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.
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.
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.
The techniques and opinions on how to start aircraft engines are as varied as the airplanes, engines, and operators that encompass this topic.
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