Delta Air Lines Flight 158

The examination of the engine confirmed that there were three holes in the LPT case. But the holes were under the LPT cooling air manifolds and plenum that did not have any damage. In addition, there was no damage to the inside of the engine's cowlings nor did any debris fall out when the cowlings were opened. There was no damage to the LPT cooling air manifolds and plenum that were over the holes and was there was no debris in the cowlings indicating that no debris passed through the LPT case that made this a contained event rather than uncontained.

The disassembly of the engine revealed one 3rd stage turbine vane cluster, No. 29, was missing although the cluster's bolt hole tab with the retaining nut were still in place on the inner transition duct. The missing cluster's inner shroud was found in the bottom of the engine in the path of the 3rd stage turbine blades. The metallurgical examination of the inner shroud revealed fatigue, however the full extent of the fatigue could not be determined because the end of the fracture surface was smeared.

The examination of the remainder of the LPT revealed all the other turbine vane clusters were complete and in place or the inner and outer shrouds were in place with just the airfoils missing. All the LPT blades were fractured and the fracture surfaces were coarse and grainy indicating an overload fracture. The examination of the remainder of the engine between the fan and high-pressure turbine did not reveal any damage. The extensive damage to the LPT and the absence of damage throughout the remainder of the engine indicated that the damage to the engine originated within the LPT.

The visual examination and a dimensional inspection of the LPT case revealed the 3rd stage turbine vane hooks had extensive wear that varied significantly between adjacent hooks. The dimensional inspection revealed the hook for 3rd stage turbine vane cluster No. 29, the missing vane cluster, had the most wear and that wear was tapered. The tapered wear on the hook and indicates that the vane cluster's outer foot disengaged from the LPT case and tilted rearward. It was not possible to determine the cause of the tapered wear on the LPT case vane hook that led to the vane cluster disengaging. The finding of fatigue on the inner shroud further supports that the vane cluster's outer foot disengaged from the LPT case initially and fatigue was caused by either the transfer of the loads to the inner shroud or from the cluster being strummed by the passing 3rd stage turbine blades.

The review of the engine's maintenance showed that it had last been overhauled in October 2005 and since accumulated 35,545 hours and 3,532 cycles of service. Although the engine was overhauled in October 2005, the LPT module's maintenance records show that it had been swapped from another engine and accumulated 17,441 hours and 2,184 cycles since its previous overhaul. So, at that time of event, the LPT module accumulated 52,986 hours and 6,546 cycles since it had been last overhauled. Except for the low cycle fatigue life limits for specific rotating parts that are outlined in the engine manual, there is no prohibition for an engine or LPT module to have operated as long as this module had been in service. The review of the LPT module's maintenance records from the previous overhaul show that the LPT case modification to the anti-rotation slots had been previously complied with. In addition, the records show that the modification to the 3rd stage turbine vane clusters to remove material from the outer platform gussets had been complied with as well. Because of previous contained and uncontained PW4000 LPT events, P&W has revised the engine manual to add extensive inspections and repairs to LPT components as well as limiting the number of strip and recoat repairs that can be done to PW4000 LPT airfoils. The inspections and repairs that were subsequently adopted into an airworthiness directive (AD) included a visual and dimensional inspection of the LPT case's vane hooks. The tapered wear that was noted on the LPT case's 3rd stage turbine vane hooks occurred over time. The records do not list any work on the LPT case's vane hooks at the last overhaul, so it cannot be determined if the wear that resulted in the disengagement of the 3rd stage turbine vane cluster had existed only from the last overhaul or had existed prior to that overhaul. However, the revised inspection and repair procedures that are now mandated by an AD likely would have captured the wear and required it to be repaired or the case replaced.

Verbatim NTSB analysis and probable cause from the NTSB dataset