Incident Overview
On April 10, 2015, at about 1447 Pacific daylight time, a Boeing 707-338C, registration N624RH, experienced a nacelle uncontained release of turbine material from the No. 1 engine during climb after departing from the Pt. Mugu Naval Air Station (NTD), Port Hueneme, California. The airplane was operated by Omega Aerial Refueling Services, Inc. and was being ferried from NTD to Brunswick Golden Isles Airport (BQK), Brunswick, Georgia, to replace the No. 1 engine due to excessive oil consumption. The flight was conducted under 14 CFR Part 91 on an instrument flight rules flight plan.
Flight History
During the takeoff roll, after the airplane had accelerated past V1, the No. 1 engine's exhaust gas temperature (EGT) began to increase toward the red line limit. The flight engineer had to retard the No. 1 engine's throttle several times to keep the EGT below the red line limit during the takeoff roll and climb. As the airplane climbed through 17,000 feet, the No. 1 engine's low oil pressure light illuminated. The crew stopped the climb at flight level 210 to reduce power on the No. 1 engine. Following checklist procedures, the captain retarded the No. 1 engine's throttle to idle. While the throttle was being retarded, a vibration began and became severe within a few seconds. The pilots shut down the No. 1 engine, declared an emergency, and requested to divert to Victorville Airport (VCV), Victorville, California. Mechanics on board observed the No. 1 engine vibrating on the pylon, with smoke and debris trailing from the engine's exhaust. The airplane landed at VCV without further incident.
There were no reported injuries to the two pilots, the flight engineer, or the two mechanics on board. The airplane sustained a hole in the bottom of the No. 1 engine's nacelle and a small hole in the underside of the left outboard aileron trim tab. No fire damage occurred.
Engine Examination Findings
The No. 1 engine, a Pratt & Whitney JT3D-3B turbofan (serial number 667883), was removed from the airplane and shipped to Aero Engines Ireland, Ltd. (AEI) for disassembly and examination. Disassembly revealed one 1st stage turbine blade (part number 819501) fractured transversely across the airfoil about an inch above the blade root platform. The fracture surface was smooth and planar for about 0.48 inches from the leading edge, with the remainder coarse and grainy.
Metallurgical examination at the NTSB and Pratt & Whitney materials laboratories determined the blade had fractured due to a high cycle fatigue (HCF) fracture initiating from multiple origins along the leading edge at the interface between the coating and the blade's base metal. The crack progressed rearward in HCF for about 0.22 inches, then a mixed mode of HCF and overload for about 0.26 inches, with the remainder in overload. The blade's material and coating conformed to engineering drawing and engine manual requirements. The blade's leading edge radius at the fracture location also met requirements. No evidence of an over-temperature condition was found. A secondary crack was observed directly below the primary fracture, with a similar morphology. Energy dispersive spectroscopy showed phosphorous on the fatigue portions, consistent with turbine engine oil, but not on the overload portion.
Extensive damage was found throughout the low-pressure turbine (LPT). All 2nd, 3rd, and 4th stage turbine blades remained in their disks but exhibited varying damage. One 2nd stage blade was missing its outer end. All 108 3rd stage blades were broken about 6 inches from the blade platform, with missing tip shrouds except five broken between 1.75 and 5 inches. Of the 80 4th stage blades, 20 were full length; the remainder were broken about 7.5 inches from the platform, missing tip shrouds, with many bent opposite rotation direction. The LPT case had no holes, but the turbine exhaust case had four splits and three holes around its circumference, about 3.5 to 4 inches aft of the front flange and in the plane of rotation of the 4th stage blades. The largest hole, at the bottom, was about 8 inches long. A hole in the side and bottom of the right-hand thrust reverser actuator contained an imprint of a turbine blade tip shroud.
Metallurgical Analysis
Dimensional inspection of five randomly selected 1st stage turbine blades removed from the 1st stage disk showed all blades had untwisted beyond overhaul manual limits, with tip shroud twist angles ranging from 0°12' to 1°22' below minimum. Cross shroud dimensions were above and below limits, and cross notch dimensions were mostly below minimum. During the hot section inspection (HSI) performed by AEI in March 2011, all 1st stage turbine blades had been replaced. AEI purchased 130 blades from Western Aero Services, which had obtained them from the Royal Australian Air Force. The blades were sent to EC Technologies (now MT Texas) for inspection and repair in January 2000, including shroud twist restoration, cross shroud and cross notch weld repair, and other work per the JT3D overhaul manual.