CEN12LA578

The partial deflation of the powered parachute canopy when the pilot flew the aircraft into an area with gusty wind conditions. Contributing to the accident were the modifications that changed the flying characteristics of the parachute.

The pilot was flying his powered parachute on a local flight when he encountered a strong and gusty tailwind. A video showed that the powered parachute cart then rocked fore and aft, while the parachute canopy moved fore and aft above the cart until the left side of the parachute canopy deformed and collapsed. The powered parachute then entered a descending left spiral. During the descent the left side of the parachute reinflated, and the powered parachute impacted terrain nose down with a partially inflated canopy. A postaccident examination of the wreckage and the video of the accident revealed no evidence of preimpact anomalies that would have precluded normal operation. The powered parachute ram-air canopy retains its airfoil shape because of the relative wind airflow entering its front openings. Examination of the accident powered parachute revealed that modifications to lower the canopy's angle of attack had been made, to allow for quicker rotation and additional forward speed. However, these modifications decreased the canopy's angle of attack such that it would partially collapse when wind gusts were encountered.

An autopsy was performed on the pilot on August 25, 2012, by Spectrum Health of Grand Rapids, Michigan. The autopsy listed multiple blunt force injuries as the cause of death. The FAA Civil Aerospace Medical Institute prepared a Final Forensic Toxicology Accident Report. The report was negative for the tests performed. The FAA Powered Parachute Flying Handbook, in part, stated: The powered parachute ram-air wing retains its airfoil shape due to the air pressurizing the inside cells via the relative wind airflow being rammed into the front openings of the canopy - thus the term "ram-air wing." The pressure inside the wing is much higher than the outside top and bottom because the dynamic pressure from the relative wind is converted to static pressure to pressurize the wing. The greater the speed, the greater the pressure inside the wing and the more rigid the wing. A powered parachute flight instructor obtained a copy of the accident video from a source outside the investigative parties. He reviewed the video and wrote the NTSB investigator in charge an e-mail in reference to the accident flight. His communication, in part, stated: It is clear from the pictures that there were two additional links (per side) added to the rear risers, this would raise the rear of the chute by approximately 3 inches over its intended connection point. In addition to the extra links the front lines of the chute have apparently been knotted (perhaps in an attempt to shorten them, as these lines tend to stretch faster then the other lines, although I have never seen this done before). As I mentioned before, this practice of adding links became popular with some due to the stretching of the forward lines and increased difficulty of inflating the chute (the chute would not fully rotate for takeoff) it lowered the angle of attack creating a quicker rotation and additional forward speed. Watching the video of the accident seconds before the collapse you can clearly see the chute heave backwards as the first gust hits the chute, you can then see the chute lunge forward, about that time, you can see the effects of a second gust hitting the chute and pushes it back even further creating a pendulum effect. When the chute tries to correct itself, it will overfly the cart as it pendulums. It is my belief that due to the additional links being added and lines being knotted that it decreased the angle of attack to the point it collapsed when confronted with the gusts and subsequent downward pitch of the wing. An Accredited Representative from the State Of Israel, Ministry Of Transport, Aviation Incidents & Accidents Investigation office reviewed the report and submitted comments. The comments stated that from review of the video, pictures, and reports, it was clear that a major modification was done to the length of the lines, decreasing the angle of attack of the wing. The knot on the front lines was performed on the line ends, where the line thickness is doubled. In this location, the knot shortens the line by 15 to 20 millimeter (mm) (A knot on a single line shortens the line by 10 mm). He also indicated that it is important to note that a knot reduces the strength of the line by up to 40 percent. The two additional links on the back riser added approximately 130 mm to the length of the lines. He stated that this is a significant change from the original length of the line. Simple calculation shows that with the above changes to line length (150 mm total), the angle of attack is reduced by approximately 3 degrees in the center of the wing and 6.5 degrees towards the wing tip. A normal angle of attack is around 16 to 17 degrees. With the line extension, the reduction in angle of attack is 30 to 40 percent and even higher towards the edges of the wings where sensitivity to down wind is high. The wing modification reduced inflation difficulties of the wing and increased flight speed. With the reduction of the angle of attack, the speed of the wing increases by about 10 to 15 kilometers per hour (kph), allowing the pilot to fly in stronger winds, or to fly faster from point A to point B. This change is an increase of up to 20 percent of the original wing speed and performance. In this type of soft wing, that uses Reflex Wing Technology, the angle of attack plays a major role in the stability of the wing and its ability to handle down winds. The modification hinders collapse resistance and pitch stability. The reduced angle of attack reduces the air intake capability of the wing. The air flows at a lower angle and faces smaller openings on the leading edge. The representative stated that the observed behavior of the wing after the collapse was more violent than normal. A rotation is expected in such a collapse, but not as fast as seen here. The reduction of the angle of attack, lower air intake and lower pressure on the under foil of the wing, all reduce the ability of the wing to recover itself from a collapse. The representative indicated that most soft wings are built in a way that even without pilot intervention the wing inflates itself back from a collapse and returns to normal flight. The video of the accident showed that the pilot, immediately following the collapse, pushed both legs almost all the way forward. The representative stated that this is done for two reasons. The leg on the right is used to maintain a straight flight and to prevent a spiral dive. The leg on the left is used to assist in re-inflating the wing by building more internal pressure in the cells. In this case, the leg pressure in the early stages of the collapse did not allow the wing to maintain a straight flight, thus contributing to the rotation and spiral dive. The video also showed that both legs were pushed all the way to the front, from the beginning of the event until impact with the ground and that no 'pumping' movement is seen. In such situations, instead of pushing both legs, only the right leg should be pushed and even deeper. After stopping or slowing the wing rotation, preventing it from spiraling, the left leg should be pumped deeply to assist in wing re-inflation. In most cases, by the time the direction of the wing is stabilized, the collapsed side inflates by itself without need for pilot intervention. The representative indicated that the continuous pressure by the left leg in this case, did not allow the application of right leg pressure to correct the wing rotation. The representative indicated that in the video, flags were clearly visible and showed strong flapping that indicate 20 to 30 kph winds. Other evidence of the higher wind speed is the delta speed between the downwind flight, compared to the last section of the flight, which was with a headwind. Additionally, the location of the collapse is between two hills at the point of the collapse, the plane altitude is almost level with the hill top, putting it in an area prone to turbulence from wind coming over the hill and down into the valley behind it. The Accredited Representative also listed the following recommendations: In the event of a collapse, do not allow the wing to rotate and dive. Keep the leg counter to the rotation direction as far as needed to stop or slow the rotation. Only then, if still required perform deep pumps with the leg on the collapsed wing side. Warn the pilot community in regards to making modifications on stock wings. Do not add links to one riser. Do not extend or shorten wing lines. Do not make fixes without consulting an expert. Specifically for Chiron wing owners, verify that the practice of adding links to ease inflation and takeoff is not used. There are better means of fixing this problematic situation by modifications of the wing attachment to the plane's body. Advise authorities to verify that the Chiron wings that are s