Delta B744 near Kamchatka on Oct 2nd 2015, engine shut down after failure penetrating engine case
Last Update: October 21, 2017 / 16:33:20 GMT/Zulu time
On Oct 7th 2015 the NTSB reported, that maintenance while removing the engine from the aircraft discovered several holes in the low pressure turbine case, nicks in the right hand wing trailing edge flaps and aileron as well as nicks in the leading edge of the horizontal stabilizer downstream of the #3 engine. The occurrence was rated an incident, the NTSB have opened an investigation into the occurrence.
On Oct 19th 2017 the NTSB reported that the crew heard a loud bang that was followed by the run down of the engine #3 N1 and increase by engine #3 EGT. The crew declared emergency with Russian ATC and descended the aircraft to FL290, consulted with dispatch to determine where to divert to. A diversion to Seattle,WA (USA) was considered, however, the weather forecast for the estimated time of arrival indicated 2 miles visibility and 800 feet overcast, the crew decided to divert to Tokyo Narita where the weather forecast indicated 25nm visibility and scattered cloud. The aircraft performed a 3 engine landing into Narita Airport. The aircraft sustained nicks and dents to the underside of the right hand wing and ailerons as well as to the leading edge of the right hand horizontal stabilizer.
The #3 engine sustained 3 holes to the low pressure turbine case, however, the engine nacelle did not receive any damage and no debris was found in the nacelle when it was opened. The NTSB stated that Japan's TSB initially reported the engine failure as "uncontained" because of the holes in the LPT case.
The NTSB reported that the 3rd stage turbine vane cluster No. 29 at the 8 o'clock position was missing. Portion of the vane's inner shroud were found at the bottom of the engine in the 3rd stage turbine blades plane of rotation. The cluster's bolt hole tab however had remained in position, it showed evidence of fretting. The inner shroud showed evidence of fatigue that originated at the aft end of the forward flange and progressed forward. The wear pattern suggested the vane cluster had shifted out of position.
The torque of all the retaining nuts on bolts of the remaining clusters were checked and found tight.
The 3rd stage turbine blades were showed "heavily battered with nicks and dents on the trailing edges".
On Oct 21st 2017 the NTSB released their final report concluding the probable cause of the incident was:
The PW4056 engine experienced an in-flight loss of power because of damage to the low-pressure turbine (LPT) because of the inadequate overhaul inspection and repair instructions that existed at the time of the LPT module's last overhaul. A vane hook in the LPT case wore to the point to allow a 3rd stage turbine vane cluster to disengage and eventually fall into the path of the 3rd stage turbine blades causing extensive downstream damage to the LPT module.
The NTSB reported: "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."
This article is published under license from Avherald.com. © of text by Avherald.com.
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