Boeing 757 near Atlanta, GA — 2018-09-06

Probable cause

A right engine turbine uncontainment resulting from failure of the high pressure turbine (HPT) lenticular seal due to a fatigue crack originating from an overheated region at a knife edge tip. Contributing to the failure was Pratt & Whitney’s decision to introduce the temperature-resistant knife-edge coating as a new-part number lenticular seal on an attrition basis without the option to recoat existing lenticular seals, which delayed implementation of the more durable seal material into the fleet.

Contributing factors

Contributing to the failure was Pratt & Whitney’s decision to introduce the temperature-resistant knife-edge coating as a new-part number lenticular seal on an attrition basis without the option to recoat existing lenticular seals, which delayed implementation of the more durable seal material into the fleet.

NTSB narrative

The high-pressure turbine (HPT) uncontainment was caused by the failure of a rotating seal located between the HPT 1st-stage (S1) disk and 2nd-stage (S2) hub (the lenticular seal). The lenticular seal outer structure separated from an inner brace piece and unwound, destroying the turbine. Seal fragments breached the turbine cases. A fragment traveled forward, destroying the HPT S1 blades and penetrating the S1 turbine nozzle assembly and combustor, damaging a fuel nozzle stem, and releasing fuel. The crack initiation site was destroyed by secondary damage. Although the failure mode was not determined, PW2000 engine lenticular seal cracks are a known failure associated with knife edge tip fatigue cracks that originate at the first or second knife edge tip and propagate down the knife edge pedestal to the barrel. Past root cause investigations determined that normal knife edge rub can cause local increases in knife edge tip temperature when the knife edge coating is worn and has begun to spall. Continued rub with degraded knife edge coating creates a heat affected zone at the knife edge tip that can lead to thermo-mechanical fatigue (TMF) and tip crack initiation. TMF is more likely to develop with second run seals matched with new honeycomb material. Review of the engine service records found that the failed lenticular seal was installed new during a 2008 overhaul when the engine had accumulated 61,385 time since new (TSN) and 25,462 cycles since new (CSN). The seal was visually inspected and reinstalled during a 2013 overhaul at 74,042 TSN and 31,701 CSN. During the 2013 shop visit, 40% of the lenticular seal land honeycomb surface was replaced. The lenticular seal chromium carbide knife edge coating was not renewed, so that the second-run lenticular seal knife edges with chromium carbide coating ran against new honeycomb material. The lenticular seal failed 6,460 cycles after the 2013 shop visit. An improved-design lenticular seal with the chromium carbide knife edge coating replaced with a more durable, temperature-resistant aluminum oxide coating was released by PW2000 SB 72-754 in 2011. The new-design seal was introduced as a part replacement on an attrition basis (use down-change part until exhausted).

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Inspection and maintenance of the lenticular seal prior to the failure.

NTSB coding

• Uncontained engine failure · Initial climb

Evidence available

• 7 docket documents

Docket documents7

• PARTY FORM - DAL TAYLOR SMITHform
• PARTY FORM - KOZAform
• PARTY FORM - ZABAWAform
• PARTY FORM, BOEING, DAN MARCOTTEform
• PARTY FORM, P&W, JEFF STRAUSBAUGHform
• AIRWORTHINESS DIRECTIVE 2021-14-13other
• POWERPLANTS GROUP CHAIR'S FACTUAL REPORTreport

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