Accident Overview
On September 24, 2015, at about 1445 central daylight time, a Bell 206B helicopter, registration N49660, performed a hard landing during a hovering autorotation in a field approximately 3 miles east of Blackwell Field (71J) in Ozark, Alabama. The helicopter was operating under 14 Code of Federal Regulations Part 91 as a day visual flight rules personal flight. Visual meteorological conditions prevailed, and no flight plan was filed. The flight originated from Blackwell Field.
The helicopter was registered to SAF Aviation Services and Leasing LLC, Midland City, Alabama, and operated by Eagle Aviation Academy LLC, Ozark. The commercial pilot and four passengers were not injured. The helicopter sustained substantial damage.
Pilot Account
According to the pilot, while hovering in a field to position for departure, the helicopter moved out of ground effect, resulting in a loss of lift and causing the helicopter to settle. The left skid made contact with terrain, and the helicopter began to roll left. The pilot arrested the rolling motion with flight controls, and the helicopter climbed 10 to 15 feet above ground. Control inputs were made to correct an unusual attitude and a rocking motion induced as the helicopter departed the ground. The pilot reported that the unusual attitude caused the main rotor system to contact portions of the upper cowling, and the transmission and transmission driveshaft contacted its mounting system, inducing abnormal vibrations throughout the airframe.
Due to the abnormal vibrations and unknown damage, the pilot initiated a hovering autorotation. The helicopter landed hard, resulting in substantial damage to the tail boom. A postflight inspection revealed substantial damage to the transmission drive system. The pilot reported no pre-impact mechanical failures or malfunctions with the airframe or engine.
Wreckage and Impact
Damage photographs provided by the operator showed buckling on the right side and a perforation on the top of the tail boom sheet metal skin aft of the tail boom attachment to the fuselage. Additional damage was observed on the transmission stop mount, consistent with contact from the transmission drag pin assembly, and on the transmission isolation mount, consistent with contact from the transmission driveshaft coupling.
Additional Information
Pylon Whirl Condition
The investigation identified that Bell 206 series flight manuals were deficient in identifying and reporting the condition known as "pylon whirl." The manufacturer agreed to inform the Bell 206 community through an operations safety notice or flight manual change. The U.S. Army OH-58A/C operator's manual (similar to the Bell 206 series) describes pylon whirl as a condition occurring after blade flapping and mast bumping, resulting in elliptical pylon motion. It states that if the motion frequency coincides with a natural frequency of the helicopter, damaging vibrations can occur in the aft tailboom, potentially during touchdown autorotations if operational limits are exceeded.
Spike Knock Condition
The investigation also identified deficiencies in Bell 206 series flight manuals regarding the "spike knock" condition. The manufacturer similarly agreed to issue a safety notice or manual change. The OH-58A/C manual defines spike knock as occurring when the round pin in the drag-pin fitting contacts the side of the square hole of the pylon stop, creating a loud noise. Factors include low rotor RPM, extreme asymmetric loading, poor autorotational landing execution, and low G maneuvers below +0.5 Gs. The manual notes that spike knock itself is not hazardous but is an indicator of a potentially hazardous condition, requiring inspection by maintenance personnel.
Bell 206A/B Series Maintenance Manual
The Bell 206A/B maintenance manual briefly discusses pylon whirl and spike knock in a pylon whirl inspection flowchart. It states that a pylon whirl inspection should follow pilot reports or evidence of abnormal landing, excessive slope landing, severe turbulence, low rotor RPM, rapid cyclic inputs, excessive spike knock, or main driveshaft contact with the isolation mount.
Abnormal Vibrations
The FAA Helicopter Flying Handbook explains that vibrations are inherent in helicopters due to rotating parts. Abnormal vibrations can cause premature wear or structural failure. Low-frequency vibrations (100–500 cpm) typically originate from the main rotor, medium-frequency (1000–2000 cpm) range between main rotor and engine/tail rotor frequencies, and high-frequency (2100 cpm or higher) from the engine and tail rotor.