FAA Ref: FAA-S-ACS-6C Area I Task G (Operation of Systems)
Scenario & Physics Summary
Diagnosing a live P-lead ground failure or hot magneto during pre-flight engine run-up checks.
Interactive 3D Visual Plates & Scenario Media
Plate 1: Ignition Magneto Check: No RPM Drop Indication
Plate 2: Ignition Magneto Check: No RPM Drop Indication
Plate 3: Ignition Magneto Check: No RPM Drop Indication
Plate 4: Ignition Magneto Check: No RPM Drop Indication
Aerodynamic & Operational Physics
Aircraft magnetos use a primary P-lead wire connected to the ignition switch to ground out spark voltage when switched OFF.
During run-up, switching from BOTH to LEFT should cause a 50-150 RPM drop because only one spark plug per cylinder is firing.
Zero RPM drop indicates either: 1) The P-lead is broken (magneto is HOT / live constantly), or 2) Ignition switch failure. A hot magneto can start the engine if prop is moved by hand.
FAA Exam Gotcha Alert
FAA Exam Trap: A magneto with no RPM drop during check is DANGEROUS because it cannot be turned off. Moving the prop during preflight will fire the engine.
Frequently Asked Questions & Exam Explanations
Why is zero RPM drop during a magneto check dangerous?
It indicates a broken P-lead ground wire, meaning the magneto is live constantly and could start the engine if the propeller is moved.
Master Ignition Magneto Check: No RPM Drop Indication in the Simulator
Drill targeted FAA knowledge test questions with step-by-step mathematical decoders and regulatory clause links.
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