Magnetos
Component Maintenance
We are approved as a Combined Airworthiness Organisation (under Annex Vd - Part CAO) to carry out maintenance on the components listed in the Scope of Work section of our Combined Airworthiness Exposition (CAE). Reference: IE.CAO.119. See page Magnetos - Scope of Work.
Aircraft level Maintenance
External timing check
We check the external timing (of the magneto to the engine). Both left and right magnetos need to be firing their respective plugs at exactly the same time (unless the engine/airframe manual says otherwise - some engines have slightly different timing for Left and Right - e.g. 30 and 28 degrees before TDC) and at the correct point in the engine cycle for maximum performance. A magneto timing light gives visual and audible indication of the opening of the points within the magneto and this needs to coincide with the timing marks on the flywheel/ring gear.
Magneto external wiring (e.g. the P lead) is light gauge and easily damaged. A broken P (Primary) lead has exactly the same effect as a Magneto switch in the "ON" position - it means the magneto isn't grounded and is Hot - i.e. it can create sparks if the propeller is turned by hand. A similar issue arises with ACS or Gerdes ignition switches found in many GA aircraft whereby the key can be in an intermediate position and not actually OFF. Again the Magneto will be Hot. AD 93-05-06 relates to this and requires inspection and lubrication every 2000 hours. Additionally it is rather important to check whether it is possible to withdraw the key from the cylinder in any position other than "OFF". If it can be removed, then the key/cylinder is excessively worn and needs to be replaced. (New key switch shown in the photo below).
