LAX01IA118

The air carrier airplane experienced severe vibrations and a loss of right engine power following a partial loss of 4th stage turbine blades during the takeoff sequence. According to the flight crew's statements, they noticed the vibration and loss of hydraulic fluid quantity and pressure shortly after VR. They then observed a loss of right engine pressure ratio (EPR) and the right engine thrust reverser indication lights. The flight crew reduced right engine power and landed uneventfully at the departure airport. The post-incident examination of the right engine revealed the engine event was caused by the imbalance of the low pressure turbine (LPT) rotor due to a significant loss of the 4th stage blade airfoil and shroud material. This imbalance resulted in rub between the LPT shaft and the high pressure turbine (HPT) shaft. A hole was eventually rubbed through the LPT shaft. Oil in the vicinity of this shaft rub was ignited, giving the LPT shaft the appearance of burn through. The extent of the damage to other systems (hydraulic lines, EPR reference lines, fire warning systems, etc.) was attributed to the duration of excessive vibration experienced until the engine was safely shutdown. Prior to the incident, the right engine was placed on a 50-cycle continue-in-service limitation in accordance with a P&W internal engineering notice for missing 4th stage turbine blade shroud/shrouds. At the time of the event, the right engine had accumulated 13 cycles of the 50-cycle limit. The post-incident blade material loss observed was well above that allowed under the continue-in-service limits, and the imbalance was most likely the result of another event that occurred after the continue-in-service shroud inspection. The cause of the 4th stage material loss could not be conclusively determined.

History of flight

On March 15, 2001, at 1239 mountain standard time, a McDonnell Douglas MD-88 twin turbo-fan airplane, N996DL, experienced a loss of right engine power during takeoff from the Tucson International Airport, Tucson, Arizona. The airplane sustained minor damage; however, the 2 crewmembers, 3 flight attendants, and 96 passengers were not injured. The airplane was registered to and operated by Delta Air Lines, Inc., Atlanta, Georgia, as flight 1634, a scheduled domestic passenger flight, under the provisions of 14 CFR Part 121 when the incident occurred. The flight was originating in Tucson at the time of the incident and was scheduled to terminate in Cincinnati, Ohio. Visual meteorological conditions prevailed and an instrument flight rules (IFR) flight plan had been filed and activated.

According to the captain's written statement, he first felt a vibration at VR (rotation speed) during the takeoff roll from runway 11R. The crew then noted a loss of right hydraulic system quantity and pressure. The captain conducted a turn to downwind while the first officer performed the right hydraulic system failure checklist. During the turn to downwind the amber "reverse thrust unlock" light for the right engine flickered, followed by a steady illumination of the right engine's blue "reverse thrust on" light. The captain brought the right engine to an idle power setting. He reported the N1 (low rotor rotation speed), N2 (high rotor rotation speed), and exhaust gas temperature (EGT) gauge readings appeared normal; however, the engine pressure ratio (EPR) was reading low. The flight crew declared an emergency, performed an abbreviated landing checklist, and landed the airplane on runway 11L at 138,000 pounds gross weight. Touchdown was accomplished at an indicated airspeed of between 120 and 130 knots, and with a final rate of descent of less than 360 fpm.

The captain used the number one engine thrust reverser along with asymmetric braking to slow the airplane while both main landing gear doors dragged along the runway. During the braking process he attempted to maintain runway alignment with the nose wheel tiller until he was able to bring the airplane to a stop on the runway. After the airplane came to a stop, the flight crew secured the right engine, and the airplane was towed to the gate. The passengers deplaned via the jet way.

The first officer's written statement was similar to that of the captain's.

The lead flight attendant's statement indicated as the airplane was on takeoff roll, she received a call from one of the flight attendants seated near the aft end of the airplane. The aft-seated flight attendant requested that the lead attendant call the captain to report vibrations in the airplane's aft end. She added the vibrations were making it difficult to stay on the flight attendant jump seat. The airplane was beginning to rotate when the lead attendant contacted the flight crew. The flight crew requested that the lead attendant check out the vibration. As the lead attendant proceeded to the aft end of the airplane, she heard and felt the vibration as she passed row 15. It increased significantly the further aft she went. She reported her observations to the flight crew, but was told to stand-by as the flight crew completed a checklist. The flight crew then reported they were returning to the gate and made several public announcements (PA) for the passengers to remain seated. The lead attendant then heard a rough sputtering noise shortly after the first PA, and assumed the engine was being shutdown. She added the landing was smooth, but it "seemed forever before we stopped."

The flight data recorder (FDR) was removed from the airplane and shipped to the National Transportation Safety Board Vehicle Recorder Laboratory, Washington, D.C., for readout. The following information was extracted from the FDR factual report. During takeoff (1239:25; release of brake pressure was used as the start of the takeoff roll), the EPR was set at 2.08, and the hydraulic system depicted a "normal" situation. At 1239:36, the right engine's EPR began to decrease from its takeoff setting. At 1239:38, the air/ground squat switch changed from a "ground" indication to "air." At 1240:43, the FDR recorded low hydraulic pressure for the right side system. Throughout the entire event, the FDR recorded the left and right thrust reversers/thrust reverser locks to be in the stowed/unlock position, with the exception of the landing sequence (1248:52 - 1249:13), when both the left and right thrust reversers/thrust reverser locks were deployed and unlocked.

Aircraft information

The airplane was equipped with two Pratt & Whitney (P&W) JT8D-219 turbofan engines. The JT8D-219 engine is of a dual-spool design, with the low pressure compressor (LPC) and high pressure compressor (HPC) rotating independently of each other. The LPC drive shaft (connecting the low pressure compressor to the low pressure turbine) is located coaxially within the HPC drive shaft (which connects the high pressure compressor to the high pressure turbine). The LPC drive is supported within the HPC shaft via a roller bearing (No. 4 1/2 bearing).

The number 2 (right) engine (serial number 725394) was on a 50-cycle continue-in-service limit for fractured 4th stage turbine blade shroud/shrouds (Internal Engineering Notice, IEN 00HCC12). According to Delta, at the time of the incident, the engine had accumulated 13 cycles of the 50-cycle limit.

The engineering notice addressed several P&W JT8D-217C and 219 series engines that were found with broken 4th stage turbine blades. The notice indicated that all of the broken blades were found during routine torque check inspections or routine maintenance, and none were found as a result of operational discrepancies. According to the IEN, the blades were found fractured at the "convex side airfoil-to-shroud fillet radius, which liberated the convex portion of the shroud." The cause of the failures had been determined and a new shroud was being designed/manufactured; however, was not available at the time of the event. In the interim, maintenance personnel, who found broken blades, were to adhere to a field management plan, which placed a 50-hour continue-in-service limitation that allowed for no more than two fractured shrouds that had to be at least 10 blades apart.

The No. 2 hydraulic system consists of a single-stage hydraulic pump that produces 3,000 psi. The system's fluid capacity is 11.36 gallons. The number two hydraulic system exclusively operates the airplane's main landing gear, aft air stairs, number two thrust reverser, outboard spoilers, and the number one and three flap actuators.

Wreckage and impact information

The airplane/right engine area was examined the same day as the incident by Delta Air Lines maintenance personnel. The post flight inspection of the number two engine revealed there were several breaks in the pressure tailpipe (PT7) line (which also supplies EPR data), the thrust reverser position switch was displaced, and the fire warning indicator light wiring was separated. The number two engine tail cone was missing; however, all the attaching bolts were undamaged and in place. One of the bolts still had a torn fragment of the tail cone attached. The number two thrust reverser outer fairing was missing and was later found and recovered on the runway.

There was no obvious foreign object ingestion damage in the engine inlet. Metal fragments were found in the tailpipe, the 4th low pressure turbine (LPT) blades were found damaged, and the mixer support struts were missing.

Initial attempts to rotate the N1 section immediately after the incident were unsuccessful; however, on the next day, the N1 section was successfully rotated by hand. The starter was removed and the N2 section was also successfully rotated with the aid of a splined wrench. T