By Harry Fenton, Business Development and Product Support
Lots of older Bendix magnetos are returned to Kelly Aero either as cores or for a custom overhaul with large external capacitors attached to the outside of the magneto. Much to the surprise of the customer, this big capacitor is not replaced or provided as new as part of a Kelly Aero overhauled magneto. In fact, this capacitor is not shown in the magneto overhaul manual or parts list, and there is little to no documentation as to how these parts are installed with the magneto. So…what is this thing?
This big capacitor is called a “Radio Noise Filter” and its intended purpose is to reduce radio noise generated by the magneto. It is a relic of a bygone era as an attempt to fix radio static problems experienced in aircraft radios used in general aviation airplanes of the 1950s and 1960s. While not useful for current aircraft and avionics, these capacitors are still sold by Kelly Aero as a spare part.
NOISE FILTER- WHY IT WAS NEEDED, WHY IT IS NOT NEEDED NOW
When the magneto is operating, the contact points close to allow the magneto to build a charge, and then open to allow the current to discharge which creates the ignition spark. The result is a pulsating voltage produced at the internal magneto capacitor, and this voltage is carried by the P-Lead and switch.
For the most part, this voltage has no effect on the engine or magneto and is simply a byproduct of normal operation. However, the pulsating voltage in the P-Lead can result in a low amplitude radiated signal that has the potential to create a signal that can be picked up by radio.
Because this signal is not a structured signal in the form of voice or morse code to identify VOR stations, it is termed “noise” and it is interpreted as interference. Radio noise is a rare occurrence and is as described: A steady whine, a general static or a staccato clicking sound as the spark plugs fire that is heard while tuned into aviation radio frequencies.
A lot has changed in the world of avionics from the 1950s to now. One of the serious problems in the 1950s and 1960s as radios became more common to the airplane cockpit, was “radio noise”. Early radios worked on dozen frequencies for voice and navigation and the radio designs were based on frequency crystals and vacuum tubes. The old radios were very susceptible to radio noise due to the limits of the components and circuit designs available at the time. On top of that, all of the components like lights, strobes, generators, spark plugs, ignition harnesses, and magnetos generated some sort of radio noise. From the radio standpoint, it was a noisy environment.
Noise filters, in the form of capacitors, were attached to all sorts of wires connected to lights, generators, and magnetos. The idea was that the filter would simply change or diminish the radio frequency of the noise generated by these devices to something outside of the aircraft radio frequency range.
Modern avionics manufactured since the late 1980s onward are digital, microprocessor controlled with much better filtering and much greater capability to reject noise. Additionally, FAA certification required that airborne emitters of electrical noise- like magneto ignition systems- keep the level of radiated electrical noise within acceptable levels.
For the most part, airplanes and engines built from the 1980s onward eliminated the noise filters as an extra part that was subject to maintenance or failure just wasn’t needed. Radio noise was cured by better shielding of airframe wires, lower noise emissions of magnetos, ignition wire, and spark plugs. The noise filters just were no longer required as they were Band-Aids to other issues such as unshielded P-Leads. A subtle issue is that the extra noise filter is in-line with the P-Lead and just another component subject to failure that could have a negative effect on magneto operation. In short, just another part that is subject to failure, so if it is not needed, it can easily be removed from the ignition system for greater overall system reliability.
NOT REQUIRED, BUT STILL IN DEMAND
Having said that, Kelly Aero still sells a lot of the MF3A Ignition Noise Filters. Why? We are not exactly sure, but there is a continuing demand for these parts. One reason may be that A&P mechanics are conservative and want to reinstall the replacement magneto to match, in every exact detail, the magneto that was removed. If the magneto was originally fitted with a noise filter, then a new Kelly Aero MF3A noise filter is installed to replace the old, worn-out noise filter. Admittedly, the noise filter may serve a purpose on some older airframes or experimental airplanes where the wire shielding and ground paths are not as well designed towards reducing the effects of radio noise. Mechanics and amateur aircraft builders can use the filter as a tool to isolate the root cause of radio noise interference.
P-Lead connections are likely the most practical reason why the noise filter continues on in service. Most of the older Bendix magnetos used “bayonet connectors” for P-Leads. The bayonet connector is configured with a large nut, and an insulator, looking much like the spark plug connector on an ignition wire. The airframe P-Lead is configured with a #8 ring terminal. If the filter is discarded, then airframe P-Lead needs to be re-configured from the ring terminal to the bayonet-style hardware. The easiest path is the just re-use the old noise filter on the replacement magneto. This is discouraged by Kelly Aero as, in effect, an untested, decades-old part of unknown history is attached to a freshly overhauled magneto. There is a lot of risk to degrading the service life of the magneto with the worn-out and unneeded noise filter.
But, if an installer wants to install a noise filter, then the Kelly Aero MF3A is the best option. The current Kelly Aero MF3A noise filter is configured with a ring terminal on the wire that connects to the magneto. This connection works with all current manufacture Bendix short cover 20/200 Series, Bendix 1200 Series, and all Slick 4300/6300 and Kelly ES4300/6300 Series magnetos. But, long cover Bendix 20/200 Series magnetos will need the ring terminal on the MF3A to be replaced with bayonet hardware as pictured below.
One final note: Remove the noise filter before returning the core magneto back to the parts supplier or back to Kelly Aero. While the filter is not required, it is useful to have to get the replacement magneto quickly installed until the airframe P-Leads can be reconfigured or a new manufacture noise filter like the Kelly Aero MF3A is installed.
By Harry Fenton, Director of Business Development and Product Support, Kelly Aero Routine maintenance requirements and all special service actions required by Service Bulletins and Airworthiness Directives typically require that the magneto serial number or part number be referenced to verify compliance. The OEM data found on the magneto data plates is always the baseline to establish the applicability of the compliance of a required maintenance or safety action. However, what is required when an OEM data plate has been replaced by a company that is not the OEM for the magneto? What if a serial number is added to the data plate that is different than the OEM serial number? Do OEM requirements still apply? The bigger question: Is the magneto considered an FAA legal part if the OEM data plate has been removed and replaced by a non-OEM data plate? Replacement Data Plates- Is This Legal? The simple answer is “yes”, Kelly Aero replaces worn and damaged OEM magneto data plates with new Kelly Aero data plates. But, the “yes” answer is not as simple as it sounds. The FAA considers the management and disposition of OEM data plates attached to FAA-PMA articles as serious business. The real world for Kelly Aero is that there is no option: worn data plates must be replaced when magnetos are inspected or overhauled. In most cases, magnetos returned to Kelly as cores for overhaul, or for the 500-hour inspection, may be legible, but not in a useable condition to continue on in service.
The continued use of the OEM data plate for ongoing service and compliance becomes a safety issue if the information on the data plate becomes illegible. The only practical and safe solution is to make a new data plate stamped with the information from the old, worn data plate. The FAA has strict regulations against the removal and replacement of airframe, engine, propeller, and life-limited parts, though. They spell out what can, and cannot be done in the Federal Code of Regulations, Part 45. The FAA states, specifically, that data plates of Type Certificated or Life Limited items may be removed in the course of maintenance and must be reattached to the item from which they were removed. Replacement data plates for these items can only be provided by the OEM, but the FAA must approve and accept that process. This concept has been drilled into mechanics thinking by the FAA and industry guidance. The general understanding is that all data plates are forbidden to be replaced.
The data plates of component articles that are FAA-PMA approved, such as magnetos, are not as strictly controlled and the FAA can approve a repair process that does not require OEM approval for data plate replacement. Kelly Aero’s process to replace the worn OEM data plate with a new Kelly Aero data plate
is part of our FAA-approved Repair Station Quality Manual. This manual details the process to replace data plates, but also documents the strict record-keeping and traceability procedure required to preserve the OEM data plate and magneto model information. Bendix and Slick magnetos worked on by Kelly Aero for overhaul or 500-hour inspection are completely disassembled and all parts are cleaned and inspected to make the magneto look and work like new. During the teardown and inspection process, the OEM data plate is removed from the magneto frame in order to strip the paint from the magneto and repaint it to a new condition. The OEM serial number and magneto part number are permanently coded onto the magneto frame to preserve the record of that part. All of the information on the data plate becomes part of the extensive overhaul record of the magneto as it progresses through the Kelly Quality system. When the inspection and re-work steps are completed, a list of parts to overhaul or to complete the 500 hour inspection of the magneto is generated and added to the magneto record. At every step in the process, from disassembly to completion of the overhaul, the OEM data is part of the documentation record.
After the magneto assembly is completed, a new Kelly Aero data plate is made for the magneto. The data plate will show the OEM serial number, the Kelly Aero Overhaul serial number, and the part number of the magneto. The new Kelly Aero data plate is attached and the inspected magneto looks as good as new. An FAA Form 8130-3 Authorized Release Certificate is generated to document and release the 500-hour inspection magneto to service. If the magneto is overhauled, the data plate marking will be slightly different as compared to the 500 hour inspection. Kelly Aero Overhauled magnetos are treated more like new production magnetos and a unique Kelly Aero serial number is assigned to the magneto. The OEM serial number is also retained and both the Kelly Aero and OEM serial numbers are engraved on the new magneto data plate.
The Dual Identity of a magneto overhauled by Kelly Aero After overhaul, an FAA Form 8130-3 Authorized Release Certificate is generated to document completion of the overhaul process and to authorize the release to service of this part by Kelly Aero. It is at this point that the magneto develops a dual identity based on the specific requirements that apply to the OEM serial number or the Kelly assigned a serial number. Both serial numbers become equally important in terms of ongoing maintenance compliance. The most common misconception is that the Kelly Aero data plate changes the requirements to comply with OEM maintenance guidance. This perception is 100% wrong. All OEM Service Bulletins and Airworthiness Directives continue to apply to the magneto based on the underlying OEM magneto data, regardless if it was overhauled by Kelly Aero or by any other company. However, the Kelly Aero overhaul uses Kelly manufactured FAA-PMA parts and additional processes of inspection and workmanship which are not part of the basic OEM minimum requirements. These additional features make the Kelly Aero overhaul unique. These unique features may be affected by inspections or service needs different than, or in addition to, than OEM requirements. To track service requirements of the content added by Kelly to an overhauled magneto, a unique Kelly Aero assigned serial number is added to the data plate and is listed on the FAA Form 8130-3 Authorized Release Certificate supplied with the magneto.
Given the “dual Identity” of the magnetos serviced by Kelly Aero, the replacement Kelly Aero data plate is clearly more than just a simple cosmetic replacement. The Kelly Aero data plate ensures legibility for the service cycle of the magneto, which is required for ongoing compliance and safety requirements. The Kelly Aero data plate also adds the extra layer of traceability of the Kelly added components and workmanship. Ultimately, ALL applicable OEM and Kelly Aero service requirements must be complied with after the magneto is released to service, and the new Kelly Aero data plate assures that action can be accomplished. Do you have a question about Kelly Aero products or piston engine ignition systems? Contact us at https://kellyaero.com/about/contact-us/
Magneto technology is some of the oldest and most trusted in the aviation industry! However, what happens when a pilot has magneto failure? When one magneto fails, there are a few things a pilot might notice. First, there will be a decrease in engine power, but the engine will remain running thanks to the second system being independent of the first. Second, sometimes a magneto can malfunction in its failure to ignite the spark plugs, or the internal timing may malfunction. In this instance, the pilot will notice the engine beginning to run rough and will need to switch to the remaining magneto and see if that clears the problem. Even though an engine will continue to run with the loss of one magneto, there are several problems with having a magneto failure mid-flight. With the loss of one magneto comes the security of redundancy. They are a vital piece of equipment, and there is a reason that aircraft are built with two. Additionally, the loss of engine power can be potentially problematic, though in most cases does not pose a serious threat to the safety of pilots and guests.
Even in the case of magneto failure, only a very few accidents are a direct result of magneto failure, according to statistics provided by the National Transportation Safety Board. NRSB reports 16 recorded cases of aviation accidents due to magneto failure, with two of those sixteen being fatal. Most of these failures were listed as being caused by a lack of maintenance or non-compliance with servicing and inspections.
With attention to detail and a history of reliability, Kelly Aero has made sure that the magnetos produced are dependable and safe. Our products are designed and engineered using in-house resources, with additional tooling and manufacturing for aftermarket magnetos occurring at our facilities. We offer an FAA and PMA approved quality system, which is readily available with low wait times. We both meet and exceed the standard industry testing with our own set of criteria for the product, whether it be the full system or a replacement part. We want to work with customers to ensure the delivery of a product that is first and foremost high quality and reliable, and that meets their needs.
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