FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsHard
A radial engine magneto drive incorporates a shear-type coupling between the accessory gear train and the magneto shaft. What is the primary purpose of this shear coupling?
- ATo automatically restore correct internal timing if the magneto rotor shifts position
- BTo increase the gear ratio so the magneto turns faster than the crankshaft
- CTo provide a mechanical failure point that shears if the magneto internal parts seize, protecting the accessory gear train from damage
- DTo allow the magneto to be removed without removing the accessory case cover
Show answer & explanationAnswer & explanation
Correct answer: C. To provide a mechanical failure point that shears if the magneto internal parts seize, protecting the accessory gear train from damage
The shear coupling (drive shear section) is designed as a deliberate weak point. If the magneto seizes internally, the coupling shears before the sudden shock load can damage the more expensive and harder-to-replace accessory gear train components.
Why the other options are wrong
- A. Shear couplings do not restore timing; they only protect against mechanical shock.
- B. Gear ratio is set by the accessory drive gears, not the shear coupling itself.
- D. Removal procedures depend on mounting hardware, not the shear coupling's purpose.
Magneto Drive Shear Coupling
A deliberately weak drive coupling between the accessory gear train and magneto shaft that shears if the magneto seizes, preventing damage to the gear train.
- Acts as a mechanical fuse for the magneto drive
- Prevents costly gear train damage
- Failure indicates magneto internal seizure
- Must be replaced along with magneto after shearing
Memory trick: Shear pin = a sacrificial soldier saving the gear army.