CONROE, TX, USA
N235FC
CARTER SPACEWALKER II
The homebuilt airplane (TT 114.3 hours & 22.4 hrs since last condition inspection) was destroyed following an in-flight separation of the left wing in level flight and the subsequent uncontrolled flight into terrain. The two-place (tandem) seating, open cockpit, low wing, fabric covered airplane incorporated a tubular steel welded fuselage and empennage, with wooden wings. The left wing separation points were at the wing root where the wooden main wing spar and rear wing spar passed through the tubular brackets of the fuselage. Examination of the failed surfaces revealed that the left wing separated from the aircraft in a predominately upward direction. The main spar was the first to separate in an upward direction. The rear spar then separated in an upward direction with a slight movement towards the rear. The probable cause of the separation was the choice of basswood for the spars instead of the specified spruce. According to the designer, aircraft grade spruce is the only wood specified for the spars. Builder records indicated the use of basswood for spar construction. Examination confirmed the wood spars were basswood. Basswood is considerably weaker and less stiff than spruce. The FAA Advisory Circular 43.13-1B/2A, Chapter 1, Wood Structures, Table 1-1. Selection and Properties of Aircraft Wood, indicates that spruce is excellent for all uses and is considered as the standard by the FAA. Table 1-1 does not list basswood as a selection of aircraft wood. Chapter 1 Paragraph 1-41 states in part: The type of wood used is critical to the structural strength of the aircraft.
HISTORY OF FLIGHT On September 25, 1999, at 1715 central daylight time, a Carter Spacewalker II homebuilt experimental airplane, N235FC, was destroyed when it impacted the terrain following an in-flight separation of the left wing in level flight at the Montgomery County Airport near Conroe, Texas. The airplane was registered to a private individual and operated under 14 Code of Federal Regulations (CFR) Part 91. The commercial pilot and the passenger received fatal injuries. Visual meteorological conditions prevailed for the personal flight, and a flight plan was not filed. The local flight originated from the Montgomery County Airport at 1712. The owner reported that when the preflight was performed, the fuel tank was 1/2 full (maximum capacity 18 gallons). The owner and pilot performed two takeoff and landings on runway 14 at the Montgomery County Airport. Upon taxiing to the fixed base operator (FBO) ramp, the pilot moved to the aft seat and the passenger boarded the front seat. The owner assisted the passenger in securing his three point safety harness and briefed the passenger to remain clear of the cockpit flight controls. The pilot taxied the airplane to runway 14 for departure. Following the departure, the airplane entered a left turn, circled in a climbing turn, and flew back over the airport. The owner stated that the airplane was in level flight on about a 200 degree magnetic heading, midfield of the airport, at 1,500 feet agl when the "left wing broke away and [the] fuselage did [a] snap rotation to left." The engine was at "full power and continued to run until the impact." The unicom operator, who witnessed the accident, reported that the "plane began to rock slightly from left to right. The plane then pitched up and then down again. At that time, the wing separated from the fuselage and the plane went into a spin." The spin occurred from about "1,000 feet agl." Another witness stated that the "right wing folded straight up against the fuselage and sheared away. The plane went into a spin and crashed into the south end of the field." Another witness reported that the airplane "appeared flying level or slight climb to south at an est[imated] alt[itude] of 500 [feet] agl. Heard loud noise of wing separation. Saw the wing flutter horizontally, then spiral away from aircraft." The engine "rpm's went to what seemed to be max[imum]. The airplane went into a tight corkscrew making 3-4 or 5 turns and impacted." PERSONNEL INFORMATION A second class medical, with a limitation for corrective lenses, was issued to the pilot on February 1, 1999. The available FAA records revealed that the pilot had accumulated 4,500 hours of flight time. He held commercial privileges in airplane single-engine land and multi-engine land with an instrument rating. He held private privileges in single-engine sea airplanes. The pilot held a mechanic certificate with airframe and powerplant ratings. The owner stated that the pilot had approximately 15 hours in the make and model of the accident aircraft. No additional information on the pilot's flight time was obtained. AIRCRAFT INFORMATION The Spacewalker II was designed by Anglin Engineering of Rutherfordton, North Carolina. The two-place (tandem) seating, open cockpit, low wing, fabric covered airplane incorporated a tubular steel welded fuselage and empennage, with wooden wings. A copy of the plans to build a Spacewalker II were sold by Anglin Engineering to the builder of the accident airplane. The designer's specifications indicated a maximum gross weight of 1,200 pounds (lbs) with a basic empty weight of 700 lbs. Builder records for N235FC indicated a basic empty weight of 898 lbs, and a maximum gross weight of 1,300 lbs. Utilizing the builder's basic empty weight, the pilot's weight (214 lbs) on his last medical certificate, the passenger's estimated weight (175 lbs), with an estimated 8 gallons of fuel (48 lbs), the gross weight of the aircraft at the time of the accident was 1,335 lbs. According to the builder of the accident airplane "there are four basic components that make up the spar. The first being a solid wood, I used a material known as basswood. The second item would be marine plywood followed by aircraft grade plywood. All parts were bonded together with a two part adhesive (T-88)." According to the designer, aircraft grade spruce wood was specified in the plans for the wing spars and the main wing spar was designed to carry 7 G's with a safety margin. According to the designer, aircraft grade spruce is the only recommended wood for the main spar and the rear spar. The designer further stated that the welded steel fuselage tubing that forms the main and rear spar carry through brackets is designed oversized vertically, which permits the builder to set the angle of incidence for the wings. Once the angle of incidence is determined and set, the oversized area is intended to be filled with wedges of wood glued to the spar so that no space remains within the tubular bracket. There was no physical evidence found that the space had been filled on the accident aircraft. However, the designer indicated that he was aware of other aircraft on which the space had not been filled and that inspection of these aircraft revealed no problems. In April 1991, a 115 horsepower Lycoming O-235-C1 engine, overhauled for experimental use only, with a 2-bladed Sensenich propeller, was installed in the airframe. On December 5, 1991, the aircraft was issued the FAA special airworthiness certificate in the experimental category under CFR Part 21.191. In September 1993, the aircraft was disassembled for transport in an enclosed truck when it was sold to an individual in California. In May 1996, the aircraft was disassembled for trucking when it was sold to an individual in Montana. In June 1998, the aircraft was assembled from container shipment when it was sold to an individual in Florida. In 1999, the current owner purchased the airplane and flew it to Texas. The last condition inspection was performed on June 30, 1999, by the accident pilot/mechanic. At the time of the inspection, the total aircraft time was recorded as 91.9 hours, with a tachometer reading of 39.9 hours. The tachometer reading at the accident site was 62.3 hours. By subtracting 39.9 hours from 62.3 hours, it was determined that the aircraft had flown 22.4 hours since the last inspection. By adding 91.9 hours and the 22.4 hours, the total time on the aircraft was calculated as 114.3 hours. These calculations are in agreement with the owner, who stated that the airplane had flown "about 22 hours in the previous 3 months since his ownership, and the total aircraft time was about 115 hours." During a personal interview, conducted by the NTSB investigator-in-charge (IIC), the owner stated that numerous people had flown the airplane since his ownership. The owner and pilots, who had flown in the airplane and responded to the NTSB IIC inquiry, stated that to their knowledge, the airplane did not have a history of aerobatic maneuvers. One pilot stated that the airplane was involved in a "hard landing" the week prior to the accident, that "damage ensued", and that the owner was "engaged in repairing a bent landing gear fitting and the aircraft was on jacks and unflyable for the week prior to the accident." The owner stated that about 2 weeks before the accident, during taxi at the Montgomery County Airport, he noted that the left wing was a few inches below the right wing. Upon examination of the left main landing gear, he found that a bracket needed repairing. Subsequently, he repaired the left main landing gear bracket; however, an entry had not been made in the maintenance records. One of the pilots, who had flown in the accident airplane, reported observing the airplane undergoing maintenance midday Friday, September 24, 1999. The main landing gear were being reattached to the aircraft and he observed the aircraft "jacked up by the use of two pieces of 2X4 lumber which was inserted, on their sides, through the space between the center and outer wing sections. Under the front spar and on top of the rear spar. The 2x4's were supported in front of the wings on boxes and were not supported at the rear. The tail rested on the tail wheel. When the starboard gear was reattached, the 2x4 supporting the starboard wing was removed. In order to obtain clearance for the port gear to be attached, the right gear was jacked up approximately 15" [inches] at the axle. After the port gear was reattached the 2x4 supporting the port wing was also removed." According to the designer, the lifting points for the aircraft are where the main landing gears attach to the fuselage, not where the outer wing sections attach to the wing center section. AERODROME INFORMATION The Montgomery County Airport (CXO), at an elevation of 245 feet, is a non-towered airport with hard surface (asphalt) runways 14-32 (6,000 feet X 150 feet) and 01-19 (3,974 feet X 100 feet). The airport communications common traffic advisory/unicom frequency (CTAF) is 122.95 Megahertz (MHz). WRECKAGE AND IMPACT INFORMATION During the examination of the accident site at the Montgomery County Airport, the left wing was found resting 80 feet west of the centerline of runway 19, and at a distance of 1,145 feet on a measured magnetic heading of 028 degrees from the main wreckage. The site of the main wreckage was in a grassy area located between Taxiway Delta and Runway 19 in front of Hangar 19. The distribution path of the main wreckage was on a measured magnetic heading of 212 degrees. The initial impact point was a longitudinal ground scar extending for approximately 8 feet and ending at a 12 inch deep crater that contained the propeller and components of the destroyed engine. The firewall and the right wing were approximately 7 feet beyond the crater. The cockpit and the fuselage/empennage came to rest on a measured magnetic heading of 051 degrees approximately 21 feet beyond the crater. Numerous components were scattered around the main wreckage area; however, no components were found between the left wing and the initial ground scar. All flight control surfaces were found at the accident site; however, in-flight separation and impact damage precluded verification of flight control continuity. Fracture surfaces of the flight control cables exhibited angled, rough surfaces consistent with overload separations. The cockpit flight instruments were found destroyed. The propeller remained attached to the crankshaft flange, and both blades of the propeller were bent aft and exhibited chordwise striations and scrapes. One blade was bent and shaped in an "S" and the second blade was twisted. Cockpit engine controls were found in the midrange position, and the fuel primer was in and locked, with the carburetor heat in the cold position. The left wing separation points were at the wing root where the main spar and rear spar pass through the tubular brackets of the fuselage. The fractures from the left wing main spar, and rear spar, along with the fuselage tubular brackets, and miscellaneous pieces of wood were forwarded to the NTSB Materials Laboratory for examination. MEDICAL AND PATHOLOGICAL INFORMATION The autopsy for the pilot was performed by the Harris County, Texas, Medical Examiner. Toxicological testing was performed by the FAA Civil Aeromedical Institute's (CAMI) Forensic Toxicology and Accident Research Center at Oklahoma City, Oklahoma. The toxicological tests were positive for ethanol, acetaldehyde, N-butanol, and isobutanol. The FAA Southwest Regional Flight Surgeon stated that these toxicological findings were "not significant." TEST AND RESEARCH The NTSB metallurgists transported the portions of wooden spar to the USDA Forest Products Laboratory in Madison, Wisconsin, for identification and fracture examination. Before the failure surfaces were examined, shavings were removed from the main spar flanges and from the aft spar pieces. Utilizing a light microscope to identify the wood species, it was determined that the characteristics of the cellular structure of the flanges were consistent with those of basswood. The failure surfaces of the left wing main spar displayed the characteristics of a brash compression failure from the top to about the midpoint of the top flange. The rest of the failure surface of the left wing main spar displayed characteristics of a fibrous tension failure. The failure surfaces of the left wing aft spar indicate that the top of the spar failed in compression and the bottom had the fibrous failure in tension. The inboard piece of the aft spar had a very irregular failure pattern. The jagged bristled failure surface indicated that the bottom failed due to tensile stress. The NTSB Metallurgists examination of the fractured fuselage tubular brackets revealed features that were consistent with overstress separations. ADDITIONAL INFORMATION The FAA Aircraft Inspections, Repair, and Alterations Advisor Circular (AC 43.13-1B/2A) Chapter 1. Wood Structures, Table 1-1. Selection and Properties of Aircraft Wood indicates that spruce is excellent for all uses and is considered as the standard by the FAA. Table 1-1 does not list basswood as a selection of aircraft wood. Chapter 1 Paragraph 1-41 states in part: The dimensions and type of wood used are critical to the structural strength of the aircraft. Care should be taken that any replacement spars accurately match the manufacturer's original design. The airplane, except for pieces of the left wing main spar and rear spar, both rear spar brackets, and the aircraft records, was released to the registered owner on September 27, 1999. The retained pieces were released to the registered owner on April 21, 2000. The aircraft records were released to the registered owner on January 5, 2001.
The in-flight separation of the left wing due to the failure of the wing spars resulting from the builder's improper design change utilizing basswood for the spars instead of the specified spruce.
Source: NTSB Aviation Accident Database
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