N401GRRotorsport Uk CAVALON · 2019

On December 4, 2022, about 1135 eastern standard time, a Rotorsport UK Cavalon gyroplane, N401GR, was substantially damaged when it was involved in an accident near Beverly, Massachusetts. The sport pilot was fatally injured. The gyroplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot, who was also the owner of the gyroplane, acquired his sport pilot certificate about 6 months before the accident and had about 120 total hours of flight experience, all of which were in the gyroplane. The pilot had just received the gyroplane back after about three months in maintenance, and according to witnesses, he had not flown during that period. On the day of the accident, the pilot was planning to conduct a local flight; after engine start, he taxied from his hangar to runway 34, where he was cleared for takeoff by a tower controller.

Security video showed that, during the initial climb, the gyroplane climbed to about 40 ft above ground level, where it leveled off briefly, then resumed a shallow climb while in a left yaw. As the gyroplane continued to climb, it began to tilt to the right followed by a rapidly developing right roll. (see figure 1.) The aircraft completed one full 360° roll to the right as it descended out of control and impacted the runway on its right side.

Figure 1. Security camera footage screen grabs showing the gyrocopter during climb as it entered a left yaw followed by a roll to the right.

Two electronic flight displays with recording capability were retained and sent to the NTSB Recorders Laboratory for download. Review of nonvolatile memory revealed 27 parameters that were captured over a period of 20 minutes leading up to the accident. During the takeoff roll, all engine parameters, including but not limited to engine rpm, oil pressure, fuel flow, temperature, pitch, acceleration and airspeed were normal as the aircraft rolled down the runway. As the aircraft lifted off the runway surface, the airspeed was 53 kts with a vertical speed of 371 ft per minute (fpm) and a pitch of about 9.7°. As the aircraft climbed through 75 ft mean sea level (msl) and a GPS altitude of 150 ft, the airspeed was at 58 kts, the vertical speed was 782 fpm and the pitch was at 5.5° with a left roll of 10°. About 6 seconds later, while climbing through 150 ft msl, the aircraft yawed hard to the left and continued to do so until the loss of control. The pitch increased to 15° and over the next 3 seconds, the aircraft climbed through 193 ft msl and the vertical speed increased to 2,692 fpm as the aircraft rolled to the right and descended to impact.

The aircraft impacted the paved runway surface and skidded off the right side of the runway into the grass. The wreckage path extended about 245 ft from the initial impact point to the main wreckage and was oriented on a 340° heading. During impact, the rotor head, containing the two aluminum-skinned rotor blades, hub bar, teeter bolt, and ring gear was impact-separated from the mast below the ring gear. All damage to the mast and rotor head was consistent with overstress due to impact. The rotor assembly was discovered about 170 ft south of the main wreckage.

The rotor assembly was removed from the rotor blades, placed on top of the rotor mast, and attached with zip ties for examination. Pitch control continuity was confirmed from the flight controls in the cockpit to the rotor mast and head through an overload separation of the rotor head pitch pushrod and pitch control cable. Roll control continuity was confirmed from the cockpit control to the rotor mast and head through an overload separation at the rotor head connection point.

Manual manipulation of the teetering function was normal and there was no evidence of rotor blade contact with any of the fuselage or control surfaces. The teetering stops were unremarkable with no bending or impact observed.

The right side of the fuselage was crushed upwards into the cockpit. Control continuity was established through the airframe to all control surfaces; the rudder and vertical stabilizer remained intact and functional. The manual static rudder trim was in the 30° right position (left rudder deflection), was undamaged, and did not exhibit any damage features or witness marks.

The engine remained intact and operated at nearly full power for about two minutes after impact, until fire rescue personnel were able to shut it down. The four-bladed composite pusher propeller was splintered and delaminated on the outer 6-inches of all four blades in a uniform pattern and remained attached to the flange and hub.

Subsequent examination of the engine revealed crankshaft and valvetrain continuity. Examination of the engine and airframe revealed no evidence of any pre-impact anomalies that would have precluded normal operation.

The Office of the Chief Medical Examiner (OCME), Commonwealth of Massachusetts, performed the pilot’s autopsy. According to the autopsy report, the pilot’s cause of death was blunt force injuries. The manner of death was accident.

The autopsy revealed an enlarged heart with thickening of both cardiac ventricles. Mild atherosclerosis of the aorta and iliac arteries was also noted. Examination of the brain, lungs, and remainder of the heart revealed no evidence of other significant natural disease. At the request of the OCME, NMS Labs performed postmortem testing of the pilot’s femoral blood.

Delta-9-THC was detected in femoral blood at 4.6 ng/mL. 11-hydroxy-delta-9-THC was detected at 1.1 ng/mL. Amlodipine and caffeine were presumptively detected.

The Federal Aviation Administration Forensic Sciences Laboratory also performed toxicological testing of postmortem specimens from the pilot. Delta-9-THC was detected in two femoral blood specimens at 5 ng/mL and 5.5 ng/mL, respectively. Delta-9-THC was also detected in cavity blood at 4.1 ng/mL and in urine at 1.4 ng/mL. 11-hydroxy-THC was detected in urine at 27.1 ng/mL but was not detected in femoral or cavity blood. Carboxy-delta-9-THC was not detected in femoral blood but was detected in cavity blood.

According to an excerpt from the Pilot Information Letter issued by the manufacturer, titled, “Gyroplane Yaw Management and Effects on Controls” (AG-PIL-2023-01-EN), “The effect of excessive sideslip on gyroplane pitch and roll control, is, in general, poorly understood, which can lead to a risk to safe gyroplane operation.” The letter stated,

Excessive sideslip may be created in a gyroplane deliberately, typically using rudder pedal inputs, or as an aircraft’s reaction (secondary effect) to another input such as engine power changes. The result of either input is the same; the aircraft will yaw but the aircraft will initially continue in the original flight direction which creates sideslip. Although the aircraft’s directional stability, caused by airflow over the fin, will resist the creation of sideslip, the fin is ineffective at low airspeeds and can be overpowered. Further engine thrust can be used to overpower the fin. In extremis the aircraft could be turned as much as 90 degrees to the direction of flight.

1. Gyroplane stick pitch control stops typically range from a few degrees forward (‘nose down’) to around 20 degrees aft. During flight the stick/rotor is typically positioned around 10 degrees aft, with an in-flight stick movement of only a few degrees. The extra movement is required for take-off (stick fully back) or for stopping the rotor after landing (stick fully forward).

2. Gyroplane stick roll control stops range normally around 8-9 degrees either side of vertical. This amount of movement is never used in flight, it is only used on the ground for safe handling in side-winds. In-flight roll movement is only a few degrees.

3. These limit stops are always built into the rotor head, normally with secondary limit stops on the stick.

Yaw as a result of an extreme pedal input

It is important to remember that a gyroplane body hangs ‘free’ under the rotating wing or rotor disc. This freedom enables the body to be moved in all axes under and with respect to the rotor.

A 90 degree yaw will place the body at 90deg to the oncoming airflow. This then means that the roll axis has effectively become the pitch axis. The rotor disc pitch angle in the direction of travel will tend to the nominal 10 deg aft orientation typical of forward flight at normal cruise speeds. However, in the roll axis the roll control stops will aim to limit the disc pitch angle to 8-9 degrees. In a light aeroplane or helicopter the wing or rotor disc would normally roll away from such sideslip. This characteristic on autogyros is very weak and easily overpowered, for example by the aerodynamic load induced on the gyroplane body in sideslip.

Consider then also that the aircraft is now flying sideways, and the side of the aircraft is facing the oncoming airflow. This airflow will create an increased drag force which will tend to swing the fuselage away from direction of flight. Because the rotor disc is already against the limit stops, and because the force from the body is considerable, the rotor disc is pitched down into the oncoming airflow. With the airflow now above rather than below the rotor disc the aircraft will rapidly roll over into the direction of flight, unless urgent corrective action is taken. Whilst this is an extreme example, AutoGyro is aware of a small number of such occurrences around the world.

Side-slipping gyroplanes without careful control is inherently dangerous. Recovery must be made promptly using pedal yaw input (effective provided there is propeller wash over the rudder), and lowering the nose to force the body into the direction of flight and maintain airflow through the rotor disc.

The letter also illustrates yaw as a result of engine thrust increase or reduction, one being the corkscrew effect:

During take-off, the air being accelerated through the propeller is rotated, and with the forward movement of the gyro, forms a corkscrew shaped flow. The spiraling air hits the centre vertical stabilizer fin and rudder on the port (left) side (on AutoGyro aircraft), exerting pressure on them and yawing the nose left. This is particularly relevant due to the close proximity of the propeller to the tail unit.” The second is asymmetrical propeller loading in which “the airflow entering the propeller disc is modified by the airflow through the rotor disc. The downward moving propeller blade has a greater attack angle than the upward moving one. This means the downward sweeping blade creates more lift (thrust) than the upward moving one and induces a port (left) yaw effect.” The third is gyroscopic precession; when force is applied to a spinning disc (gyroscope). During take-off, when the gyroplane’s nose is lifted, force (airflow) is applied to the bottom of the disc (the propeller) and the resultant reactive force is felt 90° in direction of rotation. In this case with an anti-clockwise rotating propeller, on the starboard (right) side. This creates a yaw motion to the left.