19990925030469A

(.19) On September 25, 1999, at 1715 central daylight time, a franklin space walker ii homebuilt airplane, N235FC, 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 owned by a private individual and operated under 14 code of federal regulations (CFR) part 91. The commercial pilot and the passenger received fatal injuries, and the airplane was destroyed. Visual meteorological conditions prevailed for the local personal flight, and a flight plan was not filed. The flight originated from the airport several minutes before the accident. The owner reported that he and the pilot performed 2 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 fuselage did 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 reported that the airplane "appeared flying level or slight climb to south at an EST[imated] alt[itude] of 500 [feet] AGL. 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." During examination of the accident site, the left wing was found 1,145 feet from the main wreckage. The measured magnetic heading from the left wing to the main wreckage was 208 degrees. The initial impact point at the main wreckage, on a measured magneti c heading of 051 degrees at 21 feet from the initial ground scar. Numerous components were scattered around the main wreckage area; however, no components were found between the left wing and the initial ground scar. The owner stated that the airplane had been flown about 22 hours in the previous 3 months. Total time on the aircraft was about 115 hours. Following the on scene examination, all components, except the left wing and the maintenance records, were released to the owner. (-23) at approx. 17:20 hours on September 25, 1999, amateur built aircraft, spacewalker ii, registration number N235FC, while in level flight, suffered an in flight structural failure and subsequent loss of the left wing. The a/c impacted the ground approx. 1124' in a southwesterly direction from where the left wing settled. The commercially rated pilot and his passenger received fatal injuries. The a/c was completely destroyed. The a/c had received the required annual condition inspection in June of 1999. (.4)the homebuilt airplane (tt 114.3 hours & 22.4 hrs since last condition inspection) was destroyed following an in-flight separation of the left win g 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.