CFM56-7B HPT Blade Distress Found on Borescope
During a borescope inspection of a CFM56-7B engine on a Boeing 737-800 at a NZ line station, the engineer finds three adjacent HPT blades with tip-corner burning and one blade with a 2.0 mm notch on the trailing edge. The HPT nozzle guide vanes show minor axial cracks below the AMM limits. The engine has 9,800 cycles since new, EGT margin is 18°C, and the last shop visit was 3,200 cycles ago. The aircraft is scheduled for a 3-hour Part 121 flight with 160 passengers. The operator's MEL does not cover HPT blade distress. The AMM provides blend limits, serviceable limits and a replace limit; engine trend shows no recent EGT rise. The engineer has four hours before the flight and no spare engine.
1. How do you classify the borescope findings and decide whether the aircraft can dispatch? Include the logic of limits, adjacent blades and performance.
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Classify each finding against the AMM: HPT blade tip-corner burning and a 2.0 mm trailing-edge notch must be compared with the serviceable, blendable and replace limits. Minor NGV cracks below the AMM limits may be serviceable, but they do not excuse the blade damage. Three adjacent blades with tip burning suggests a local hot streak from a fuel nozzle or combustor, so the cause must be found, not just the blades blended. An EGT margin of 18°C is positive but not generous; no recent EGT rise is reassuring but not decisive, because a mechanical notch can propagate without a trend change. The MEL does not cover this, so there is no dispatch relief. My decision is to not dispatch on the passenger flight until an approved assessment is done. If the notch is within blend limits and the blades can be blended per the AMM, and the hot-streak cause is corrected, a return to service may be possible with a repeat borescope. If the notch exceeds the blend limit, or there is any crack, the blade set must be replaced or the engine removed.
2. What additional inspections or data would you require before making a decision?
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Repeat the borescope with a second engineer, document blade positions and dimensions, and inspect all HPT blades, NGVs, combustor liners and fuel nozzles. Check the previous borescope report to see whether the notch is new or stable. Review the engine trend, EGT margin, fuel flow and vibration. Check for applicable ADs and SBs and the AMM repair limits. Inspect the oil filter and magnetic chip detector for metal, because HPT distress can shed material. Check the fuel nozzles for coking or spray pattern, since adjacent tip burning suggests a hot streak. If possible, use a borescope with measurement capability and compare the notch with the blend limit. Do not rely on a verbal report. If the notch is at or beyond the limit, or if the blade is cracked, stop and plan replacement. If within limits, obtain engineering/CAMO approval for blend and set a shorter repeat inspection.
3. If the notch is within blend limits, what maintenance and release requirements apply?
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Blend only to the AMM procedure: the limits specify depth, length, number of blades per stage and minimum wall or tip dimensions. Record the blade position, before and after dimensions, and the blend method. After blending, verify that the blade is within limits and that blending has not removed coating beyond allowance. Reinstall and inspect per the manual, run the engine or perform a performance check as required, and confirm no new exceedance. Release under CAA NZ Part 43 with a maintenance release that references the AMM and the blend record. Because three adjacent blades were affected, correct the likely hot-streak cause, such as a fuel nozzle or combustor, and repeat the borescope at a reduced interval, for example the next 100 hours or earlier, as agreed with the CAMO. If the cause is not corrected, the damage will recur. Do not release on a blend alone if the NGV cracks or the hot streak are outside limits.