What happened
During the takeoff sequence, an air carrier airplane experienced severe vibrations and a reduction in right engine power. According to statements from the flight crew, the vibration and a drop in hydraulic fluid quantity and pressure were detected shortly after reaching rotation speed (VR). Following these indications, the crew observed a loss of right engine pressure ratio (EPR) and noted the activation of the right engine thrust reverser indication lights.
In response to the engine issues, the flight crew reduced power to the right engine. The aircraft subsequently returned to the departure airport and landed uneventfully.
The investigation
A post-incident examination of the right engine determined that the event was triggered by an imbalance in the low pressure turbine (LPT) rotor. This imbalance was caused by a significant loss of material from the 4th stage blade airfoil and shroud.
The resulting imbalance led to mechanical contact, or rubbing, between the LPT shaft and the high pressure turbine (HPT) shaft. This friction eventually rubbed a hole through the LPT shaft. The investigation found that oil in the area of the shaft rub ignited, which created the appearance of a burn-through on the LPT shaft.
Damage to various other systems, including hydraulic lines, EPR reference lines, and fire warning systems, was attributed to the duration of the excessive vibrations experienced before the engine could be safely shut down.
Prior to this incident, the right engine had been subject to a 50-cycle continue-in-service limitation following a Pratt & Whitney internal engineering notice regarding missing 4th stage turbine blade shroud material. At the time of the event, the engine had completed 13 cycles of that 50-cycle limit. The investigation noted that the amount of blade material loss observed was significantly greater than what was permitted under the continue-in-service limits. It is believed the imbalance resulted from an unrecorded event occurring after the most recent shroud inspection, though the specific cause of the 4th stage material loss could not be conclusively determined.