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
Today’s world is dominated by modern, high-tech smart electronics that can be found in every device imaginable from toothbrushes to spacecraft operating billions of miles away from Earth. General Aviation airplanes are equipped with the latest glass panel, GPS driven avionics that have more computing capability than any manned space vehicle that was sent to the moon. Aviation has historically been on the cutting edge of the newest and best technology found in the cockpit, so the expectation is that there should be an equally new technology applied to the aircraft engine and its systems. But, cutting-edge engine technology has been stubbornly slow to change piston aircraft engines.
In particular, why is it that mechanical magnetos- which have been in use on reciprocating engines for over 125 years- are still being used as the primary ignition systems for piston-engine aircraft? With all of the modern technology at our fingertips, why isn’t there something better? Auto engines have not used contact points for a few generations. It is likely that the parents of the high school students learning to drive today never drove or owned a car with an engine using a mechanical ignition system. Mention Magneto to these generations and the only reference they will have is a Marvel comic book character. Yet, magnetos remain the most prevalent ignition system used for aircraft engines. If asked, most aviation enthusiasts believe that aircraft engines use magnetos because that is the only ignition system approved by FAA Regulations. It is true that the Civil Aeronautics Authority, the predecessor to the FAA, defined the standards for piston-engine aircraft ignition systems nearly 85 years ago. The wording in the current regulations has remained virtually unchanged since then, and reads as follows: CFR 14, 33.37, PART 33—AIRWORTHINESS STANDARDS: AIRCRAFT ENGINES 33.37 Ignition System Each spark-ignition engine must have a dual ignition system with at least two spark plugs for each cylinder and two separate electric circuits with separate sources of electrical energy, or have an ignition system of equivalent in-flight reliability. Interesting….where are the words that say, specifically, magnetos must be used on piston-engine aircraft engines? The truth is there is no specific guidelines set forth by the FAA that piston engines must use magnetos as the primary ignition source for piston engines. So, why are magnetos used as the most prevalent ignition system used on aircraft piston engines when more modern technology for ignition is available?
BLAME IT ON WRIGHTS- AND CHARLES TAYLOR
The discussion of aircraft magnetos needs to begin with some sort of historical context as to how magnetos were first designed onto aircraft engines. As with virtually all the basics of flight, magnetos can be directly traced back to the engine used in the very first Wright Flyer of 1903. Everyone knows that the Wright Brothers built and flew the first successful powered airplane. But, very few people know that the Wrights also built the very first piston engine specifically designed for airplanes. The engine used by the Wrights was one of the most important, but overlooked elements that made their airplane successful.
The Wrights were not the first to fly or developing sophisticated aircraft designs. George Cayley, Otto Lilienthal, Octave Chanute had flown manned gliders many years ahead of the Wrights and had proven the concept of heavier than air flight. Samuel Langley made successful flights with an unmanned airplane powered by a steam engine. Langley unsuccessfully launched a steam engine-powered, man-carrying airplane in October 1903, 3 months prior to the Wright’s historic flight. The important point is, numerous inventors had developed flight-capable airframes at the time that the Wrights were experimenting with flight. However, the airframes lacked a suitable powerplant of the right power and weight that could propel the airplane in powered flight. The engine used by the Wrights solved the propulsion problem and directly contributed to their accomplishment to demonstrate controllable, powered flight of a heavier than air machine. The Wrights were assisted in their aircraft engine development work by their in-house master bicycle mechanic, Charlie Taylor. Keep in mind that gasoline-fueled piston engines were an emerging science of the time, and not common at all. The vast majority of people living at the time had never seen nor heard a piston engine and very likely had no knowledge of how a piston engine worked. In 1903, when the Wrights completed their first powered flight, Henry Ford was still 5 years away from producing the very first Model T car, so gasoline-powered engines used in vehicles were a rarity. Incredibly, with no formal engineering background, using only the skills he had learned as a toolmaker and bicycle repair mechanic, Charlie Taylor built the first successful airplane engine in only six weeks! He did follow established engineering concepts for piston engines of the time and used design elements from existing, successful engines. For the ignition system, he used what all other engine manufacturers were using- a magneto! What inspired Charles Taylor to use a magneto? Was there a better solution to be found in the automotive world? In a word- No. In 1903, the magneto was state of the art for ignition systems, was the very best solution for a lightweight, simple, self-contained generator of electrical energy. The magneto did not require any external sources of power to make it generate spark energy. The engine flywheel turned a magnetic rotor shaft in the magneto, and an electrical charge was generated. That energy fired the spark plug to ignite engine combustion which made the engine run to turn the propellers. The only other option available to Taylor was a battery ignition system that supplied power to an external coil and contact point mechanism to distribute the spark. The dilemma for Taylor was that batteries and generators of the time were extremely heavy, with all-up weight in the many hundreds of pounds. The size of the batteries also would have required considerable physical space, extra structure- and resulting weight- to support the batteries in the Flyer. The magneto solution used by Taylor was an engineering marvel. The average magneto weight was 20- 25lbs. and made a spark any time that the engine was running. The empty weight of the Wright Flyer was just over 600 lbs., meaning the magneto system was just under 5% of the total weight of the airplane. A battery ignition system probably would have added 30% more weight to the Wright Flyer, which would have clearly prevented it from flying. Is it a stretch to suggest that the Wrights were successful as the first to fly a controllable airplane due to the magneto? While that is an interesting idea, it is safe to say that the magneto certainly contributed to the overall success of the Wright Flyer and the Taylor engine.
THE GOVERNMENT REQUIRES DUAL IGNITION
The current day Federal Aviation Administration, or FAA, can find trace its roots through a number of government agencies that were focused on defining the regulations for aviation safety. The Civil Aeronautics Authority of the 1930s put into effect more stringent regulations to improve the safety of aircraft and engines. The early Civil Aviation Regulations became the later Federal Aviation Regulations and established the basis for rulemaking and safety standards for aircraft and engine design. Through the 1920s and early 1930s, the aircraft engine continued to rely upon single magnetos as the primary ignition system. As the CAR’s developed to improve upon aircraft engine design and safety, the Regulations for a dual, independent ignition system made the spark generating magnetos a perfect solution to comply with this government requirement. Dual ignition systems became the standard design, and for good reason. If one ignition system malfunctioned, then the remaining ignition could keep the engine running so that the flight could continue and be landed normally, under complete control.
The magneto also made sense as it was uncommon for aircraft of the time to be fitted with electrical systems or starters. Aircraft electrical systems did not develop as quickly as they did for cars, primarily due to weight, complexity, and expense. Batteries, starters, and alternators were still very heavy and not particularly reliable. The added weight of an aircraft electrical system could easily add 200 lbs. to the aircraft weight or about the weight of a passenger and personal baggage. For the most part, pilots of the time did not care that there were no electrical systems on airplanes. With no electric starters, aircraft engines used the “Armstrong Method” to get engines started: The propeller was swung by a person using their arms, the mags were switched to on, and the engine started. No worry about dead batteries, no worry about the cost of maintaining starters and electrical systems, no worry about getting stranded due to a failed electrical system. If the pilot could swing the propeller, the magneto sparked, and the engine would run. Magnetos provided the perfect solution to provide simplified system installation, good starting characteristics, and low cost of operation.
IT IS ALL ABOUT THE TIMING
In the early 30s, aircraft ignitions and automotive ignitions took different paths in terms of development. Automakers favored battery-driven contact point/coil/distributor systems and aircraft engines remained steadfast with the magneto. There are numerous technical reasons why each system worked better in some way for either the automotive or aircraft application. First, the mission profile of automotive engines and airplane engines became distinctly different. Auto engines are subject to a frequent change of RPM to speed up and slow down, sometimes driving in stop-and-go traffic, sometimes driving fast for long distances. Because of this, auto engine ignition systems and timing to the engine were biased to improve starting, idle, and low to mid RPM acceleration. Magnetos are limited to “fixed” advance ignition timing for all operations other than starting the engine. The fixed advanced timing works for airplane engines because full power is required at takeoff, and engine RPM does not vary for all inflight operations after takeoff and landing. The requirement for the aircraft engine to make full power is critical to flying safety. Airplanes must carry their certified load at takeoff. Not only that, but they must takeoff within a specific length of the runway and climb at a rate sufficient to clear obstacles or terrain within the vicinity of the airport. Auto engines rarely need to run at full power for extended periods, and rarely run at greater than 30% to 40% power most of the time. Car drivers never worry about having enough power to clear a hill or the power required to drive with light or heavy loads. Full engine power is rarely if ever, required for a typical passenger car. Because auto engine RPM varies when driving and the engine accelerates or decelerates randomly, the fixed timing of the magneto does not provide the best overall performance. Magnetos were not optimum and automakers devised distributors with “variable advance” mechanisms that changed timing based on the centrifugal force applied to the advance mechanism as the engine RPM changed.
The advanced mechanisms had the potential for failure modes which could affect ignition reliability, though. The advance mechanism itself adds many extra components to the system, all of which are subject to maintenance, or in the worst case, failure. Auto ignitions were designed to default to an engine timing point not at full power, but to low power, idle timing position. The automotive failsafe provided for “limp home” capability, but at the cost of reduced power. The default timing for the aircraft engine magneto is the maximum power timing point, which provides for the safest situation should the engine be required to continue running after one of the ignition systems fails in flight. The lack of the advanced mechanism is a benefit in terms of maintenance and overall cost due to lower parts count. The picture comes into focus that aircraft engines and automotive engines have distinctly different “mission profiles” relative to how the engine develops power relative to ignition timing. Automotive engines timing is designed to provide the best starting spark and optimize spark at less than full power engine loads by varying engine timing o The limp home default in the event of a component malfunction is for reduced engine power
Aircraft engines timing is optimized to perform best at full power engine loads by keeping ignition system timing at a fixed point with no variability o The limp home default is normal operation provided by the remaining ignition system should one system fail
MAGNETOS ARE THE CHOICE
The FAA’s single-minded goal for safety does not necessarily inhibit innovation, but it can encourage aircraft engine manufacturers to follow conservative, simple design paths of engineering. However, is a conservative path, wrong, or just as pragmatic as a mindset of “not re-inventing the wheel?” There have been at least a half dozen electronic ignitions specifically designed to replace magnetos introduced into the aviation market since 1986. But, none of these ignitions have shown the potential to be the “perfect” replacement for magnetos. Incredibly, many of the electronic ignitions sold today require that a magneto be retained as part of the system for failsafe backup. When the electronic ignition fails, the old technology, tried and true magneto will save the day so that the aircraft engine continues to run safely
In the final analysis, electronic ignitions are challenged to match the simplicity of installation and repair support that exists for magnetos. By design, electronic ignitions are more complicated installations with numerous components and wiring connections that all have to be not only installed correctly but maintained correctly. Troubleshooting of electronic ignitions requires the ability to think in more abstract terms of electronics and component interactions. Magnetos are mechanical, maintenance and troubleshooting do not require any extraordinary troubleshooting skills. The vast majority of aircraft mechanics in the world know how to install, maintain and repair magnetos. Out of the hundreds of thousands of aircraft mechanics in the world, only a few hundred may have experience with installing maintaining, and servicing aircraft electronic ignitions. Parts and service support for magnetos are unparalleled. Magnetos, parts, and companies that can service magnetos can be found worldwide. Anywhere in the world where a piston engine airplane can take off or land, there are magnetos parts or support available within a one-day shipping time. In most cases, maintenance shops based at airports with higher levels of airplane activity will have parts in stock and mechanics available immediately to provide service for the magneto. Due to the very low population of electronic ignitions, virtually no repair parts are easily found in the worldwide market. Parts are stocked at a handful of locations, or available as a special order from the manufacturer. In some cases, electronic ignitions sold and installed at some point in the past are simply no longer supported by the manufacturer. The recommendation from the manufacturer is to replace that electronic ignition with magnetos should it need service! So why is it that mechanical magnetos- which have been in use on reciprocating engines for over 125 years- are still being used as the primary ignition systems for piston-engine aircraft? When all of the advantages and disadvantages are summed up, the very reason that Charlie Taylor selected the magneto for the first piston aircraft engine remains as true today as it was 125 years ago: The magneto is a self-contained generator of electricity and ultimately the least complicated, most common sense the choice for reliability and performance for aircraft engines.