NTSB Factual Report

HISTORY OF FLIGHT

On September 8, 2013, about 1130 Pacific daylight time, an experimental amateur-built Cantrell LW Challenger II SP airplane, N65VG, was substantially damaged when it impacted terrain near the Prineville Airport (S39), near Prineville, Oregon. The airplane was registered to and operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91. The sport pilot, sole occupant of the airplane, was fatally injured. Visual meteorological conditions prevailed, and no flight plan had been filed for the local personal flight, which originated from S39 about 1100.

Witnesses located adjacent to the accident site reported observing the accident airplane fly near their location at a low altitude. The witnesses stated that the airplane oscillated upward and downward three times and the tail flight control surfaces appeared to be moving erratically before the airplane descended nose down into terrain. One witness near the accident site reported that he thought the engine had lost power prior to losing sight of the airplane. Another witness, who was located adjacent to the accident site, reported that the propeller appeared to be operating as it flew over his position, however, the engine didn't sound normal.

Examination of the accident site by a Federal Aviation Administration (FAA) inspector revealed that the airplane came to rest on its left side. The wing structure was separated and found just forward of the main wreckage. All major structural components of the airplane were located within about 20 feet of the main wreckage. The wreckage was transported to a secure location for further examination.

The FAA inspector reported that during an interview with the son of the pilot, he reported that both his father and he knew the airplane was experiencing abnormally high vibrations from the tail section during high speed flight, when approaching about 90 miles per hour, and the control stick would slam forward. The son of the pilot reported to the FAA inspector that after doing research on the internet, they found an article that spoke about adjusting the elevator 3 degrees would help alleviate the problem, and that they did not do any troubleshooting to discover and correct the cause of the vibrations.

The FAA inspector further reported that the last condition inspection was performed June 10, 2013, by a local airframe and powerplant mechanic, who also conducted flight training for the pilot. The mechanic said that there was no abnormal wear on the elevator flight controls that he could find during the inspection. The mechanic flew the accident airplane after the inspection, and noticed a buffet during and after throttling back in descent, but attributed it to the pusher engine and propeller. The mechanic informed the FAA inspector that the pilot had mentioned the same issue, but at approximately 90 mph, he also experienced heavy vibrations and the control stick slamming forward. The mechanic and pilot inserted gap strips between the horizontal stabilizer and elevators, and that the pilot had increased the angle of incidence on the elevators, however, he was not sure how he did it.

PERSONNEL INFORMATION

The pilot, age 46, held a sport pilot certificate. A second-class airman medical certificate was issued to the pilot on June 27, 2013, with the limitation stated "must have glasses available for near vision." Information provided by local law enforcement revealed that the pilot's logbook was located, and a total of 73 hours of flight time was logged as of September 1, 2013.

AIRCRAFT INFORMATION

The amateur built experiential two-seat, high-wing, fixed-gear tailwheel equipped airplane, serial number (S/N) CW1165, was completed in 1997. It was powered by a Rotax 503 engine, serial number 4838245, rated at 50 horse power. Review of FAA registration records revealed that the pilot and his son had purchased the airplane on April 9, 2012.

METEOROLOGICAL INFORMATION

A review of recorded data from the Redmond Municipal Airport (RDM) automated weather observation station, located about 11 miles west of the accident site, revealed at 1156 conditions were wind variable at 6 knots, visibility 10 statute miles, clear sky, temperature 22 degrees Celsius, dew point 9 degrees Celsius, and an altimeter setting of 30.12 inches of mercury.

MEDICAL AND PATHOLOGICAL INFORMATION

The Oregon State Medical Examiner conducted an autopsy on the pilot on September 10, 2013. The cause of death was determined to be "multiple blunt force traumatic injuries."

The FAA's Civil Aeromedical Institute (CAMI) in Oklahoma City, Oklahoma, performed toxicology tests on the pilot. According to CAMI's report, carbon monoxide, cyanide, volatiles, and drugs were tested, and had negative results.

TESTS AND RESEARCH

Examination of the recovered wreckage was conducted by the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on October 22, 2013. The examination revealed that both wings were separated to facilitate wreckage transport. The ailerons, rudder, and elevator remained attached to their respective mounts. Both the left and right elevator control torque tubes were fractured. The left side torque tube exhibited chaffing on the tube near the fracture area. Additional striations were observed on the fuselage tube structure adjacent to the torque tube and along the left side of the fuselage tube. Additional impact marks were observed on the bottom left side of the vertical stabilizer, almost directly above the striations on the fuselage tube. The bottom side of the left horizontal stabilizer exhibited impact marks, missing paint, and puncture holes in the fabric almost directly in line with the aft area of chafing on the fuselage tube. A trim tab installed on the left elevator remained attached to its mount, however, the bolt that attaches the trim tab to the actuating arm was found loose within the trim tab.

The trim tab and both the left and right elevator torque tubes were removed and subsequently sent to the NTSB Materials Laboratory for further examination.

Examination of the recovered engine revealed that the engine remained attached to the airframe and the carburetor was separated from its mount. The spark plugs were removed, and exhibited normal operational signatures. The crankshaft was rotated by hand using the propeller. Thumb compression was obtained on all cylinders. The carburetors were disassembled, and were unremarkable. Fuel was found within both carburetor float bowls, and was found to be free of debris. No evidence of any preimpact mechanical malfunction was observed with the engine that would have precluded normal operation.

The trim tab and both the left and right elevator torque tubes were examined at the NTSB Materials Laboratory. Pieces of the left and right elevator control torque tube assemblies are fabricated from aluminum alloy telescoping tubes that are riveted together with pairs of rivets located 180 degrees circumferentially from each other. The locations of the rivet pairs are along the length of the control tubes, which include rod ends that are riveted to the terminal ends of the elevator tube assemblies. Two additional areas were riveted near the ends of a torque tube sleeve, which connected the smaller diameter torque tubes together.

Both control tube assemblies exhibited fractures in the tubes located between the aft rod end and the center tube. The fractures occurred adjacent to the aft end of the outer torque tube sleeve. Examination of the fracture surfaces on the tubes using a 5X to 50X stereo-zoom microscope revealed fractographic features (such as 45° circumferential shear lips) consistent with overstress fracture. The shape of the crack path and the deformation in the tubes was consistent with the application of bending forces on the tubes.

The qualitative composition of the rivets was determined by x-ray fluorescence spectroscopy (XRF) using a portable hand held XRF analyzer with imaging camera and 8 mm spot size. The generic material type for each rivet pair at a given location were of the same generic material type. The rivets connecting the torque tube to the rod ends (both left and right sides) were an austenitic stainless steel material, and both the forward and aft set of rivets on the right control tube and the aft rivet on the left control tube that connect the outer sleeve to the smaller portions of the control torque tubes were an aluminum alloy material.

The forward rivet for the left control tube sleeve was not located. Two through-holes (located 180 degrees circumferentially from each other) were present in the telescoping tube pairs at the rivet location. Although no rivets were present at that location, the outside surface of the center tube near the rivet holes exhibited marks consistent with the previous installation and removal of rivets. The diameter of the through-holes measured nominally 0.125 inch using a dial caliper.

The upper and lower control surfaces of the elevator trim tab were examined. A through-bolt and bracket assembly that appeared to be part of a control horn was loose in a hole. The enlarged through-hole is due to the separation of a plate of trim tab material consistent with cantilever flexing of the bolt-bracket horn assembly. The edges of the hole were deformed consistent with impact wear and batter from contact with the bolt assembly.

Review of the Quad City Challenger II Assembly Manual, section III manual, fuselage construction and engine installation, page 13, revealed that step 8 stated in part "…assemble telescoping elevator push rods (do not rivet yet) and connect the push rods between elevator control horn and stainless steel elevator bell cranks on fuselage (drill 3/16" holds to ¼" where required). Be sure the elevator bell cranks are positioned in the vertical position (as shown below). When proper length of the push rod is found, rivet in place with short stainless steel rivets (SSD-42). Make sure that the two smaller tubes telescope of an equal amount into the outer sleeve." In addition, the manual contained a diagram outlying the installation of 1/8" stainless steel rivets in four positions of the elevator torque tube.

The airplane maintenance records were not located, and it was not determined when the torque tube rivets were last installed or when maintenance was last performed on the elevator flight controls.

NTSB Probable Cause

The in-flight failure of the left elevator control torque tube. Contributing to the accident was the improper assembly of the elevator control torque tubes.