Event Details
On September 5, 2018, about 2332 EDT, a Delta Air Lines Boeing B757-232, registration N668DN, experienced a right engine failure during climb after departing Hartsfield-Jackson International Airport (ATL) in Atlanta, Georgia. The flight crew heard a loud bang and noted airplane vibration and engine failure indications. The right engine was shut down, and the flight returned to ATL for an uneventful single-engine landing. The airplane sustained minor damage, and no injuries were reported.
Aircraft and Engine Damage
Post-incident inspection revealed a hole in the inboard side of the right nacelle and minor impact damage to the right wing, fuselage, and horizontal stabilizer. A piece of high pressure turbine (HPT) lenticular seal material was found protruding from the engine case. Subsequent disassembly revealed a 14-inch circumferential by 3-inch axial hole in the HPT case, a 7-inch by 2.5-inch hole in the low pressure turbine (LPT) case, and a 6-inch by 1.5-inch hole in the HPT case. All HPT S1 blades were fractured to an inch or less above their platforms. The HPT S1 blade outer air seals (BOAS) were liberated between 5 and 1 o'clock. The HPT S2 vanes between 3 and 11 o'clock were liberated. The LPT gas path components from the S3 vanes to the S5 vane leading edges were thermally consumed over a 30° arc centered on 9 o'clock.
Investigation Findings
Digital flight data recorder (DFDR) data showed no abnormal engine operation or indications prior to the failure. Engine maintenance records indicated that the lenticular seal was installed new during a 2008 overhaul and was visually inspected and reinstalled during a 2013 overhaul. The seal failed 6,460 cycles after the 2013 shop visit. Metallurgical examination identified an axial separation through the seal barrel as the most likely primary fracture. The fracture surface showed evidence of progression from the area of the knife edges toward the aft edge, but no origin area was identified due to oxidation and deposits. The seal material conformed to IN-100 alloy specifications.
Previous Occurrences and Corrective Actions
There were 39 previous PW2000 lenticular seal failures, all attributed to fatigue cracks originating at knife edge tips. An improved-design seal with a more durable aluminum oxide coating was released in 2011 via PW2000 Service Bulletin (SB) 72-754. Subsequent revisions changed compliance requirements. Pratt & Whitney developed a borescope inspection technique in 2018 and issued SB 72-773 requiring inspection of certain lenticular seals. The FAA issued Airworthiness Directive 2021-14-13 in 2021, requiring removal of chromium carbide coating and fluorescent penetrant inspection at piece part opportunities, and replacement of affected seals at next shop visit.
Conclusion
The investigation determined that the engine failure resulted from a fracture of the lenticular seal, a rotating seal between the HPT S1 disk and S2 hub. No probable cause was explicitly stated in the source.