High-tension Magneto

Aircraft high-tension magneto is a self-contained unit delivers high voltage to Spark plug, incorporating a coil, points and distributor. Coil having primary and secondary winding, step up the voltage to a spark-generating level, turn it into timed high tension electrical pulses, and sends it to the appropriate spark plug

Magnetos are fixed with manufacturer’s data plate  with either ‘L’ or ‘R’. The L or R does not refer to the position on the engine, but to the magnetos direction of rotation. In one instance, a left-hand rotation magneto was installed in the position where a right-hand rotation was specified. The result was a rough running engine with reduced power and the associated possibility of destructive detonation.

It is important to install serviceable magnetos with the correct part, model and configuration details for the intended engine. The correct magneto configuration should be verified against the approved maintenance data.

Magnetos can be overhauled and refurbished many times but some of the components have inherently limited lives. The plastic gears that turn the distributor are among these and eventually, due to heat and fatigue, will suffer from brittleness and susceptibility to fracture as the plastic degrades. When they snap, shear, or degrade into dust, the magneto stops.

A failed distributor gear can create another problem within the magneto, of electrical arcing, when the mechanism stops turning. This is because the high-tension electricity is still being generated as the magneto continues to operate, and if the electrical energy cannot discharge at the spark plug, it will seek (by arcing and/or burning) another path to earth.

Any event which places a thermal or impact shock on the engine, such as overheating or prop strike, has the potential to also damage the magneto. Oil contamination can enter a magneto through worn or inadequate magneto drive seals or in mist form, from an engine that has crankcase ventilation problems. Once inside a magneto, engine oil accelerates its failure.

Magneto drive rubbers or cushions can become hard and brittle over many hours and years of normal operation. Also, it has been found that abnormal torsional engine vibration (e.g. de-tuned crankshaft dampers) may cause magneto drive rubbers to fragment.

High cycle fatigue cracking can begin from small corrosion pits in the magneto shaft or in the area of the Woodruff key. This shaft can also respond to engine vibration which, under certain conditions, may induce a bending or wave motion response typical of shafts rotating at critical speed, making the shafts vulnerable to any surface defect. Shear failure often soon follows.

Magnetos contain capacitors which are essential to store electric current briefly each time the breaker point opens. Age and/or high temperature may cause the capacitor to change value or break down. The result can include a partial short, which can lower the voltage in the primary coil. Signs of high temperature on the contact spring or severe breaker point erosion are signs of a failing capacitor.

Cracks and corrosion in elevator torque tubes on Cessna

SAIB CE-17-25 issued by FAA on  cracks and corrosion in elevator torque tubes on Cessna 172, 175, 180, 182, 185, 188, and 208 airplanes.

An elevator torque tube removed from a Cessna Model 172C airplane during an annual inspection for cracks, corrosion and improper repairs. The airplane had spent 24 years in Florida (a high corrosion area). During the annual inspection, a blind rivet installation (not approved) was found. The date of this blind rivet installation could not be determined.

The Cessna 100 Series Service Manual, 1962 and Prior, in Section 2, Airframe Inspection item 34 states “Elevators for security of attachment, smooth operation, security of balance weights, cracks, corrosion, and skin or structural damage.

These elevator torque tubes have been installed on Cessna 100 airplanes since the 1950s and continue to be installed on production Cessna 172 and 182 airplanes. The tubes are made of aluminum. They are exposed to wheel spray during landings or spray from floats during water landings. The tube is oriented horizontally so it tends to retain water. Exposure to moisture over many years leads to corrosion damage. Airplanes used in coastal areas are especially prone to corrosion.

The SIDs state: “Visually inspect the torque tube for corrosion and rivet security. Pay particular attention to the flange riveted onto the torque tube near the airplane centerline for corrosion.

(1) Clean area before inspecting if grime or debris is present.”

For the 180/185 and 100 airplanes built between 1953 and 1968: Initial inspection compliance is recommended at 5,000 hours or 20 years. Repeat inspection intervals are recommended at 2,000 hours or 5 years.

 For the 172, 182, and 188 airplanes built after 1968: Initial inspection compliance is recommended at 10,000 hours or 20 years. Repeat inspection intervals are recommended at 3,000 hours or 5 years.

            Recommendations – For Cessna 100 airplanes and Cessna 208/208B airplanes, FAA recommend adherence to the applicable SIDs and maintenance manuals for corrosion inspections. Airplanes based or operated in high corrosion areas are recommended to be inspected more frequently. Pilots should check this area for corrosion or obvious damage during preflight inspections. If minor surface corrosion is found, remove the corrosion in accordance with Textron Aviation procedures. If cracks or severe corrosion is found, replace the affected parts.