ORLANDO, FL, USA
N500RT
HUGHES 369HS
DURING CRUISE FLIGHT, THE ENGINE EXPERIENCED A PARTIAL LOSS OF POWER. DURING THE DESCENT FOR A PRECUATIONARY LANDING, THE ENGINE EXPEREINCED A TOTAL LOSS OF POWER. THE PILOT INITIATED AN EXAGGERATED DECELERATION MANEUVER DUE TO UNSUITABLE TERRAIN ENCOUNTERED DURING THE AUTOROTATIVE FORCED LANDING. EXAMINATION OF THE ENGINE REVEALED THAT THE NO. 2 BEARING FAILED DUE TO UNDETERMINED REASONS ALLOWING THE COMPRESSOR IMPELLER TO CONTACT THE COMPRESSOR SHROUD. THE FAILED BEARING MATERIAL WAS FOUND ON BOTH MAGNETIC PLUGS LOCATED IN THE GEARBOX. THERE WAS NO EVIDENCE OF OIL STARVATION TO THE BEARING AND IT WAS NOTED THAT THE BEARING WAS MANUFACTURED FROM CORRECT MATERIAL.
On June 15, 1993, about 1350 eastern daylight time, a Hughes 369HS, N500RT, registered to Heli Tech, Inc., leased to and operated by Magic Helicopters, Inc., was substantially damaged during a forced landing near Orlando, Florida, while on a sightseeing flight conducted under 14 CFR Part 91. Visual meteorological conditions prevailed at the time and no flight plan was filed for the local flight. The commercial pilot and one passenger sustained minor injuries. One passenger was not injured. The flight originated about 3 minutes earlier. The pilot stated that during cruise flight about 800 feet above ground level at 110 knots indicated airspeed, the engine chip detection light illuminated. Shortly thereafter, the exhaust gas temperature increased substantially. He immediately lowered collective and initiated a descent for a precautionary landing. While descending through 200 feet, the engine experienced a total loss of power. He immediately executed the procedures for an autorotative landing and when the flight was just above ground level, the pilot observed unsuitable terrain. The pilot initiated an exaggerated deceleration maneuver during which, the tail rotor blades contacted the ground. The helicopter then touched down on the skids and the main rotor blades flexed down and severed the tail boom. The engine was removed from the helicopter and sent to the manufacturer for further examination. Visual examination of the external portion of the engine revealed that the compressor shroud had several holes. The engine was disassembled which revealed that the compressor impeller and compressor shroud were damaged. Additionally, the No. 2 bearing had failed. According to the engine manufacturer, the failure of the No.2 bearing allowed the compressor to move forward and contact the compressor shroud. Two oil samples which were returned with the engine were tested and determined to be MIL-L-23699 which according to the engine manufacturer is acceptable for use in this engine. The oil delivery tube which supplies oil to the No. 2 bearing was removed, examined and flow tested which revealed no failure or malfunction. The No. 2 bearing was removed from the engine for further examination. Additionally, the two magnetic plugs located in the engine gearbox were removed and found to contain material. Analysis of the material revealed chemical elements associated with the components of the failed No. 2 bearing. According to the engine manufacturer, the No. 2 bearing is comprised of a 2-piece inner ring, a separator, 13 balls, and an outer ring. Visual examination of the bearing revealed that the separator was cracked in several places from the forward side of the separator aft toward the ball pockets. One of the cracks extended from the forward side of the separator to the aft side. All 13 balls were accounted for but were irregularly shaped. The forward side of the separator was heat distressed and the two piece inner ring was fused. The outer and inner rings ball contact surfaces were found to be worn and material transfer was present. The outer ring was cut and the separator was removed for metallurgical examination. Metallurgical examination of the separator revealed a fatigue crack near 1 of the 13 ball pockets. Heat damage to the separator prevented Brinell testing. Semi-Quantitative X-Ray Dispersive Analysis (XEDA) of the separator confirmed the type material per engineering drawing requirements. There was no evidence of subsurface spalling of the outer or inner rings or the balls. The metallurgical report states that the severe damage sustained by the bearing prevented an accurate determination of the failure mode. The metallurgical report was reviewed by the NTSB materials laboratory which concurred that the damage to the bearing precluded an accurate determination of the cause of the bearing failure. Review of the engine logbook revealed that a new No. 2 bearing (serial number not indicated), lock key, slinger, and nut were installed in the compressor assembly on January 6, 1973. There were no further entries regarding replacement of the No. 2 bearing. At the time of the accident the bearing had accumulated about 7,643 hours since installation. The engine was removed for inspection in November 1991, and was installed in this helicopter on June 12, 1993. Review of the rotorcraft log revealed that a 100-hour inspection was performed on June 12, 1993. At that time the engine oil was drained and replaced. At the time of the accident the helicopter had accumulated 3.3 hours since the 100-hour inspection. According to the engine manufacturer, the serial number of the failed bearing was installed in an engine that was sold to the military in 1977. Review of the engine records revealed that a new No. 2 bearing was installed in the compressor assembly on January 6, 1973, but the entry did not specify the serial number of the bearing. There were no other entries indicating that the No. 2 bearing was replaced. Additionally, a representative of the engine manufacturer stated that analysis of five failed No.2 bearings which were returned revealed that two failed due to spalling. One of the five failed due to spalling fatigue and the remaining two failed due to oil starvation. There were no records available from the maintenance facility that replaced the No. 2 bearing on January 6, 1973. The retained engine assembly was released to Mr. Tyler Dedman of Sample International, Inc., on May 5, 1994.
UNSUITABLE TERRAIN ENCOUNTERED DURING THE AUTOROTATIVE FORCED LANDING. CONTRIBUTING TO THE ACCIDENT WAS TOTAL LOSS OF ENGINE POWER DUE TO SEIZURE OF THE COMPRESSOR IMPELLER FOLLOWING FAILURE OF THE NO.2 BEARING FOR UNDETERMINED REASONS.
Source: NTSB Aviation Accident Database
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