main-rotor blades remained attached to their hub, and while the fire had consumed the main-rotor gearbox housing, the gear train remained intact. The manufacturer conducted an extensive
The engine
compressor following the accident (above). Thermal damage to
the compressor case halves (below) prevented the
investigative team from determining
whether the stage 3 and 6 blades might
have rubbed against the case and its
plastic coating, which could have initiated
the fatigue fractures, the NTSB noted in its report. (NTSB Photos)
examination of the engine core, which survived the fire. The first- and second-stage compressor blades were largely undamaged and showed no evidence of damage from foreign object debris (FOD). Two blades of the third-stage compres- sor wheel had fractured near their roots and weren’t located; the remaining third-stage com- pressor blades were damaged primarily along their trailing edges. The fourth-, fifth-, and sixth-stage compres- sor blades were all missing, fractured at their roots. Fragments of some axial compressor blades were found in the axial compressor sec- tion and the impeller inducer, and “the impeller inducer exhibited evidence of hard body debris ingestion.” The roots of the missing stage 3 compressor
blades “exhibited signatures consistent with fatigue.” The fractures began near the pressure side of the blades’ trailing edges, but impact damage made it impossible to determine how the fatigue originated. Not surprisingly, the NTSB found the accident’s probable cause to be “a total loss of engine power due to fatigue failure of two of the stage 3 compressor blades.” But what caused the fatigue, and how did it escape detection? The teardown inspection found “generalized corro- sion … on the inner and outer diameters of the com- pressor wheels for stages 2–3 and stages 4 and 5, but no pitting corrosion.” Generalized corrosion was also present on both the inner and outer diameters of the stage 6 compressor wheel, and the fracture surfaces of all but one of the stage 6 blades showed “signatures of fatigue with multiple origins near the suction-side crown root.”
Sixteen of the stage 3 stator vanes in the compressor
case were missing, along with all the stator vanes from stages 4–6. The remaining stage 3 vanes were flattened, and all surviving vanes, including the undamaged vanes in the first two stages, had generalized corrosion.
The Takeaway The NTSB’s finding of probable cause states that “con- tributing to the failure of the compressor blades was the failure of maintenance personnel to inspect the com- pressor at the recommended interval for operation in
56 ROTOR JUNE 2023
corrosive environments.”
Not reported is whether this lapse represented a con- scious decision or a lack of information. Providers of ser- vices in tourist areas often operate on thin margins; the combination of the added maintenance expense and air- craft downtime might have seemed prohibitive, at least during prime visitor season.
One can imagine the operator deciding to at least postpone the inspection until bookings slowed enough to accommodate the inevitable pause between flights. Without knowing the background of the technicians who worked on the aircraft or the completeness of the paper- work that arrived with it, however, one can’t exclude the possibility that they simply weren’t aware of the CSL. Maintaining airworthiness requires researching all rel-
evant airworthiness directives (ADs) and confirming or attaining compliance, but CSLs, service bulletins, and the like aren’t subject to the same mandate as ADs. While obtaining all the manufacturer’s advisories might seem like cheap insurance, in a busy, understaffed shop it could also be seen as a distraction from more urgent tasks.
Whatever the reason, not performing the 300-hour engine inspection proved to be a very costly false econ- omy. The fact that one wasn’t technically required didn’t make skipping it a good idea.
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