ATL97IA035

After takeoff, the left engine experienced an uncontained failure of the Stage 1 High Pressure Turbine (HPT). The airplane returned safely and made a precautionary landing. After further examination of the engine, it was determined that the 'HPT disk blade retention lug had separated because of a low cycle fatigue fracture'. Both a Service Bulletin, PW2000 A72-592, and an Airworthiness Directive, TAD 97-11-51T, were issued to address this problem in this and other Pratt and Whitney engines.

On January 31, 1997, about 1610 eastern standard time, a Boeing 757, N611DL, experienced an uncontained failure of the number one (left) engine during climb, near Atlanta, Georgia. The airplane, Flight 602, was operated by Delta Air Lines under instrument flight rules, and the provisions of Title 14 CFR Part 121, as a scheduled, domestic, passenger flight. An instrument flight plan was activated. Visual meteorological conditions prevailed. There were no injuries to the two flight crewmembers, the five flight attendants, and the 161 passengers, and the airplane incurred minor damage. Origination of the flight was Atlanta, Georgia, about 1600 on the same day. The flight was destined for Dallas, Texas.

According the captain, the airplane was climbing through about 15,000 feet when "the engine failed abruptly". The cockpit and aft cabin filled partially with smoke which cleared quickly. The captain requested an immediate return to Atlanta's William B. Hartsfield International Airport. The landing was uneventful. After being visually inspected for fire, the airplane was able to taxi back to the gate without further incident.

The left engine was a Pratt and Whitney PW2037 turbofan, serial number 716582. Delta's records indicate the engine had accumulated 19,243 hours total time and 9,823 cycles since new. The engine had operated 3,518 hours and 1,719 cycles since its last heavy maintenance visit on September 26, 1996.

An examination by Delta, Pratt and Whitney, and the Powerplants Group of the National Transportation Safety Board revealed the Stage 1 high pressure turbine (HPT) disk, Part Number 1B3621, had fractured and was missing the lug between two blade root slots. According to the report by the Powerplants Group, this rupture liberated one Stage 1 lug and two Stage 1 HPT blades. One blade was recovered from the cowling and determined to be "battered and fractured transversely across the airfoil adjacent to the blade root platform".

A visual examination showed the fracture surface was "smooth and had a purple discoloration from the front side that faded to a gold color towards the rear" of the disk. According to the Powerplants Group report, the "fir tree serrations on the adjacent lugs were intact." At the time of separation, according to Delta's records, the Stage 1 HPT disk had accumulated 9,825 cycles since new, and 1,719 cycles since the last heavy maintenance. Part Number 1B3621 Stage 1 HPT disks are life limited to 15,000 cycles.

The remaining rotating parts of the HPT were intact, with impact damage on the blades. The low pressure turbine (LPT) was intact. Its vanes and blades both had "nicks and dents". The remainder of the engine appeared normal and was not disassembled.

A metallurgical examination was completed by Pratt and Whitney's material laboratory in February, 1997. The examination showed that the "HPT disk blade retention lug had separated because of a low cycle fatigue (LCF) fracture that had initiated from multiple origins along the front sideplate snap radius and propagated axially rearward. The origins of the fatigue fracture were parallel to circumferential machining marks that were just inboard of the snap radius".

After this and other similar HPT failures, Pratt and Whitney produced an Alert Service Bulletin (ASB). This bulletin, PW2000 A72-592, described a modification of the HPT disk assembly for all PW2037, PW2037(M), PW2040, PW2240, and PW2337 engines. This Service Bulletin attempted to prevent further blade attachment lug liberations by enlarging the front and rear sideplate snap radii. This modification was designed to reduce the stresses and eliminate the cracking in the radii which was allowing the blades to be liberated.

From Pratt and Whitney's Alert Service Bulletin, the FAA produced a Telegraphic Airworthiness Directive (TAD) 97-11-51T, which required serviceable disks to be operated in accordance with Pratt and Whitney's ASB No. PW2000 A72-592.