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What is likely to be the cause if the engine runs unusually rough while checking the magnetos?

  • A

    Starter is faulty.

  • B

    Sparking plug is defective.

  • C

    Short circuit on the ground cable.

  • D

    Ignition switch is faulty.

All aero piston engines are fitted with dual ignition, that is to say, two electrically independent ignition systems. Each engine cylinder has two sparking plugs fed by two separate magnetos. This reduces the risk of engine failure caused by faulty ignition and increases the power output of the engine by igniting the cylinder charge at two points (reducing combustion time).

Magnetos are self-contained engine-driven electrical generators. They produce a series of Extra High Tension (EHT) electrical sparks at the sparking plugs, in the correct firing sequence, for ignition of the petrol and air mixture.

Ignition Switch provides complete control of the engine’s magneto circuit, the magneto being made inoperative by earthing the primary circuit. In the ‘OFF’ position the switch is closed and this short-circuits the contact breaker points, which therefore no longer make and break the primary circuit. In the ‘ON’ position the switch is open and the primary circuit is controlled by the action of the contact breaker.

Grounding Wire is used to switch off the magneto. If the grounding wire breaks when the engine is running there will be no apparent changes in the engine’s performance. If the grounding wire breaks and touches the engine-body or airframe then this is the equivalent of grounding the primary circuit, and the magneto is switched off.

Magneto Checks: The Dead Cut Check is carried out at slow running. This check ensures that the pilot has control of the ignition before carrying out further ignition checks at higher engine speeds. RPM MUST DROP BUT ENGINE MUST NOT STOP WHILE SWITCHING ONE MAGNETO OFF AT A TIME.

Consider the situation which would exist with an engine running with the pilot unaware that only one magneto was working. If that live magneto was switched ‘OFF’ during a high rpm magneto check the engine would die.

The automatic reaction of the pilot would be to switch the ignition switch quickly back to ‘BOTH’. The engine suddenly bursting into life with the throttle still at the check position would set up a high torque reaction between the airframe and engine, possibly causing extensive damage.

The Live Magneto Check is not normally required, as evidence of a live magneto is usually found at the Dead Cut Check simply by observing a change in rpm as the switch is operated.

The Magneto RPM Drop Check is carried out at approximately 75% of the maximum engine speed. This checks that the magneto and sparking plugs are functioning correctly.

As each magneto is switched off in turn, a check for a drop in RPM is made and this drop must be within the limits laid down by the manufacturers. The fall in RPM is due to the increased time taken for the mixture to burn in the cylinders, as a magneto, and consequently a plug in each cylinder is switched off.

If the engine runs unusually rough while checking the magnetos, the cause is likely to be a defective sparking plug.

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