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...


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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.

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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."

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