History of Flight
On January 17, 2010, at 1250 Pacific standard time, a Eurocopter AS350 B3, registration N904CF, landed hard on the helicopter pad at Renown Regional Medical Center, Reno, Nevada. The helicopter was operated by Air Methods, doing business as Care Flight, under Part 91 of the Federal Aviation Regulations as a positioning flight. The pilot and two medical crew members were not injured, and the helicopter sustained substantial damage. Visual meteorological conditions prevailed, and a company visual flight plan had been filed. The flight originated at the Renown Regional Medical Center.
The pilot reported that the purpose of the helicopter emergency medical service (HEMS) flight was to fly to Humboldt General Hospital, Winnemucca, Nevada, to pick up a patient for an inter-facility transfer to Renown Regional Medical Center. He lifted from the Renown helipad and positioned the helicopter into a high hover at 25 feet, per normal operating procedures. As the pilot began transitioning to forward flight, he heard a loud bang and the helicopter experienced a partial power loss. The pilot lowered the collective slightly and landed hard on the helipad.
Two surveillance video cameras from the medical center captured the accident sequence. The videos showed the helicopter on the pad with the nose pointed northwest; a windsock was limp at the time of rotor engagement. The helicopter lifted into approximately a 30-foot hover over the pad. About 10 seconds after lift-off, the helicopter's nose abruptly rotated right 90 degrees, then it descended vertically with the nose pitched down about 20 degrees. The forward portion of the skids and cockpit chin bubble impacted the ground. The helicopter rebounded and came to rest on collapsed landing skids, facing 180 degrees from the original takeoff orientation. The main rotor stopped turning after 1 minute 25 seconds, and the crew began to egress about 2 minutes after the accident.
Personnel Information
The pilot, age 61, held a commercial pilot certificate with rotorcraft-helicopter and instrument helicopter ratings, issued October 23, 2008. He held a first-class medical certificate with the limitation that he wear corrective lenses and possess glasses for near and intermediate vision, issued October 14, 2009. The pilot reported 14,300 total flight hours in helicopters, including 9,544 hours in the AS350. Over the previous 90 days he had logged 46 hours, with 11 hours in the last 30 days.
Aircraft Information
The AS350B3, serial number 3676, was manufactured in 2003. It is a main rotor-tail rotor configured single-engine helicopter. The interior was configured with the pilot seat on the right; the patient litter was on the left side, extending over the area where the copilot seat would traditionally be. Two crew seats were mounted along the rear cabin bulkhead. The helicopter was powered by a Turbomeca Arriel 2B1 engine rated at 848 shaft horsepower. The engine power output is directed through a reduction gearbox and a drive shaft connected to the main gearbox at the forward end and the tail rotor drive shaft at the aft end.
Review of maintenance records revealed the airframe had 3,939.5 hours at the time of the accident. The most recent inspections were the A21 daily airworthiness inspection, B61 20-hour engine inspection, and tail rotor AD 2001-26-55, performed on January 17, 2010. These were conducted according to the Air Methods Approved Airworthiness Inspection Program.
On August 3, 2009, at 3,880.4 total airframe hours, the engine had been removed and the airframe sent for paint finishing. On October 29, 2009, the engine was reinstalled at the Air Methods certified repair station in Englewood, Colorado. On November 2, 2009, a 100-hour inspection was performed, including the Eurocopter 'S' interval maintenance, which involved inspection of both flex couplings on the engine-to-main gearbox shaft. On November 11, 2009, an engine power assurance check was performed with a 'Checked Good' logbook entry. The helicopter was approved for return to service on December 1, 2009.
Wreckage and Impact Information
The helicopter was removed from the Renown helipad and transported to the Air Methods maintenance facility in Reno, Nevada, for examination. On January 25, 2010, technical representatives from American Eurocopter, Turbomeca USA, the Federal Aviation Administration (FAA), and Air Methods Corporation inspected the helicopter under the supervision of the National Transportation Safety Board (NTSB) investigator-in-charge.
The left skid was collapsed into the forward fuselage area. The fuselage-tailboom junction exhibited buckled skin. The main fuselage structural beams were deformed. Full control continuity was established for cyclic, collective, and anti-torque controls. External power was supplied to the cockpit avionics, and the Vehicle and Engine Multifunction Display (VEMD) energized. The VEMD data displayed the accident flight as flight number 7211, with total time of 2 minutes and 8 seconds. Three failures were recorded: test reference 69-OAT (left module), test reference 69-OAT (right module), test reference 124-amber gov OAT VEMD-OAT sensor FADEC (left module), and an NF over limit of 510 rpm. The outside air temperature (OAT) sensor, located under the helicopter's nose, was impact damaged.
No main rotor blade damage was observed. The main gearbox rotated freely in both directions with corresponding rotation of the main gearbox input flange, but without corresponding rotation of the engine and tail rotor. Hardware associated with the engine-to-main gearbox drive shaft flex coupling was identified on the engine deck. The engine-to-main gearbox shaft coupling tube was removed, revealing that the engine-side flex coupling was detached from the engine-to-main gearbox shaft. Metal shavings were identified inside the perimeter of the aft side of the coupling tube. The flex coupling remained attached to the engine output spline. The three bolts that connect the flex coupling to the transmission shaft appeared sheared, and the bolt fracture surfaces appeared smeared. The three nuts and two associated washers were loose and located clumped together just forward of the gimbal ring in the transmission input housing. No cotter pins were located on the bolts or in the main gearbox input housing. The three bolts attaching the flex coupling to the splined engine flange were present, with nuts and cotter pins in place.
The engine was attached to the airframe at both front and rear mounts with no apparent damage. The engine deck was clean, with no fluid leaks. Aircraft-to-engine interface connections were intact and secure. All engine pipes, hoses, and harnesses were intact and properly connected. The fuel shutoff lever in the cockpit was in the closed position. Oil was visible through the sight glass and appeared within limits.
The engine was removed to facilitate examination of the transmission shaft. The oil filter and fuel filter by-pass indicators were not popped. The electrical magnetic plug, oil filter, and fuel filter were clean. A small metallic sliver was found on the accessory gearbox (MO1) magnetic plug. The fuel pump and metering unit appeared undamaged. The freewheel shaft assembly rotated smoothly in both driving and freewheeling directions. Contact damage to the front face of the clutch housing was observed. The engine drive shaft splined nut and lock exhibited rotational scarring. Removal of the engine front support revealed damage to the inside surface. The nose cone and axial compressor blades were unremarkable. The bleed valve was in the open position. The containment ring was unremarkable. The power turbine wheel assembly rotated easily by hand. The free turbine blades were unremarkable. The reduction gearbox casing (MO5) was unremarkable. The reduction gearbox (MO5) magnetic plug was clean. The flexible coupling to the tail rotor drive shaft was intact.
Tests and Research
The engine-to-main gearbox drive shaft and flex coupling assembly were sent to the NTSB Materials Laboratory and examined from April 14-16, 2010.
When assembled, three bolts attach the flex coupling assembly to the main gearbox drive shaft. Castellated nuts are to be attached to the threaded end of the bolts, with cotter pins as an additional safety feature. Bolts were found inserted in the through holes of the flex coupling. One bolt was intact. The other two bolts contained a fracture that intersected the cotter pin holes, and a thread fragment separated from these two bolts. The cotter pin holes for the three bolts did not contain a cotter pin or fragment. Scanning electron microscope examination of the fracture faces from the bolt fragments revealed micro-void coalescence features typical of overstress separation with no evidence of fatigue cracking. The bolts, washers, and nuts specified for the flex coupling-to-main gearbox drive shaft connection were accounted for and recovered.
The flange portion of the main gearbox drive shaft contained three arms, each with an impact mark at the same respective position. The impact marks are consistent with the threaded portions of the bolts from the flex coupling assembly and nuts impacting the flange portion while the flex coupling assembly was rotating clockwise looking forward with respect to the main gearbox drive shaft.
Bolts were manually removed from their respective holes with ease. The bolts and recovered nuts showed no evidence of a crack. Examination of the aft face of the flex coupling assembly revealed the exposed bolt holes contained deformation on one side at the same respective position for each hole. The main gearbox drive shaft on the aft face of the flange portion also contained deformation marks on one side of the bolt holes at the same respective position, corresponding to the marks on the aft face of the flex coupling. The forward and aft faces of each washer, the corresponding areas on the coupler assembly, and the underside portion of each nut contained no evidence of fretting damage. The first thread adjacent to the underside portion of each nut had partially separated and fractured. The inner portion of the recovered nuts contained an elastomer in the bore portion at the castellated end, indicating the nuts were self-locking. The forward face of the flex coupling contained cracks that intersected each bolt hole.
Additional Information
An FAA inspector interviewed the mechanics who removed and reinstalled the engine. The inspector reported that the engine-to-main gearbox drive shaft had been disconnected from the splined engine flange by the mechanic who removed the engine on August 3, 2009. This step was not per the AS350B3 Maintenance Manual. The removing mechanic did not tag the disconnected flex coupling and partially reinstalled it by finger-tightening the nuts onto the bolts. The removing mechanic made the following discrepancy logbook entry: "Removed engine for inspection," and the following continuity logbook entry for step #9: "Removed fwd shaft from eng side." (Note: The discrepancy logbook records a particular maintenance action; the continuity logbook records each step used to complete that action.) The engine was reinstalled by a different mechanic on October 29, 2009. The installing mechanic entered the corrective action for step 9 in the continuity log as "Installed fwd end short shaft per AS350MM ch 65." The 'short shaft' is common terminology for the first length of the tail rotor drive shaft that connects to the engine and is located in the aft area of the engine. The installing mechanic did not realize the engine-to-main gearbox drive shaft needed to be reconnected because there is no step in the AS350 B3 maintenance manual to disconnect it to remove or install the engine. The engine reinstallation write-up was signed off by the Quality Assurance inspector without a visual inspection of the engine-to-main gearbox flex coupling.
The engine-to-main gearbox drive shaft flex coupling is covered by the flared coupling housing and coupling tube. Inspection of the flex coupling involves using an inspection mirror positioned through an inspection hole in the flared coupling housing to visually observe that the nuts are on the bolts and cotter-pinned.
On the Arriel 2B1 engine, the drive shaft can be installed on the engine into the splined engine flange without removing the flex coupling cotter pins and nuts. This is the method described by the Eurocopter maintenance manual.