During a borescope inspection of a turbine engine, a technician notices rub marks with metal transfer on the tips of several turbine blades where they contact the shroud. What is the most appropriate next step?
- AIncrease the fuel schedule to reduce turbine speed and eliminate future rubbing
- BImmediately replace all turbine blades regardless of the extent of rub
- CDisregard the finding since minor tip rub is expected during every engine start
- DInvestigate further for possible bearing wear, rotor imbalance, or case distortion causing the rub
Show answer & explanationAnswer & explanation
Correct answer: D. Investigate further for possible bearing wear, rotor imbalance, or case distortion causing the rub
Turbine blade tip rub with metal transfer suggests the rotor is no longer running with proper clearance to the shroud, which can result from worn bearings, rotor imbalance, thermal case distortion, or FOD-induced shaft bending. Because these underlying causes can worsen and lead to more serious failures, further investigation is required rather than dismissing the finding, blindly replacing all blades, or adjusting fuel schedule (which does not correct a mechanical clearance problem).
Why the other options are wrong
- A. Fuel schedule changes affect thrust/temperature, not mechanical rotor-to-shroud clearance.
- B. Blade replacement decisions depend on severity and manual limits, not an automatic full replacement.
- C. Some very light rub during initial run-in may be normal, but distinct rub marks with metal transfer warrant investigation, not dismissal.
Turbine Blade Tip Rub
Contact wear marks with metal transfer found on turbine blade tips where they have rubbed against the surrounding shroud, often signaling a mechanical clearance problem.
- Found during borescope inspection of the turbine section
- Can result from bearing wear, rotor imbalance, or case distortion
- Requires root-cause investigation, not just cosmetic repair
- Left unaddressed, can progress to more serious rotor or case damage
Memory trick: Scraped tips tell tales — something shifted the spinning wheel.