Clines Corner, NM, USA
N555AP
JONKER SAILPLANES CC JS 1B
While in soaring flight, the pilot heard a loud metallic sound come from the area near the rudder pedals and subsequently lost contact between his feet and the pedals, which resulted in a temporary loss of glider control. Although the pilot regained control of the glider, it was descending. When the glider had descended to about 10,000 feet mean sea level, the pilot chose to bail out with a parachute due to the descent rate and the unfavorable terrain for landing. The glider subsequently impacted a mesa and sustained substantial damage. Subsequent examination of the adjustable rudder pedal system revealed that the right rudder cable had fractured at the forward opening of the S-tube on the right rudder pedal. Examination of the rudder pedal design configuration revealed that full-forward deflection of the rudder pedal forced the cable to sharply bend at the point where it exited the forward opening of the S-tube, which was confirmed by a corresponding wear pattern on the accident glider’s S-tube. The repeated bending during normal operation resulted in the fatigue failure of 92 percent of the individual right rudder cable wires and, subsequently, final overstress failure of the remaining wires. After the accident, the owner of another glider of the same design examined the rudder cable on his glider and found similar damage to the cable. Subsequently, he conducted ground testing, which confirmed that repeated operation of the rudder pedals resulted in further damage to the rudder cables, indicating a progressive failure. Therefore, it is likely that the right rudder cable failed during the flight and resulted in the loss of rudder control. The glider manufacturer subsequently issued service bulletins and service letters to address the rudder pedal system deficiencies discovered during the accident investigation. Flight testing conducted by the manufacturer in an exemplar glider confirmed that the glider would have been controllable if the pilot had not chosen to bail. However, the flight testing was conducted under controlled conditions, and the test pilot was fully aware of the modifications made to the exemplar glider to simulate the cable failure. Further, despite these modifications, the test pilot was still able to fully control the rudder, if necessary, during the flight test. Since the accident pilot did not have knowledge of the nature of the control problem and did not have the ability to gain full rudder control, his decision to bail out of the airplane was understandable.
HISTORY OF FLIGHT On April 29, 2012, about 1300 mountain daylight time, a Jonker Sailplanes model JS 1B glider, N555AP, collided with terrain following an in-flight loss of control near Clines Corner, New Mexico. The pilot bailed out of the glider and received minor injuries during his parachute landing. The glider sustained damage to the fuselage and both wings. The aircraft was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The flight originated from the Moriarty Airport (0E0), Moriarty, New Mexico, about 1230. The pilot reported that while in-flight at about 11,300 feet above mean sea level (msl), he heard a loud metallic noise from the area near the rudder pedals and lost contact between his feet and the rudder pedals. The pilot stated that he thought that the pedal adjustment mechanism had given way and attempted to adjust the pedals through the use of the adjustment handle. When he pulled on the adjustment handle it had no tension and did not retract as designed. - He maintained partial control of the glider using aileron and elevator control; however, the glider was descending in a right yaw condition. When the glider had descended to about 10,000 feet msl, the pilot elected to bail out because the surrounding terrain was unfavorable for a forced landing. The glider was equipped with a Clear Nav global positioning system (GPS) receiver. The data downloaded from the receiver depicted the entire accident flight. The data showed the glider was heading north-northeast when it made a climbing left turn of about 360 degrees. During the 40-second turn, the average rate of climb was calculated to be 388 feet per minute. After the completion of that turn, at 1904:40, the glider was about 11,538 feet GPS altitude. As the glider continued to turn left, the altitude dropped during the next approximately 90-degrees of turn and at 1904:52 the glider was about 11,171 feet pressure altitude. The calculated descent rate during this portion of the turn was 1,835 feet per minute. The glider continued in the left turn and climbed back up during the next approximately 90 degrees of turn where it was about 11,414 feet pressure altitude at 1905:08. After this point, the glider began a descending right turn. During the next 21 seconds the glider’s average rate of descent was calculated to be 2,437 feet per minute. The glider then began an abrupt descent to the ground. During the 59-second descent, the glider’s course varied, but was predominately in an easterly direction. The glider’s average rate of descent was 4,044 feet per minute during this time period. The pilot stated that the glider was in a right turn when the mishap occurred, but the GPS data showed the glider in a left turn. In subsequent telephone conversations, the pilot could not explain why his recollection is different from the recorded data. PERSONNEL INFORMATION The 68-year old pilot held a commercial pilot certificate with an airplane single-engine land, airplane multi-engine land, instrument airplane, and glider ratings. The pilot reported having 7,000 total flight hours, including 100 hours in the accident glider. He reported that his most recent flight review as required by 14 CFR 61.56 was completed on March 20, 2011. He was issued a third-class airman medical certificate, with a restriction for corrective lenses, on January 12, 2012. AIRCRAFT INFORMATION The accident glider was a Jonker Sailplanes model JS-1B, serial number 24. It was a single-seat, high-performance sailplane, constructed mainly of composite materials. Registration data indicated that the glider was manufactured in 2011 and was issued an airworthiness certificate on July 27, 2011. The pilot reported that the airframe had accumulated approximately 100 hours total time in-service at the time of the accident. METEOROLOGICAL INFORMATION Weather conditions recorded by the 0E0 Automated Weather Observing System (AWOS), located about 22 miles south-southwest of the accident site, at 1255, were: wind from 230 degrees at 6 knots, visibility 10 miles, scattered clouds at 7,500 feet agl, temperature 21 degrees Celsius, dew point 1 degree Celsius, and altimeter 30.04 inches of mercury. WRECKAGE AND IMPACT INFORMATION The glider impacted a mesa in rural New Mexico 23 miles and 23 degrees from 0E0. Examination of the glider on-scene revealed that the empennage was partially separated from the tail boom at a point near the leading edge of the vertical stabilizer. The tail boom had a longitudinal split from the point of separation with the tail running forward toward the cockpit. The forward fuselage had crushing damage in the cockpit area. The wings were predominately intact. The left wing had a split in the composite structure on the bottom skin that started from the joint between the removable wing tip running inboard. The control system was intact except for a break in the right rudder cable at a point where the cable exited the forward exit of the “S”-tube on the adjustable rudder pedals. The rudder pedal assembly and cables were removed for further examination. ADDITIONAL INFORMATION Examination of the design configuration revealed that the rudder pedals were mounted on the rudder pedal assembly by sliding the lower pivot tube of the pedal itself onto a smaller diameter tube on the adjustment assembly. The pedal was retained laterally by a large diameter washer and a nut. The retaining nut was an “acorn” style nut and did not have any internal locking capability, nor was any other locking device installed. Examination of the rudder pedal system from N555AP was conducted by the NTSB Materials Laboratory. The right rudder cable was found to be severed at a location corresponding to the forward exit of the S-tube on the adjustable rudder pedal assembly. The cable was composed of 7 strands, each of which had 19 wires. Eleven (8 percent) of the wires exhibited signatures consistent with overstress failure, and the remaining wires (92 percent) exhibited signatures consistent with fatigue failure due to bending stress. The left rudder cable was also examined and 22 wires were found to be broken. All of the breaks were similar to the fatigue fracture breaks found when examining the right rudder cable. The exit of the S-tube where the right rudder cable break was located had wear damage consistent with the diameter and location of the rudder cable. The S-tube was welded to the side of the rudder pedal and had cut ends that were not flared. A mockup of the rudder system revealed that with the rudder deflected fully forward, the cable was forced to make a sharp bend at the forward exit of the S-tube. Additionally, the wear pattern noted on the S-tube was consistent with the position of the cable during full deflection. After the accident, the owner of an exemplar glider, N60VR, examined the rudder control cables on his glider and found that the cables had damage in the same location as on N555AP. The owner of N60VR elected to conduct ground based testing to determine the progressive nature of the rudder cable damage. While in a normal seating position wearing a backpack parachute, the rudder pedals were cycled in one minute intervals, with an observation period between cycles. The study was concluded after two cycles of one minute each, , followed by one cycle of three minutes (a total of five minutes) and it was believed that further cycling would have completely severed the right rudder cable. During the three minute cycle, an increasing lateral movement of the rudder pedals on the rudder pedal shaft was noted. Observation revealed the right side securing nut on the shaft had backed off allowing the rudder pedal to slide laterally on the pivot tube. The owner of N60VR provided documentation of the testing performed including photographs taken during the testing. The glider manufacturer conducted flight testing to evaluate the flight characteristics of the glider with a simulated break of the right rudder cable. The method employed to simulate the break was to remove the rudder pedal return spring on the right rudder pedal of an exemplar glider, which would allow the rudder to deflect fully to the left if the pilot removed foot pressure from both pedals. If necessary, the test pilot could regain full rudder control at any time by re-applying foot pressure to the rudder pedals. The testing included side-slip and spin evaluations. The spin testing confirmed that the glider could be placed into spin in either direction and recovery accomplished using only control stick movements while the rudder was fully deflected due to the simulated cable break. The exemplar glider was subsequently flown from the testing area to an airport for landing. The glider was reportedly controllable with minimal difficulty with the simulated cable failure. A complete traffic pattern was flown with the simulated failed cable and rudder was only used during the final portion of the landing due to crosswinds at the airport. In the wake of the accident, Jonker Sailplanes issued one Service Bulletin (SB.JS-007) and three Technical Notes (TN.JS-009, TN.JS-010 and TN.JS-011) pertaining to the rudder control system cables and locking mechanism. SB.JS-007 specifies regular cable and rudder pedal mechanism inspection intervals and techniques. It refers to TN.JS-009 and TN.JS-010 for instructions in replacing the rudder cables and pedal locking mechanism, respectively. Finally, TN.SJ-010 specifies the availability of a refined rudder pedal S-tube design, to reduce wear damage to the rudder cables. The new pedal S-tubes exhibit four wear reducing changes: 1) Redesign of the S-tube geometry to reduce the distance from the lower S-tube opening to the pedal axis of rotation 2) Flanges at both openings of the S-tube 3) No cutoff at the lower S-tube opening 4) A tubular nylon lining between the S-tube and the cable
The fatigue failure of the rudder cable due to the inadequate design of the rudder pedal system.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
