Magnetos Under Pressure

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...

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.


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.)


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


We Recommend