N49660Bell 206B · 1975

On September 24, 2015, about 1445 central daylight time, a Bell 206B helicopter, N49660, landed hard during a hovering autorotation in a field, about 3 miles to the east of Blackwell Field (71J) in Ozark, Alabama. The commercial pilot and four passengers were not injured, and the helicopter sustained substantial damage. The helicopter was registered to SAF Aviation Services and Leasing LLC, Midland City, Alabama and operated by Eagle Aviation Academy LLC, Ozark, under the provisions of 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 (71J), Ozark.

According to the pilot, he was hovering in a field to position the helicopter for departure. The helicopter "moved out of ground effect" and a "loss of lift" occurred allowing the helicopter to settle. The left skid contacted terrain and the helicopter began to roll to the left. The pilot arrested the rolling motion with the flight control inputs and the helicopter climbed 10 to 15 feet above the ground. Flight control inputs were made to correct for the unusual attitude and a "rocking motion" that was induced as the helicopter departed the ground. The pilot reported that the unusual attitude resulted in the main rotor system contacting portions of the upper cowling, and the transmission and the transmission driveshaft contacting its mounting system, which induced abnormal vibrations throughout the airframe.

The pilot reported that due to the abnormal vibrations and the unknown damage at the time, he initiated a hovering autorotation. The helicopter landed hard resulting in substantial damage to the tail boom. A postflight inspection also revealed substantial damage to the transmission drive system.

The pilot reported there were no pre-impact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.

The damage photos supplied by the operator displayed buckling on the right side and a perforation on the top of the tail boom in the sheet metal skin aft of where the tail boom attaches to the fuselage. Additionally, the photos displayed damage sustained to the transmission stop mount, consistent with impact from the transmission drag pin assembly. The photos also displayed damage to the transmission isolation mount, consistent with impact from the transmission driveshaft coupling.

Pylon Whirl Condition

During the course of the accident investigation, it was identified that the Bell 206 series flight manuals were deficient with the identification and reporting of the condition known as "pylon whirl." The manufacturer has agreed to make the Bell 206 community aware of this condition through either an operations safety notice and/or a change to the flight manuals.

The operator's manual for the U.S. Army OH-58A/C (a single engine, observation type military helicopter produced by Bell, similar to the Bell 206 series), Technical Manual 55-1520-228-10, discusses the pylon whirl condition. This manual states in part;

"Pylon whirl is a condition which occurs after blade flapping and mast bumping. The resultant motion of the pylon is elliptical, and spike knock is apt to occur. If the frequency of motion coincides with a particular natural frequency of the helicopter, and the amplitude and direction of the force is large enough, damaging vibrations can occur in the aft section tailboom of the helicopter. Motion of this type could occur during touchdown autorotations, if operational limits are exceeded."

Spike Knock Condition

During the course of the accident investigation, it was identified that the Bell 206 series flight manuals were deficient with the identification and reporting of the condition known as "spike knock." The manufacturer has agreed to make the Bell 206 community aware of this condition through either an operations safety notice and/or a change to the flight manuals.

The operator's manual for the U.S. Army OH-58A/C (a single engine, observation type military helicopter produced by Bell, similar to the Bell 206 series), Technical Manual 55-1520-228-10, discusses the spike knock condition. This manual states in part;

"Spike knock occurs when the round pin in the drag-pin fitting contacts the side of the square hole of the pylon stop, which is mounted to the roof. It creates a loud noise and will occur during a rocking of the pylon. The following factors can cause a spike knock: low rotor revolutions per minute (RPM), extreme asymmetric loading, poor execution of an autorotational landing and low G maneuvers below +0.5 Gs."

"Spike knock will be more prevalent during zero ground run autorotational landings than for sliding autorotational landings and running landings."

"Spike knock in itself is not hazardous but is an indicator of a condition that could be hazardous. If spike knock is encountered, an entry must be made on Department of Army Form 2408-13-1, to include the flight conditions under which the spike knock has occurred. An inspection will be performed by maintenance personnel before continuing."

"During landing, starting, and rotor coastdown, spike knock could also occur, especially if there are high winds and/or the elastomeric damper is deteriorated. This type of spike knock is not considered damaging to the aircraft."

Bell 206A/B Series Maintenance Manual

The Bell 206A/B series maintenance manual does briefly discuss the pylon whirl condition and the spike knock condition in a pylon whirl inspection flowchart in Chapter 63 – Main Rotor Drive System. This manual states in part;

"Perform pylon whirl inspection following pilot report or evidence of abnormal landing, excessive slope landing, operation in severe turbulence, low rotor RPM during flight (power on or off), rapid and extreme cyclic input, excessive spike knock, or main driveshaft contact with isolation mount."

Abnormal Vibrations

The Federal Aviation Administration's Helicopter Flying Handbook (FAA-H-8083-21A) discusses abnormal vibrations associated with helicopters. This handbook states in part;

"With the many rotating parts found in helicopters, some vibration is inherent. A pilot needs to understand the cause and effect of helicopter vibrations because abnormal vibrations cause premature component wear and may even result in structural failure. With experience, a pilot learns what vibrations are normal and those that are abnormal, and can then decide whether continued flight is safe or not. Helicopter vibrations are categorized into low, medium, or high frequency."

"Low-frequency vibrations (100–500 cycles per minute) usually originate from the main rotor system. The main rotor operational range, depending on the helicopter, is usually between 320 and 500 revolutions per minute."

"Medium-frequency vibrations (1000–2000 cycles per minute) range between the low frequencies of the main rotor (100–500 cycles per minute) and the high frequencies (2100 cycles per minute or higher) of the engine and tail rotor."