CHI05FA192

The loose tail rotor drive shaft coupling due to its improper installation by the operator's maintenance personnel, which resulted in the failure of the tail rotor drive shaft. An additional cause was the inability of the pilot to maintain control of the helicopter in the hover following the drive shaft failure.

The helicopter was substantially damaged when it struck the helipad during an uncommanded yaw encountered during the initial hover after liftoff from a roof-top hospital heliport. The pilot reported that he picked up into a 4 to 6-foot hover and initiated a right pedal turn. He stated that as the helicopter reached a west heading "the aircraft would not turn any more" despite his continued application of right pedal. He stated: "As I continued to apply right pedal the aircraft then went into [a] sudden and uncommanded yaw to the left. I was unable to stop the yaw." The helicopter subsequently impacted the helipad and roof structure. It came to rest at the east edge of the helipad oriented on a southeast heading. A post accident inspection revealed that the Fenestron (tail rotor) drive shaft had failed approximately 6 inches aft of the main gearbox. The failure occurred at the point where the drive shaft entered a tunnel formed by the left and right engine firewalls. The firewalls and drive shaft segments in the vicinity of the point of failure exhibited scrape marks. Examination of the forward section of the drive shaft revealed features characteristic of an overload failure. The main gearbox output shaft assembly and rear transmission coupling connected the tail rotor drive shaft to the gearbox. Further examination revealed that the coupling flange could be moved laterally relative to the pinion approximately 3/32 (0.094) inch. Allowable lateral play in the drive flange was 1 millimeter (0.039 inch). Disassembly of the transmission coupling determined that the nut which secured the drive flange to the output assembly pinion gear was improperly installed. Wear patterns indicated that the locking tangs on the cup washer did not engage the corresponding slots on the shaft allowing the nut to loosen over time. In addition, the condition of the locking tangs indicated that they were folded over during installation causing them to separate from the cup. The resulting wear had removed material to such an extent that the contact face was no longer perpendicular to the longitudinal axis of the shaft. This allowed excessive radial play in the transmission coupling, which permitted contact between the tail rotor drive shaft and the firewalls. The FAA Rotorcraft Flying Handbook, FAA-H-8083-21, provided information related to failure of the anti-torque system on a helicopter. The handbook stated: "The loss of antitorque normally results in an immediate yawing of the helicopter's nose. The helicopter yaws to the right in a counter-clockwise rotor system and to the left in a clockwise system. . . . The severity of the yaw is proportionate to the amount of power being used and the airspeed. An antitorque failure with a high power setting at a low airspeed results in a severe yawing." The main rotor system of the accident helicopter rotated clockwise as viewed from above.

HISTORY OF FLIGHT On July 14, 2005, at 1604 central daylight time, an Aerospatiale (Eurocopter) AS365N helicopter, N365S, piloted by an airline transport pilot, was substantially damaged when it impacted the heliport after takeoff following an in-flight loss of control at the Porter Memorial Hospital Heliport (46II), Valparaiso, Indiana. The medical transportation flight was being conducted under 14 CFR Part 135. Visual meteorological conditions prevailed. The pilot, physician, nurse and patient were not injured in the accident. The intended destination was the University of Chicago Hospitals Heliport (4IS3), Chicago, Illinois. The pilot stated that the helicopter was oriented toward the southwest as it sat on the helipad prior to liftoff. He noted that "all systems were normal" after start up. He reported that winds were from the north, about 15 knots and gusting to 22 knots. The pilot reported that he picked up into a 4 to 6-foot hover and initiated a right pedal turn to a west heading. He stated that as the helicopter reached a west heading "the aircraft would not turn any more" despite his continued application of right pedal. He stated: "As I continued to apply right pedal the aircraft then went into [a] sudden and uncommanded yaw to the left. I was unable to stop the yaw and by the time I was heading 090 [degrees,] the tail of the aircraft hit a roof structure just west of the pad. The aircraft then rolled on [its] right side." The physician on-board reported that he "felt the helicopter spin quickly not long after takeoff." He stated that he felt the helicopter descend and impact the helipad. The helicopter subsequently came to a rest on its right side. The flight nurse stated: "We were lifting off the helipad when I felt a sudden twisting/turning of the aircraft [and] we came to rest on the starboard side of the aircraft." The helicopter came to rest at the east edge of the helipad oriented on a southeast heading. PERSONNEL INFORMATION The pilot held an airline transport pilot certificate with single and multi-engine land airplane, glider and helicopter ratings. The multi-engine land airplane and glider ratings were limited to commercial pilot privileges. He held a second-class airman medical certificate issued on December 27, 2004. The medical certificate required the use of corrective lenses. The pilot's total flight time was reported as 9,777 hours, with 667 hours in the same make and model as the accident helicopter. Reported flight time within the previous 90 days was 45 hours, all of which were in an AS365N helicopter. AIRCRAFT INFORMATION The accident helicopter was an Aerospatiale (Eurocopter) AS365N helicopter, S/N 6036. Two Turbomeca Arriel 1C turbo-shaft engines powered the helicopter. Each engine was rated at 700 horsepower. The helicopter was owned by Chase Equipment Leasing Inc., Columbus, Ohio, and operated by CJ Systems Aviation Group, West Mifflin, Pennsylvania, in a medical transport role. The airframe had accumulated 6,267 hours total time. The helicopter was maintained under a Federal Aviation Administration (FAA) Approved Aircraft Inspection Program. The most recent inspection was completed on February 2, 2005, at an airframe total time of 6,112 hours. The left (#1) engine, S/N 2060, had accumulated 7,620 hours at the time of the most recent inspection on February 2, 2005. The engine was installed on the accident helicopter on April 25, 2001, at 6,372 hours. The right (#2) engine, S/N 2268, was installed on the accident helicopter at the time of the previous inspection on February 2, 2005. The engine had accumulated 6,418 hours at that time. An overhauled main rotor gearbox was installed on the accident helicopter on January 31, 1999, at 4,012.7 hours. The gearbox was overhauled by the helicopter manufacturer and installed by the aircraft operator. The accident helicopter was equipped with a mechanical air conditioning system. This system was powered by a drive pulley, which was secured to the main gear box rear drive flange. Installation of the overhauled main rotor gearbox required removal and reinstallation of the drive flange to allow for installation of the air conditioning pulley. METEOROLOGICAL INFORMATION Porter County Municipal Airport (VPZ) was located 2 nm west of the accident site. At 1553, the VPZ Automated Surface Observing System (ASOS) recorded: Wind from 360 degrees at 11 knots; visibility of 7 statute miles; few clouds at 900 feet above ground level (agl) with thunderstorms in the vicinity; temperature 28 degrees Celsius, dew point 23 degrees Celsius; and the altimeter setting was 29.91 inches of mercury. Remarks attached to the 1553 weather record noted that distant lightning had been detected in all quadrants. The thunderstorms began at 31 minutes past the hour. Rain began at 31 minutes past the hour and ended at 47 minutes past the hour. According to the record, only a trace of precipitation had been detected since the previous hourly record. AIRPORT INFORMATION The helipad was located on the roof of the main building at the Porter Memorial Hospital. The five-story main building placed the helipad about six stories above street level. The helipad was located at 41 degrees 28.0 minutes north latitude, 087 degrees 03.0 minutes west longitude. The pad elevation was 864 feet. The helipad deck comprised the Touchdown-Lift Off (TLOF) area of the heliport. The aluminum deck was approximately 40 feet square and was positioned 4 feet 10-1/2 inches above the roof level. A 5-foot wide safety net was installed around the perimeter of the deck. The hospital building structure was located 41 feet east of the helipad. This portion of the building rose to a height of 6 feet 10 inches above the pad. The building ultimately rose to a height of 17 feet 3-1/2 inches above the level of the pad. A building stairway enclosure was located west of the pad. The roof of the stairway enclosure rose to a maximum height of 4 feet - 10-1/2 inches above the pad and was located within the helipad safety area. Access to the heliport was provided via a ramp to the east side of the pad. The ramp extended east from the pad to the sixth-floor hospital building structure, with elevator access to the emergency department on the first floor. Additional access was provided by a stairway located near the northeast corner of the pad. This stairway did not have handrails and did not protrude above the level of the helipad. WRECKAGE AND IMPACT INFORMATION The helicopter came to rest on the west edge of the hospital heliport, lying on its right side. The fuselage was supported by the pad itself. The tail boom extended off the pad and was supported by a brick wall adjacent to the helipad. The brick wall was part of the stairway enclosure west of the pad. The portion of the enclosure struck by the helicopter was approximately level with the helipad. The main section of the fuselage appeared intact. The left side cabin doors were separate from helicopter at the time of the on-scene examination. Both were located on the safety net surrounding the helipad deck. The landing gear was extended and appeared intact. The main rotor blades had separated from the helicopter at the respective blade sleeves. The blades were fragmented. Cyclic and collective control linkage segments were damaged, however, continuity was confirmed from the cockpit controls to the main rotor swash plate. The tail boom was buckled forward of the horizontal stabilizers at the same location along the tail boom as where it came to rest on the brick wall. The Fenestron (tail rotor) drive shaft, which ran along the top of the tail boom, was fractured at this location. The Fenestron control tube was bent but otherwise intact. Fenestron control continuity was confirmed. The Fenestron blades were intact. The Fenestron duct exhibited scrape marks adjacent to