Q&A: Eliminating the Source of Headset Noise and Installing a Davtron Clock in a C-150B

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Hi Steve,

I am sick and tired of the static noise in my headset. I have been putting up with it only because I had gotten used to it, but the other day I rode in my friend’s Cessna 210—and was I surprised! His radios were clear and loud; no background static. 

What do I need to do to cut out this annoying noise in
my plane?


—Static Sally


Dear Sally,

You shouldn’t hear any static—also called radio-frequency interference (RFI) or electromagnetic interference (EMI)—if every component is in good shape and the wiring is correctly installed. 

But it does occur; sometimes you’ll notice static in the headsets after maintenance, and sometimes it can be due to component breakdown. It can even be caused if two wires are rerouted or have sagged and become too close to each other. 

Unfortunately, it’s not always easy to determine the source. 

Static noise problems require a special kind of sleuth. I know avionics techs that are always “too loaded up with work” when approached to work on a static problem—unless they installed the avionics or components themselves.

If you’re experiencing static noise only when flying in rain or near convective activity, you’re experiencing precipitation static (P-static). Obviously steering clear of intense convective activity is the first step. 

Installing static wicks on the trailing edges of the ailerons and elevators, and installing electrical bonding wiring or braiding between the control surfaces and the wings and horizontal stabilizers provides a path for the electrical charges to dissipate. 

P-static can build to the point that navigation and communication radios become ineffective, high-pitched squeals are heard, and St. Elmo’s Fire (corona discharge) is seen on the windshield. I’ve talked to Cessna pilots that have gotten quite a shocking surprise from moving their hand or arm too close to the windshield-mounted compass or OAT probe when flying under P-static conditions. 

One way to narrow down the source is to attempt to isolate the noisy circuit. You can do this by turning off circuits one at a time to see if the noise goes away; press the on/off switches (left magneto or right magneto; strobe, beacon, etc.). 

Further narrowing can be accomplished by pulling individual circuit breakers. If the noise goes away, you’ve isolated the noise-producing circuit. That’s a good start—but since there may be a number of circuits on that circuit breaker, you haven’t yet pinpointed the source. 

Once the noise-generating circuit is located, look for broken shielding connections, loose wire connections and other wiring problems. 

If your airplane has an alternator, it can be a source of static. Another common source of static is the magnetos, and a third common source is a leaky ignition harness. 

However, your noise problem can also be caused by something as small as an improperly insulated headphone jack or because the shield of the magneto P-lead is improperly grounded or has broken. The P-lead (for “Primary”) is the wire from the magneto to the Left/Right/Both switch in the instrument panel.

I recommend this first: make sure that the headphone jacks are properly insulated. Each jack should have two small plastic washers—one with a raised shoulder, and one plain—and these should be in position to prevent the body of the jack from touching the metal of the panel. 

Magneto-generated noise is generated by Bendix magnetos. If only one of your magnetos is the source of the noise, this can be detected by switching off the magnetos—JUST ONE AT A TIME—with the engine running. 

If one magneto or both magnetos are the source of the RFI, there are external filters that are easy to install. These filters cost $70 to $100 each. (Note: Installing one of these external filters on a Slick magneto will cause magneto operational problems.)

The same on/off troubleshooting can be used to determine if the alternator is producing noise in your headset. If the noise goes away, there are filters designed for alternator noise and they cost about the same as magneto filters. 

Lone Star Aviation in Mansfield, Tex. is one well-known manufacturer of these filters. In addition to its magneto and alternator filters, the company produces an Eliminator filter that’s advertised to remove noise from the DC power feed line. All Lone Star filters have FAA PMA approval for installation.

Aircraft manufacturers go to great lengths to design systems that are noise-free. However, most GA airplanes today have been subject to many equipment and wiring changes. Wiring aircraft systems does require a working knowledge of proper systems and component power, as well as grounding and shielding techniques. 

Do some troubleshooting to determine the source of the noise; take steps to eliminate it, and—if you have to—install a filter to remove the noise. 


Happy flying.

Static wicks on the trailing edges of the ailerons and elevators also help.
Electrical bonding straps between the control surfaces and the wings and horizontal stabilizers provide a path for electrical charges to dissipate

Hi Steve,

I am the happy owner of a 1962 Cessna 150B Anniversary model. I love my fastback model 150. 

I received a real nice Davtron 811 digital clock on my birthday, and although I rarely need a clock while flying, I want to install it. Isn’t it a big deal to make any kind of change to my 150?

I am very happy to just fly my simple little 150 around, so I haven’t installed any upgrades or done anything except for annual inspections and changing worn-out parts since I bought it 17 years ago. What do I need to know?


—Worried Will


Dear Will, 

Don’t worry, Will. In 2010 the FAA sent a letter to Kevin Torresdal, president at Davtron, stating, “We consider the installation of replacement clocks (including timers and stopwatches) in non-transport category airplanes to be minor changes in accordance with Title 14 Code of Federal Regulations (CFR) section (~) 21.93(a). 

“Per CFR S 21.95, copies of this letter may be given to installing mechanics as evidence that this installation is considered minor. The installation should be documented using a maintenance log book entry referencing this letter.”

The reference code for the letter is 100S-GA-10-53. (See Resources for a link to the CFA forums where you can retrieve the PDF. —Ed.)

I have a Davtron M811 clock with the elapsed time and flight time features in my airplane. Installation directions can be found on the Davtron website after selecting “M811” under “800 Series – Clocks” in the drop-down menu on the left. All that’s required is a power wire, a ground wire and a one-amp fuse. I believe Cessna installed a one-amp fuse in your airplane to provide circuit protection for the clock circuit. 

Flight time can be turned on by connecting the blue wire from the plug to an air switch, but that adds another layer of complexity. If you want the clock to provide an approximate flight time, you can connect the blue wire to the master switch. 

Each M811 clock has a small memory (“keep-alive”) battery to provide power when the airplane power is off. Davtron recommends replacing the memory battery every two years, but one way to determine if the battery is exhausted is when the flight time feature no longer works. Cost for a new battery is around $28.


Happy flying.

The M811B digital clock by Davtron is considered by the FAA to be a minor change for non-transport category aircraft. A logbook entry referencing the letter serves as sufficient documentation.
Lone Star Aviation produces an Eliminator filter to remove noise from the DC power feed line.

Know your FAR/AIM and check with your mechanic before starting any work.

Steve Ells has been an A&P/IA for 44 years and is a commercial pilot with instrument and multi-engine ratings. Ells also loves utility and bush-style airplanes and operations. He’s a former tech rep and editor for Cessna Pilots Association and served as associate editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and lives in Templeton, Calif. with his wife Audrey. Send questions and comments to


Resources
Noise filters
Lone Star Aviation
 
M811 clock
Davtron, Inc.
 
Clock installation reference letter
Letter 100S-GA-10-53

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