For various reasons, several incidents of TCM Dual Magnetos separating from Lycoming engines during flight have occurred.
One failure occurred on the first flight after installation of a newly overhauled magneto. The failure occurred due to cracks in the magneto housing mounting flange area that were present as a result of the magneto’s prior operation. During the overhaul process, the overhaul manual only required visual inspection of the magneto.
Another failure mode has been due to the use of the wrong gasket between the magneto and the engine accessory housing. Lycoming Service Instruction (SI) 1508B clearly shows that the gasket must be circular and no part of the gasket can extend beyond the flange of the magneto housing. To be more specific, using a gasket with extensions on the outside diameter that allows the stud from the
accessory housing to go through the gasket, and position the gasket between the magneto clamp and the accessory housing, is unacceptable.
When a gasket is located between the magneto clamp and accessory housing it will compress and in time result in a loose magneto. A loose magneto will result in the magneto clamp wearing into the
magneto mounting flange. Ultimately, the flange thickness will be reduced and the flange will fail.
Improper maintenance usually occurs when magneto maintenance is performed in the field and the nuts holding the magneto clamps in position are removed or loosened. When these nuts are reinstalled, new lock washers must be used because they lose their locking ability with repetitive usage. The ability to obtain the proper torque on the nuts securing the magneto clamps or even make
them adequately tight depends on the accessibility of the nuts when the engine is installed in the airplane. Magnetos are more accessible on some airplanes than on others and obtaining the correct torque is easier. However, on some installations, the working room is limited and the magneto clamping nuts are inaccessible making proper torquing difficult. As stated above, most of the problems have occurred on Lycoming IO-360-A1B6D and IO-360-A3B6D engines installed in Mooney M20J-201 and Cessna 177RG aircraft because of inaccessibility of the nuts securing the magneto clamps.