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N6121S accident description

New Jersey map... New Jersey list
Crash location 40.969167°N, 75.002500°W
Nearest city Blairstown, NJ
40.966764°N, 74.982951°W
1.0 miles away
Tail number N6121S
Accident date 31 Jul 2001
Aircraft type Air & Space 18A
Additional details: None

NTSB Factual Report

On July 31, 2001, at 1146 eastern daylight time, an Air & Space America 18A gyroplane, N6121S, was substantially damaged while on the ground at Blairstown Airport, Blairstown, New Jersey. The certificated airline transport pilot and the passenger received minor injuries. No flight plan had been filed for the local flight. Visual meteorological condition prevailed for the personal flight conducted under 14 CFR Part 91.

In the NTSB form 6120.1/2, the pilot stated:

"Preflighted aircraft for an hour, taxi to end of runway for runup (rotors too long for taxiway)...engaged rotor system fully, advanced throttle, rotor went into ground resonance and I cut throttle."

According to an inspector from the Federal Aviation Administration (FAA), who interviewed the pilot:

"...[The pilot] completed the run-up (mags and prop checks included) without the main rotors turning. After the checks...[the pilot] moved the aircraft to the approach end of runway 7 to engage the rotors. She indicated that she engaged the rotors (fully), and had 120 rpm (on the rotor) / 1,000 rpm on the engine. As she advanced the throttle (towards full), the aircraft started to vibrate, going into ground resonance. As this started to happen...[the pilot] indicated that she de-pitched the rotor system and closed the throttle. This was confirmed by having no broken teeth on the shafts from the transmission / engine. The main tires 70 psi, the nose wheel had 35 psi, and the struts were 4 fingers exposed for the pre-flight."

According to a ground witness:

"The gyrocopter appeared to do a run-up at the west end of the runway. As the main rotor came up to speed, heavy vibration seemed to occur. The aircraft oscillated up and down, breaking off the right landing gear, rolling the aircraft first to the right, and when the rotor hit [the ground, the gyrocopter rolled] to the left."

A witness in the traffic pattern reported that he saw the gyroplane enter runway 7 and the pilot transmitted on UNICOM that the she going to do a run-up. As the rotor came up to speed, with no forward travel, oscillations began and that after the third or fourth oscillation the right wheel hit the ground and broke off. He further stated that the gyroplane rolled over and came apart.

The FAA inspector further reported that the gyroplane came to rest on its left side. The cockpit windows were shattered. The main rotor head separated from the main rotor mast. The tail boom and vertical stabilizers were bent, and the landing gear separated from the fuselage. The upper and lower droop stops had evidence of multiple impacts. The friction fit sleeve fitting located below the swash plate, which was used to change the pitch on the main rotor blades, was separated, with the bolts still in place. Examination of the bolts revealed no evidence of recent wear on the threads. The hydraulic dampers on the three main rotor blades were not recovered.

According to the maintenance manual:

"The Air & Space 18A Gyroplane is a two-place craft...The seating arrangement is tandem; the landing gear is tricycle type with oleo shock struts and a fully castering nose wheel. Lift is provided by an auto-rotating rotor.

Power is provided by a 180 horsepower Lycoming O-360 engine driving a pusher propeller. A clutch and transmission allow pre-rotation of the rotor in flat pitch to normal flight RPM for a jump take-off. A hydraulic system is used to hold the clutch engaged and the rotor in flat pitch. At the start of the take-off maneuver, the hydraulic pressure is released, disengaging the clutch and immediately allowing the blade pitch to increase to its flight position.

The rotor is of conventional three-hinge articulated design. Cyclic pitch control for pitch and roll is provided by a swashplate, and a two-position collective pitch control is provided to allow pre-rotation as described below. A...[delta] 3 angle incorporated in the flapping hinge-pitch control geometry provides the pitch change required from jump take-off to normal flight.

Yaw control is obtained from an all-moving rudder, centrally located in the triple tail unit."

"...The ground sensing device is a system for disconnecting the electrical source required to operate the hydraulic system and thereby making the hydraulic system in-operative in flight. Conversely, this same mechanism closes the circuit when the aircraft is on the ground."

According to the FAA inspector, the last annual inspection had been performed by the pilot, who also held a mechanics certificate with airframe and powerplant ratings and inspection authorization. Since the inspection, the gyroplane had accumulated 0.7 hours.

The pilot reported her total flight experience as 18,000 hours, including 6,000 hours in rotorcraft. She did not list any flight experience in make and model. The pilot declined to allow the FAA to examine her pilot logbook, and her flight experience was not verified. In addition, the pilot declined to be interviewed by the Safety Board about the accident, and it took multiple requests for her to complete the required NTSB form 6120.1/2. On her last airmen medical application dated June 14, 2001, she listed her total flight experience as 16,000 hours with 500 hours in the preceding 6 months. On her two preceding airmen medical applications dated June 26, 2000, and June 18, 1999, she listed her total flight experience as 15,000 hours, and 11,000 hours respectively, with 500 hours in the preceding 6 months each time.

According to the FAA Rotorcraft Flying Handbook, Chapter 21, Gyroplane Emergencies; Ground Resonance:

"Ground resonance is a potentially damaging aerodynamic phenomenon associated with articulated rotor systems. It develops when the rotor blades move out of phase with each over and cause the rotor disk to become unbalanced. If not corrected, ground resonance can cause serious damage in a matter of seconds.

Ground resonance can only occur while the gyroplane is on the ground. If a shock is transmitted to the rotor system such as with a hard landing on one gear or when operating on rough terrain, one or more of the blades could lag or lead and allow the rotor system's center of gravity to be displaced from the center of rotation. Subsequent shocks to the other gear aggravate the imbalance causing the rotor center-of-gravity to rotate around the hub. This phenomenon is not unlike an out of balance washing machine....

To reduce the chance of experiencing ground resonance, every preflight should include a check for proper strut inflation, tire pressure, and lag-lead damper operation. Improper strut or tire inflation can change the vibration frequency of the airframe, while improper damper settings change the vibration frequency of the rotor.

If you experience ground resonance, and the rotor r.p.m is not yet sufficient for flight, apply the rotor brake to maximum and stop the rotor as soon as possible. If ground resonance occurs during takeoff, when rotor r.p.m is sufficient for flight, lift off immediately. Ground resonance cannot occur in flight, and the rotor blades will automatically align themselves once the gyroplane is airborne. When prerotating the rotor system prior to takeoff, a slight vibration may be felt that is a very mild form of ground resonance. Should this oscillation amplify, discontinue the prerotation and apply maximum rotor brake."

According to the maintenance manual, a rotor brake was an optional item. According to the FAA inspector who examined the gyroplane, there was no rotor brake installed.

NTSB Probable Cause

An encounter with ground resonance for undetermined reasons.

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