Engine Vibration After Fan Blade Replacement
A CFM56-7B engine on a Boeing 737-800 has just had a fan blade replaced due to foreign object damage. The maintenance manual specifies that fan blades must be matched within a certain weight range and that the fan rotor must be balanced. After replacement, the engine is run and a vibration survey is conducted. The vibration level is 2.5 units, which is above the acceptable limit of 1.5 units. The AME must decide whether to perform a fan trim balance or investigate other causes.
1. List the possible causes of the high vibration and outline the steps to rectify the issue.
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Possible causes: incorrect blade matching (weight imbalance), improper blade installation (blade not seated), foreign object debris in the fan, or a faulty vibration sensor. Steps: 1) Review blade part numbers and weights to ensure they are within limits and properly matched. 2) Inspect fan blades for damage or improper seating. 3) Perform a fan trim balance per the maintenance manual. 4) If vibration persists, check the vibration sensor and wiring. 5) Repeat vibration survey after corrective actions.
2. Explain the principle of fan trim balancing and how it reduces vibration.
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Fan trim balancing involves adding small weights to the spinner or fan disk to counteract imbalance. The vibration sensor measures amplitude and phase of vibration. By adding weights at specific locations, the imbalance is reduced, lowering vibration. The process is iterative: run engine, measure, adjust weights, and repeat until vibration is within limits.