FAA Aviation Mechanic Powerplant (AMP)Engine InspectionMedium
A starter-generator is removed for scheduled brush inspection. The manufacturer specifies a minimum brush length of 0.50 inch, and the mechanic measures the longest remaining brush at 0.31 inch. What action is required?
- AThe brushes should be reseated and reinstalled with no further action
- BOnly the shortest brush needs to be replaced; the others may remain in service
- CThe brushes must be replaced because they are below the minimum allowable length
- DThe brushes are serviceable since they are more than half their original length
Show answer & explanationAnswer & explanation
Correct answer: C. The brushes must be replaced because they are below the minimum allowable length
A measured brush length of 0.31 inch is below the 0.50-inch minimum specified by the manufacturer, so the brushes are worn beyond limits and must be replaced; brushes worn below minimum length can lose spring tension and cause arcing, poor commutation, and reduced electrical output.
Why the other options are wrong
- A. Reseating does not restore lost brush material; replacement is required.
- B. Brush sets are normally replaced as a complete set, not individually, per most manufacturer instructions.
- D. 0.31 inch is below the 0.50-inch minimum, so this is incorrect regardless of comparison to original length.
Starter-Generator Brush Wear Limit
Carbon brushes in a starter-generator must be measured against a minimum length specified by the manufacturer; brushes below this limit are replaced to avoid arcing and poor contact.
- Minimum brush length is specified in the component maintenance manual
- Worn brushes reduce spring pressure, causing arcing and overheating
- Brush sets are typically replaced together, not individually
Memory trick: A short brush can't reach — replace before it fails to make contact