DeLand, FL, USA
N801DS
BELL 407
The helicopter was cruising at 120 knots and 700 feet above the ground. About 2 city blocks from the intended destination, the pilot heard a loud "bang," and the helicopter then yawed left about 30 degrees. The pilot initiated an autorotation to a nearby road that was under construction. While attempting to land, the pilot maneuvered the helicopter to avoid construction equipment, and during the landing the main rotor blades contacted the vertical stabilizer. After the helicopter came to rest, the pilot observed a fire in the engine compartment, which he extinguished using the on-board fire extinguisher. Post-accident examination of the engine revealed that one of the blades on the third stage turbine wheel had failed in fatigue, resulting in an over-speed and subsequent burst of the first stage turbine wheel. The root cause of the blade failure could not be determined.
On January 7, 2008, about 1845 eastern standard time, a Bell 407, N801DS, was substantially damaged during a forced landing near DeLand, Florida. The certificated commercial pilot and the paramedic were not injured. Visual meteorological conditions prevailed, and company flight plan was filed for the positioning flight, which departed DeLand Municipal Airport (DED) at 1831, destined for an off-airport site in DeLand Florida. The flight was conducted under 14 Code of Federal Regulations (CFR) Part 91. During a telephone interview, the pilot stated that while en route to pickup an injured pedestrian for a 14 CFR Part 135 emergency medical service flight, the helicopter was cruising at 120 knots and 700 feet above the ground. About 2 city blocks from the intended destination, the pilot heard a loud "bang," and the helicopter then yawed left about 30 degrees. He also heard "squealing or scratching" metallic sounds, the sound of the Full Authority Digital Engine Control (FADEC) warning horn, and observed 5 warning lights illuminate. About 2 seconds after the initial "bang," the low rotor rpm warning horn sounded, so the pilot initiated an autorotation, and maintained the airspeed at 60 knots. The helicopter continued to descend, and the pilot identified a roadway that was under construction for a forced landing. As the helicopter touched town, the pilot maneuvered left to avoid construction equipment, and "flared a little too high." The helicopter landed firmly, but not "hard," in a level attitude and slid along the ground. While sliding, the tail of the helicopter raised up, so the pilot applied aft cyclic. He believed that the main rotor blade may have contacted the vertical stabilizer during this maneuver. After the helicopter came to rest, the pilot and the paramedic exited the helicopter, observed a fire in the engine compartment, and attempted to extinguish it with the on-board fire extinguisher. The pilot did not recall having smelled smoke during the flight. A witness was sitting in a parking lot near where the incident occurred. The witness stated that he looked up and saw a "big orange glow [and] then sparks" emanating from the right side of the helicopter, before it began to descend toward the ground. Post-accident examination of the helicopter by Federal Aviation Administration (FAA) inspectors revealed damage to the vertical stabilizer consistent with main rotor blade contact. The helicopter’s engine was removed and shipped to the manufacturer for examination under the supervision of an FAA inspector. The manufacturer subsequently produced a report that detailed their observations. According to the report, the compressor section of the engine, including the inlet and impeller leading edge surfaces, were free from damage and unremarkable. Both the N1 and N2 gear trains turned freely and smoothly when rotated by the spur adapter gearshaft and pinion gear, respectively. Continuity of the N2 was confirmed from the pinion gear to the output shaft. Circumferential scoring was noted on the aft face of the pinion gear, consistent with contact between the pinion gear and the power turbine outer shaft forward face during operation. The N2 speed pick up displayed raised metal consistent with contact between the pickup an the number 5 bearing castellated nut. The number 5 bearing oil nozzle was scored consistent with contact between the nozzle and the forward section of the power turbine rotor. Components from the turbine section of the engine were forwarded to the engine manufacturer's Failure Analysis Laboratory for metallurgical examination. This detailed inspection found that a single third state turbine wheel blade had cracked, exhibiting signatures consistent with fatigue, from the trailing edge area toward the leading edge for about 0.5 inches, prior to finally separating in overload. Damage to the first stage turbine wheel was consistent with an over-speed burst, as a result of the fracture that had occurred through the turbine to compressor coupling. The microstructure, hardness, and chemistry of the first and third stage turbine wheels met engineering drawing requirements. Examination of maintenance data recorded by the engine's FADEC revealed that at an undetermined time, the power turbine speed reached 104.56 percent. Additionally, the power turbine had been operated above 102 percent for a cumulative 138.43 seconds. Finally, the engine torque had previously reached 108.4 percent, and was above the torque limit for 0.1 seconds. It was not possible to determine the engine total time at each of the recorded exceedances. According to the FAA airman database, the pilot held a commercial pilot certificate with a rating for rotorcraft-helicopter, and a flight instructor certificate with a rating for rotorcraft-helicopter. The pilot’s most recent FAA first-class medical certificate was issued on February 27, 2007. On that date the pilot reported 3,255 total hours of flight experience. The weather conditions reported at Daytona Beach International Airport (DAB), located about 17 nautical miles northeast of the accident site, at 1853, included winds from 070 degrees at 4 knots, few clouds at 2,000 feet, broken clouds at 7,000 feet, and 10 statute miles visibility.
A total loss of engine power due to the fatigue failure of a third stage turbine wheel blade.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
