Magnetos Under Pressure
By Harry Fenton
July 2000
One of the fundamental problems with turbocharged piston engines that operate at high altitudes is that the ignition system requires special design features to compensate for the lack of air density, due to lower ambient pressure, encountered at high altitudes. One special feature on turbocharged engines to allow them to perform at altitude is pressurized magnetos. Let's take a look at how they work.
The science
An electrical arc across the spark plug initiates each combustion event in the engine cylinder. The source of the high voltage required to start the electrical discharge across the spark plug gap is the ignition coil. The ignition coil must not only supply enough breakdown voltage to form an arc at the spark plug, but must also provide enough breakdown voltage across all of the other "gaps" in the ignition system. For an aircraft magneto, there are typically two "air gaps" that the ignition coil must arc across. One is the well-known gap between the spark plug electrodes and the other is the air gap inside the magneto between the distributor gear electrode and distributor block electrodes.
A typical normally aspirated aircraft engine operating at sea level and takeoff power may require 10 kV (10,000 volts) to breakdown the plug gap and 1 kV (1,000 volts) to breakdown the gap between the distributor gear and distributor block electrode. The total voltage needed from the ignition coil to make a spark at the spark plug is therefore 11kV. Once an arc is formed across the spark plug gap, the spark continues as long as current from the ignition coil continues to flow through the all of the air gaps in the system. Immediately after the plug gap breaks down and current flow begins, the plug gap voltage drops from the initial 10 kV to about 1 kV. There is a similar decrease in the voltage required to sustain the arc between the distributor gear and distributor block electrodes. So, once the air gaps break down and the spark plug begins to fire, the coil only needs to supply about 1 kV total to sustain the arc across the spark plug gap.
Concerning how long each of these events takes place, the ignition coil operates at the relatively high voltage (11 kV) for only a small amount of time (0.5 percent of the duration of the arc) and operates at the lower voltage for 99.5 percent of the time. Unfortunately, even though the magneto operates almost all of the time at the lower voltages, it must work reliably at the higher voltages needed to initiate the spark. The physical magneto design must therefore accommodate this highest voltage, and this subsequently drives the magneto's outside dimensions and weight.
As the air density and ambient atmospheric pressure decreases with altitude, the voltage required to breakdown the distributor gap and any other high voltage conductor spacing in the magneto, drops. For any given throttle setting in a normally aspirated engine, the cylinder pressure at the time of the ignition spark event also drops due to the reduced manifold pressure caused by lower atmospheric pressure at higher altitudes. The result is less voltage is required to fire the spark plug. When an engine is turbocharged, however, the spark plug voltage demand remains relatively high (near sea level conditions) because the air pressure in the engine cylinder does not decrease with altitude. This requires that the magnetos produce the same high voltages at high altitude as at sea level.
Calibrated Orifice
As an aircraft equipped with a turbocharged engine climbs to higher altitudes, a non-pressurized magneto's internal breakdown voltage margin would continue to diminish until, in theory, the ignition voltage no longer reached the plugs. A non-pressurized magneto could be redesigned to operate under these conditions; however, it would not be a desirable package in terms of size, weight, or cost. A more practical option is to divert some of the pressurized air available from the turbocharger and pressurize the magneto. Thus, the magneto continues to operate as if it was at a lower altitude.
While this magneto configuration is commonly called "pressurized," these magnetos are really just sealed and fitted with calibrated orifices so that the magneto actually leaks high-pressure air at a measured rate.
Pressurized magneto systems typically consist of the following basic components: pickup port, filter or check valve, magnetos and pressurized harness. Some STC'd aftermarket systems, and specifically all Lycoming designed systems, incorporate a check valve in the system that serves to regulate the pressure of the system and to act as a moisture drain to purge contaminants from the system.
Pressurized magneto systems
Magneto pressurization is not a new idea but one that has evolved since the 1930s. As piston engine bombers and fighters began conducting high-altitude operations for tactical advantage, it became necessary to pressurize the magnetos so that high altitude missions could be accomplished.
The basic concept has carried forward and contemporary aircraft such as the Piper Malibu and Mirage, Cessna P210, Cessna T303, Mooney TLS, and many other aircraft are all equipped with pressurized magnetos. Without pressurized magnetos, these aircraft could not perform as designed.
Both Lycoming and Continental engines use pressurized magneto systems. Although not consistent, the general rule of thumb is that if an engine is equipped with a turbocharger, then it is also equipped with pressurized magnetos.
There are turbocharged engines that were not originally equipped with pressurized magnetos, but in almost all cases there is an aftermarket kit supplied by the airframe and engine OEM or an aftermarket STC holder to retrofit the engine with pressurized magnetos.
Environmental concerns
Moisture has an extremely negative impact on the internal workings of the magneto and is the single most damaging element to a pressurized magneto system.
Typically, moisture is present in the induction air in the form of very low levels of water vapor. But, in certain cases, the moisture levels can be more concentrated, such as when the incoming air mass may be saturated with water from ambient weather conditions (i.e. rain).
Given enough moisture-laden airflow, the nylon components of the magnetos can become gummy and begin to dissolve. Additionally, the metal components corrode in this corrosive atmosphere.
At left is a normal distributor gear. Right is a distributor gear that has been attacked by nitric acid.
Water droplets, heavy moisture, particulate matter and other contaminants may be removed from the system by providing a purge valve at the low point of the system or via an inline filter. The filter and purge valve is provided by the engine or airframe manufacturer or by the STC holder of the pressure system. The magneto manufacturer does not supply any of the filtering or pressurizing components.
Typically, Continental engines rely upon an inline filter, and Lycoming engines utilize a check-valve positioned at the low point of the pressurization system. On Lycoming engines, this purge valve also acts as an airflow check-valve regulating airflow to the magneto while accumulated moisture is purged from the pressure system.
Consult the engine and airframe manuals for the specific magneto pressurization and filter inspection intervals as requirements vary.
Servicing pressurized magnetos and related systems
Because of the moisture and the overall harshness of high altitude operation, pressurized magnetos require strict maintenance and in some cases, more frequent maintenance than their non-pressurized counterparts.
For Slick magnetos, the maximum time interval that a pressurized magneto should operate between internal inspections is 500 hours; however, based upon operational experience, many operators elect to do inspections more frequently.
Remember, the manufacturer's recommendations reflect the minimum inspection interval; there is nothing to stop the operator from performing inspections more frequently. For example, Piper states in the Seneca manual that if the aircraft is routinely flown above 12,000 feet, the ignition system should be completely serviced every 100 hours.
Pressure input
Prior to servicing a pressurized magneto system, consult the engine, airframe, and STC holder's documents as they may have maintenance requirements specific to the needs of their application. Quite often, the requirements specified by the airframe, engine, and magneto manufacturer may not be aligned, and it is the responsibility of the operator and maintenance organization to determine the most appropriate maintenance schedule for a particular application.
In addition to special components within the system, special techniques are used to maintain and inspect a pressurized magneto system.
Lycoming Service Instruction (SI) 1308 details the testing procedure for pressurized magnetos installed on Lycoming engines. The testing should be accomplished every 100 hours and requires the use of a Kellstrom tool, P/N 11-10090. Lycoming Service Instruction SI 1308 does detail an alternative testing method if the Kellstrom tool is not available. The test involves pressurizing the magneto and checking for proper pressurization and leakdown rate.
Slick pressurized magnetos are subject to specific inspections, some of which are mandatory under Cessna airframe Airworthiness Directive 88-25-04 and Slick Service Bulletin SB1-88A. (See sidebar on page 32 for more information on the AD.)
Lycoming pressure relief valve assembly
The routine internal inspection interval for all Slick magnetos is 500 hours. At the 500-hour inspection interval the magneto is removed from the engine, opened for inspection, and repaired or replaced as required. However, due to the harsh operating environment to which the pressurized magneto is subjected, more frequent inspections may be, and frequently are, required.
To preface the discussion of inspecting pressurized magnetos, it must be stressed that the entire magneto pressurization system must be inspected if there is a fault found in the magneto due to contamination or pressurization issues. The magneto is only one of several components in the system and is the end component in the system. If moisture is not adequately filtered out of the incoming airflow to the magneto, then the magneto will be damaged. Damage to the magneto may be the immediate service problem, but the reason for the damage must be detected and corrected.
If the engine pressurization system is clean, leak-free and operating normally, and the magneto pressure seals and bleed orifice are intact, then the pressurized ignition system can provide trouble-free service.
The inspection criteria for Slick pressurized magnetos can be found in the L1037 4200/6200 Series Maintenance and Overhaul manual and in the L1363 4300/6300 Series Maintenance and Overhaul manual. Slick Service Bulletin SB1-88A provides the inspection criteria specific to Slick 6220 and 6224 magnetos installed on Cessna airframes equipped with pressurized magnetos. As always, before performing a maintenance check, be sure that you have the latest revision of these documents.
When performing maintenance on 6200 or 6300 series magnetos, it must be understood that while these magnetos are similar in design, some of the inspection techniques, assembly and repair parts are subtly different, and in some cases, not interchangeable. Specifically, the 6200 series pressurized magnetos use a metallic gasket between the housing and an individual gasket under the housing screws. The later 6300 series magnetos use an O-ring type gasket that fits into a groove machined into the magneto housing.
Inspection tips
Block has arced internally - note burn at 2 o'clock position Distributor block contamination
The inspection begins by opening the magneto to conduct an internal inspection for any outward signs of moisture contamination or corrosion. The nylon rotor gear and distributor gear will be soft and sticky if they have been exposed to moisture. Some discoloration of the nylon material occurs during routine operation, but the material should not be soft or gummy. When cleaning the nylon or plastic parts of the magneto, use soapy water. Rinse with clear water and dry with a lint-free cloth. The metal parts can be cleaned using standard shop solvent.
The Oilite® bushing should be inspected for gummy contamination. The distributor block and bearing bar should be free from yellow or white powder deposits. Replace the distributor block if the bushings are contaminated, or the base material has become soft, or cannot be cleaned. Examine the block for evidence of electrical arcing. If electrical arcing has occurred in areas other than at the distributor block electrodes, then the component must be replaced.
If the screws or any of the internal hardware are corroded, then they need to be scrapped and replaced with new parts.
While the magneto is open, inspect the coil, capacitor, bearing ,and impulse coupling for condition. Reset the contact points to e-gap. Upon reassembly scrap all of the old pressurization gaskets, seals, and housing screws and install new parts. Never re-use old pressure gaskets as the magneto may not pressurize properly and could become damaged.
Once the magneto is reassembled, the magneto must be pressure tested. To perform this test, an air source, pressure gauge and flow meter are required. The flow meter recommended by Slick is a model MMA-7, manufactured by Dwyer instruments, 219-879-8868. Configure the air supply, flow meter, and pressure gauge as detailed in Figure 1. The pressurized ignition harness must be attached to the magneto to perform the test.
Apply 15 psi to the inlet nozzle of the magneto. If the flow is excessive, reposition the housing gaskets and re-torque the housing and harness cap screws. Use of soapy water to detect leaks is discouraged, as it is normal for some leakage to occur at the various junctions of the magneto frame. In fact, the magneto needs to "breathe." The major criterion is that the airflow at 15 psi should not exceed 40 standard cubic feet per hour (scfh). However, the balance of the leakage rate is extremely important. There must be enough airflow to purge the magneto, but the airflow must be slow enough to keep contaminants from being pulled through the pressurization system into the magneto.
Ultimately, pressurized magneto systems, much like the turbocharged engines to which they are attached, will require more frequent maintenance than their less complex, normally aspirated counterparts. If routine maintenance is performed then problems can be reduced, and the service life of the magnetos will generally be as advertised.
Where's that AD?
Cessna airframe AD 88-25-04 is unusual in that, while an airframe AD, it mandates inspection of an engine accessory, namely the Slick 6220 and 6224 pressurized magnetos. Cessna T20L, T210M, T1210N, P210N, and T303 aircraft are subject to inspections detailed in this AD. The goal of the AD is to preclude moisture contamination, which could result in magneto failure.
In addition to the physical inspection detailed in Slick Service Bulletin SB1-88, the airframe must be placarded and the Pilot's Operating Handbook revised to incorporate specific operating procedures.
Within the Normal Procedures section of the Airplane Flight Manual/Pilot Operating Handbook (AFM/POH), a supplement directs the pilot to perform a magneto check before and after flight to determine magneto operation. Additionally, a placard must be fabricated and installed on the instrument panel in clear view of the pilot that states, "PRIOR TO EACH FLIGHT, CONDUCT MAGNETO CHECKS IN ACCORDANCE WITH THE AFM/POH SUPPLEMENT DATED APRIL 1, 1988."
It should be noted that while AD 88-25-04 and Slick Service Bulletin SB1-88 only apply to Cessna airframes, operators of other airframes using pressurized magnetos may, at their discretion, use these documents for guidance to establish more frequent inspection schedules to suit their specific needs.
Harry Fenton is manager of Customer Services. He is an A&P IA and has an extensive background in general aviation and product support; (815) 965-4700, extension 161, E-mail harry@ unisonindustries.com
