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A Delta Airlines Boeing 747-400, registration N662US
performing flight DL-158 from Seoul (South Korea) to Detroit,MI (USA) with
341 passengers and 17 crew, was enroute at FL320 northeast of Kamchatka
(Russia) in Russian Airspace when the crew needed to shut the #3 engine
(PW4056) down. The aircraft drifted down to FL290 and turned south to
divert to Tokyo's Narita Airport (Japan), where the aircraft landed safely
about 4.5 hours later.
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."
The missing vane cluster (Photo: NTSB):
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