiring just doesn't meet the requirements of today's equipment. If you look under your Cessna panel, you will see very little shielded wiring, if any. Without shielding , the noise from trim motors and everything else gets into the intercom and radios. This wasn't a problem in the 70s & 80s but things are different today. This interference or noise often is a problem for autopilots, especially the ARC autopilots.
At OSH, 1996 I had the opportunity of looking under the panels of the new 172 and 182. I was fortunate to be allowed to crawl around the aircraft. The first thing I noticed was the nice shielded wiring, switches that were high quality and not under rated, center point grounding and properly routed wiring harness. Cessna rewrote the book on how to wire a single engine aircraft. We see far fewer problems in the twin engine aircraft because without a doubt the wiring quality is much better. Another thing we've noticed is we have far fewer noise problems in other aircraft manufacturers such as Piper or Mooney. The reason for this in because the wiring is of a higher quality and in most cases center point grounding was used. We will explain center point grounding in future articles.
Stand by for articles to come as we tell you how to hunt down those pesky noises and what to do for the cure. In future articles we will discuss just about everything in our Cessna aircraft that causes some noise in our headsets or avionics. Picking up dots on your Stormscope or Strikefinder? How about a continuous popping sound in the headset? Got strobe or beacon noise? Next article will tackle these dragons and give you the secret cures we've found at Avionics West.
One complaint we get on noise in our Cessna aircraft is related to the strobes. Anytime you have a large inductive or capacitance load you have the opportunity of picking up noise in your headset. Strobe noise is normally determined by a regular pop in the head set. You may only hear the noise from one of the strobes. Most Cessna strobe systems consist of a strobe lamp and a power supply for each strobe. In the single engine Cessna the strobe power supply is located on the end of the wing, just inside the wing tips. This places the strobe power supply just inches from the strobe flash tube or strobe lamp if you wish to call it that. Let's look at how the strobe works. You turn on the strobe switch located on the instrument panel. This sends voltage out to the strobe power supply. Inside of the strobe power supply are a bunch of high voltage capacitors and a high voltage power supply. This high voltage at times (determined by the power supply) goes out to the strobe flash tube. The flash tube has a gas inside that when excited with a high voltage will flash. Rate of flash is determined by the manufacture of the power supply. Under a perfect world this works great. What often happens is as the strobe power supply gets old, the internal filtering breaks down and allows a pulse of energy to flow down the power line, through the strobe on/off switch and on to the aircraft buss. This causes a popping noise to be heard in the headset. On a long flight it will drive you crazy, if you haven't already reached that point. If you have an intercom, the problem is often amplified. I really can't imagine owning a Cessna without an intercom of some type.
Now you ask, "what can I do to cure this terrible problem." What I would do first is find the strobe power supplies (probably under the wing tip on singles). Then I would disconnect the power going to the strobe power supplies one at a time to determine which one is causing the havoc. This should be simple because it's normally the single wiring coming through the wing to the supply. Once you've determined which side is causing the problem half the battle is done. The strobe power supplies are interchangeable between sides and in most cases only four screws hold them in. Swap the strobe power supplies and see if the problem goes to the other side. If so, the problem is in the strobe power supply and the best thing to do is replace it. If you swapped the strobe power supplies and still have a problem, read on. Now here's where the magic comes in. You will notice two "twisted" wires going from the power supply to the strobe flasher tube. Please notice the direction the wires are twisted, this is very important! Now disconnect the connector and twist the wires together BUT in the opposite direction. This has something to do with nulling but that's well beyond the scope of this article. Now reconnect the connector and see if the noise has disappeared. This cures about 50% of strobe noise cases. Another thing to do while you have the wing tip off is make sure the strobe power supply is well grounded. The easiest way to do this is remove the power supply screws, clean up where the power supply attaches and reinstall. If you still have strobe noise you are going to hate the nest step. Crawl under the instrument panel and find the back of the strobe on/off switch. Be sure the master switch is "off"! Place an OHM meter across the terminals of the strobe on/off switch with the switch turned "ON". You should read less than one ohm of resistance. If you read more than this "often you do" you should replace the switch. This under rated switch often can cause noise, believe it or not. You might want your favorite avionics shop to help under the panel. It could be cheaper than back surgery. If the strobe on/off switch is good, read across the strobe circuit breaker (with the battery master off). You again should read less than one ohm.
In every case I've seen the above steps have led me to the problem and cured it. Some after market strobe system have different characteristics. It would be best if you call me if you have a noise problem with after market strobes.
Just a couple of strobe pointers while we are on the subject. If the strobe does not flash on the ground, be sure you try it with the engine running or external power. Strobe power supplies are often voltage sensitive. If the strobe is dead, make sure you have power going to the power supply (wire in the wing attaching to the power supply. If you have voltage to the power supply you need to swap either the flash tube or the power supply to determine which is bad. The proper method is to measure the voltage out of the power supply going to the flash tube. This is VERY HIGH VOLTAGE when the power supply is working. We have high voltage test equipment to test the strobe system. Another way you can test the power supply output is have your mother-in-law hold the wires going to the flash tube while you turn on the switch. If she is still standing, then the power supply is defective, get a new one!
Much like the Cessna strobes, our flashing beacon can also be a noise generator in our headsets. This particular noise often sounds like intermit high pitches in the headset that is related to the time the strobe flashes. If you have a Strikefinder or Stormscope you will easily see the problem. You will notice that anytime you have on the flashing beacon, a storm will always be chasing you. Of course, that seems to be normal life for me... This is easy to verify, just turn off the beacon and if the noise disappears, that's the culprit! Beacon noise is often noted in upper end intercoms but in some cases we've heard the noise even in without an intercom. The first thing to check is remove the panels that connect the horizontal with the vertical tail. They are easy to remove. Now take note of a large resistor located under these panels. Be sure a wire goes to one connection on the resistor and to airframe ground coming out of the resistor. Remove the ground wire and clean the connector and the airframe where the wire attaches. So now you wonder why the resistor, huh? Well the flasher power supply is capable of driving two beacons. Since we only use one beacon, the other load is tied to ground via the resistor. The resistor is a simulated load for the missing beacon.
If the pesky noise is still present then read on.
What is happening is a spike is running down the power wire to the breaker and on to the
radio buss. I've seen this noise so bad that it would cause the loran and Omega to
unlock at times. Often you can put an inductor in series with the power input wire and
cure the problem. I can't put in print where to install the inductor or where to
purchase it but I bet your local avionics shop could help. The inductor is only a few
dollars. Another way to get rid of flashing beacon noise is by just replacing the beacon
power supply. In most cases the cost is around $60.00 plus about a hour to replace. This
could be a cheap price to pay for one's sanity.
Oh the Cessna alternator whine. What is it? A whining noise in the headsets that change in pitch when the engine RPM is changed . The noise can often be heard when transmitting also. Often at full RPM the noise can not be heard, not because it's gone but the frequency is out of range for most of us to hear. But retard the RPM and it comes back. The problem is often amplified when the alternator is heavily loaded with landing lamps and other heavy loads. How often do we see it in Cessna piston aircraft? I ask folks in the Cessna classes I teach and about 40% is the norm for the problem.
What we will attempt to do is give you some ideas on how to verify if the noise is caused by the alternator and what to do to kill the noisy beast. First, let's discuss how the Cessna alternator works. For the most part belt driven alternators are expensive car alternators. In fact, most modern automotive alternators are around 120 amps and are about one third smaller than the Cessna alternator. True, the brushes are a little different but for the most part they are the same. Of course you can't use an automotive alternator, it's illegal and could cause some real legal problems should something go wrong during your flight. The alternator is nothing more than a three phase AC (alternating current) that uses diodes to rectify the AC to DC (direct current) for use in our Cessna aircraft. The output of the alternator will have some ripple instead of pure DC but this is allowed. The alternator manual says the maximum AC allowed out of the alternator is 1 volt AC PTP (peak to peak). DO NOT start change components! This noise must be troubleshot. Chances are you could change everything in the aircraft charging system and still have the same problem! I couldn't begin to tell you how many aircraft alternators I've seen changed by the shotgun method that didn't cure a thing. So the first thing you do if you suspect alternator noise is put an oscilloscope on the output of the alternator and see how much AC is on the line. The oscilloscope will read PTP, most multimeters will never show the AC If the AC output is more than 1 volt AC peak to peak, repair the alternator. Don't be surprised if your new alternator has AC out of it too! Most repair shops only load the alternator down to verify the proper current output, they don't look at the AC component. Whenever you send in your alternator in for repair require the shop to replace all the diodes. Don't just say test them, replace them. Any avionics shop would have an oscilloscope and could do this test in about 30 minutes. Few A&P shops have the equipment or expertise to properly test charging systems.
So you've had your local avionics shop look at alternator output and they tell you it's fine. Now what? The first thing I'd check is to make sure the alternator output has a capacitor attached to the output of the alternator. It's a large round metal can that attaches to the alternator case and the wire attaches to the alternator output. This capacitor must be present in any Cessna with any kind of avionics at all. In fact, in most cases where we find the ADF is weak, is directly related to the capacitor missing or the wire broken. If this area looks great, then I'd start checking for bonding between the alternator and the airframe. A heavy wire should go from the alternator case to the airframe, normally at the firewall. This is a must. Check the ends of this ground wire for corrosion. Remove the cable and clean the terminal and areas where the terminals fasten with fine sand paper. You want to make sure these areas are really grounded. If the bonding strap only goes to the airframe mount or engine this is not good enough. It really needs to be attached to the airframe. It's OK for the alternator to be bonded to the engine and engine mount with a strap but another strap directly to the airframe is a must. We've seen a lot of Cessnas with a difference of potential of 25 ohms between the alternator case and the airframe. What a way to cause noise and for the ground wire to heat up! Verify the terminal on the alternator output is in good shape. Not a lot of frayed strands not attached to the terminal. The alternator output wire will go either the main buss inside the aircraft or a junction on the firewall, then it goes inside. Most late 210's have the junction point, most 100 series aircraft run the alternator input direct to the buss under the instrument panel. Check these connections closely for frayed, dirty or loose connections. Check the alternator output circuit breaker. With no power on the aircraft and the battery disconnected, read the resistance of the output breaker, it should read less than .2 ohms. You will need the service of an avionics shop for this measurement too. In some Cessna's, the alternator output breaker was installed on an aluminum buss bar. Remove this bar and clean it with 600 grit sand paper and really tighten down the screws when you reinstall it. This has been a real problem area for us. If any of this work is over your head please get the aid of a professional. Never venture into an area you aren't comfortable with. I feel comfortable working on Cessnas but I'm a little uneasy performing heart surgery...
So now you've done all of this and you still got the Cessna whine. The next article will cover more possibilities.
More on Alternator Noise
In the previous article we discussed some things to check should you have an alternator whine and it's been determined the problem is not the alternator, the filter or poor grounding in the alternator system. We also discussed some of the necessary equipment you or your shop will need in order to properly troubleshoot your alternator for the pesky noise. Now you've checked everything I've mention but you still have the blasted whining especially at low engine RPM. What is one to do? A famous person once said, "At least I know what it's not"!
OK, so you are still hearing the whining noise through the headsets and at times it goes away when the alternator is turned off. They are several things you could do and the easiest is to order one of the new 172s or 182s from Cessna. All of the things they did wrong in early years, they've corrected. The new Cessna's have single point grounding throughout the aircraft and a DC distribution system. The new 182/172s DC power is much like their big brothers electrical system in the Cessna 208 Caravan. If this is not an option for you at this time, read on. Remember in the early days, we didn't have high quality intercoms, stereo and equipment that was subject to problems generated by noise. When we install an upper end stereo/ICS with CD inputs the prices can easily go over the $2,000.00 mark. When you spend this kind of money, you want to be able to enjoy the quality the system can produce without background noise. In most cases this can be accomplished.
When we exhausted most of our normal avenues for noise, this is what my boys look for next. Common point grounding for the ICS, all jacks and the aircraft radios is paramount. Ground loops are one of the biggest problems we see, especially in the 100 series Cessna. Often we will see the mike and phone jacks grounded to the instrument panel and the radios grounded to the airframe. I've found as much as 10 ohms difference between the two. Now that's a lot! Another thing we do is when possible we run the audio lines segregated from the charging system wiring. Keeping the audio from the buss wiring in most cases really helps in most cases. We try and keep the audio lines away from anything that could induce noise in the system such as magneto wiring and turn-coordinator wiring.
Another policy of our shop is to only install intercoms that require shielded wiring. If the wiring is not shielded and the shielding properly terminated, then you will get noise. We only terminate one end of the shielding. The reason we only terminate one end of the shielded wire so we will not get current flow through the shielding, thus causing noise. If you ground both ends of the shielding and have a difference of potential on each end of the shielding you will get current flow and that translates into noise. Take that one from experience. Another policy we have when installing high in intercoms is we always float the jacks. What this means is we do not let the female jack "the ones your headset plugs into" touch the airframe. This too aids in keeping ground loops from being a problem. Most modern intercoms call for placing the jacks above ground with an isolator provided by the manufacture, we require it on all installations. There have been times that we've had to add inductors to the input power of the ICS to quiet it down. Adding inductors and/or capacitors is something that should be left for the shops. You often have to experiment with different combinations to get rid of the noise.
Old wiring is often a problem. You've probably had several engines installed on your Cessna but that wiring in many cases is older than the owner! The copper wiring doesn't fail but corrosion, failing insulation, poor grounding take it's toll on the over all wiring of the aircraft. For future reference, anytime you get a new radio stack, be sure all the wiring is removed from the breakers out and replaced. True, splicing into existing wiring is easier and that translates in to cheaper but in the long run you will pay the price!
Unfortunately, noise in our Cessna's isn't easy to find or cure. It can be a long costly process but the end results can be worth it. We often see two 182s experiencing the same noise but require different fixes for the cure. Of course neither is cheap or easy. So you've got an unbearable noise and you what it fixed, what to do first. I'd recommend getting in touch with an avionics shop that know how to troubleshoot your aircraft. I didn't say change parts, the key word was troubleshoot the problem. Set down with the manager and discuss the plan of attack pertaining to your problem. If the first thing they want to do is change the alternator without troubleshooting the problem, then you are at the wrong place.
Bottom line is, noise problems can be cured. They must be troubleshot, not shotgunned and the technician must have a good knowledge of the systems and especially your Cessna. Chances are the cure will not be cheap or easy to find but there is a cure. In my fifteen years of working on Cessna's I've often found that in each case the problem was the same but the cure was different! Hope this series has helped some in your understanding of Cessna's many noises! By the way, we are getting a Cessna 182 in next month that has been to eight shops for severe noise and no one can cure it. This aircraft is said to be the "Mother of Cessna Noise". I'll give you a update later and let you know what we find. Sounds exciting to me!