FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsHard
During an engine runup at 2,300 RPM, a technician checks the magnetos and records the following: dual RPM 2,300, left magneto only 2,150 RPM, right magneto only 2,175 RPM. The engine manufacturer specifies a maximum allowable drop of 125 RPM and a maximum differential between magnetos of 50 RPM. Based on these limits, is the magneto check within limits?
- AYes, because both magnetos show a drop when switched to single ignition, which is normal and expected
- BYes, because the differential between magnetos is only 25 RPM, which is within the 50 RPM limit
- CNo, because the right magneto drop of 125 RPM exceeds the maximum allowable drop
- DNo, because the left magneto drop of 150 RPM exceeds the maximum allowable drop of 125 RPM
Show answer & explanationAnswer & explanation
Correct answer: D. No, because the left magneto drop of 150 RPM exceeds the maximum allowable drop of 125 RPM
Left magneto drop = 2,300 - 2,150 = 150 RPM, which exceeds the 125 RPM maximum allowable drop, even though the differential (150 - 125 = 25 RPM) is within the 50 RPM limit. Both the individual drop limit and the differential limit must be satisfied; since the left drop alone is excessive, the check fails.
Why the other options are wrong
- A. A drop is normal, but the magnitude here exceeds allowable limits.
- B. The differential being acceptable does not offset the left magneto's excessive individual drop.
- C. The right magneto drop of 125 RPM exactly meets, but does not exceed, the limit.
Magneto Check Limits
During runup, switching from BOTH to a single magneto should produce a drop within the manufacturer's maximum allowable drop, and the difference between the two individual magneto drops must not exceed the specified differential limit.
- Both the maximum single drop AND the differential must be satisfied
- Typical limits are around 125-150 RPM drop and 50 RPM differential (verify with AFM/POH)
- Excessive drop or differential indicates fouled plugs, bad timing, or ignition faults
Memory trick: Two rules to pass: 'Drop must be small, AND the gap between drops must be small'