FAA Aviation Mechanic Powerplant (AMP)Engine InspectionHard
During a borescope inspection of a turbine engine compressor section, a technician finds a foreign object damage (FOD) nick 0.015 inch deep on the leading edge of a rotor blade. The maintenance manual specifies that blend repairs must have a blend length at least 10 times the blend depth to avoid a stress riser. What is the minimum blend length required for this repair?
- A1.500 inch
- B0.150 inch
- C0.015 inch
- D0.030 inch
Show answer & explanationAnswer & explanation
Correct answer: B. 0.150 inch
Blend length = blend depth × 10 = 0.015 inch × 10 = 0.150 inch. This gradual blend minimizes stress concentration at the repair site, which is critical for high-speed rotating compressor blades.
Why the other options are wrong
- A. This overstates the calculation by a factor of 10.
- C. This is only the blend depth, not the required blend length.
- D. This is only double the depth, not the required 10x ratio.
Compressor Blade Blend Repair
FOD damage on compressor blades is blended out with a gradual taper whose length is a specified multiple of the damage depth to avoid creating a stress concentration point.
- Blend length is typically a multiple (e.g., 10x) of the blend depth
- Sharp-edged repairs create stress risers that can lead to fatigue cracking
- Borescope inspection allows FOD assessment without engine disassembly
- Manufacturer manuals specify exact blend ratios and depth limits
Memory trick: 'Ten Times the Depth Keeps the Stress at Bay' — always multiply depth by the blend ratio.