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No rpm drop during the magneto safety check

Aviation Mechanic Powerplant (AMP) practice question 3 of 9 on Engine Inspection. ACS element AM.III.C.R1 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONDuring the magneto safety check at approximately 1,000 rpm, moving the ignition switch from BOTH to LEFT produces no drop in rpm. What does this indicate?

  1. AThe left magneto is out of time with the right.
  2. BThe spark plugs on the left magneto are fouled.
  3. CA magneto ground connection is faulty.
Show the answer and explanation

Correct answer: C. A magneto ground connection is faulty.

  • A — incorrect A timing difference between magnetos shows up as an excessive difference in rpm drop between the two switch positions, not as no drop at all.
  • B — incorrect Fouled plugs cause roughness and a fast rpm drop when switching to a single magneto, not an absence of any drop.
  • C — correct A slight but noticeable rpm drop should occur when switching to a single magneto, showing the opposite magneto is grounded out; failure to obtain any drop means one or both ground connections are faulty, so a magneto may fire if the propeller is turned.

BACKGROUND

The magneto safety check is performed during warm-up with the propeller in the high rpm, low pitch position at approximately 1,000 rpm. The switch is moved from BOTH to RIGHT and back, BOTH to LEFT and back, then momentarily to OFF and back to BOTH. A slight but noticeable drop in rpm when switching to a single magneto shows that the opposite magneto has been properly grounded out, and complete cutting out when switching to OFF shows that both magnetos are grounded. Failure to obtain any rpm drop in a single magneto position, or failure of the engine to cut out when switching to OFF, indicates that one or both ground connections are faulty. That is a safety problem, because the magnetos are not secured at shutdown and may fire if the propeller is turned.

STUDY TIP

Separate the three symptoms: no drop means a grounding fault, a fast drop points to plugs or harness, and a slow drop points to ignition timing or valve clearance. Treat any grounding fault as a live propeller.

KEY POINTS

  • A small rpm drop on a single magneto proves the other magneto is grounded out
  • No drop at all means a faulty magneto ground connection
  • An ungrounded magneto can fire if the propeller is turned, so treat it as hot

SOURCES

  • amp-powerplant/ch10.txt "Engine Warm-Up" (magneto safety check)
Practice the Aviation Mechanic Powerplant (AMP) test →Timed 100-question exam, by ACS area, or review your misses. Free.
MORE ON ENGINE INSPECTION

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.