WPR23FA219

The pilots’ exceedance of the airplane's critical angle of attack while maneuvering to land, which resulted in an aerodynamic stall and subsequent loss of control.

The two pilots, both of whom were owners of the airplane, were returning to their home airport. Radar track information correlated to the accident airplane was consistent with the airplane departing and continuing southwest toward the destination airport before it turned south toward the accident airport. The airplane joined the left downwind leg of the traffic pattern (right traffic was specified for this runway), and after turning onto a base leg, made a sharp left turn and spiraled toward terrain, consistent with an aerodynamic stall. A witness reported hearing one of the pilots transmit "engine out" or "simulated engine-out"; however, the airport’s common traffic advisory frequency was not recorded, and the content of this transmission could not be confirmed. The witness also stated that the airplane crossed over the airport to join the traffic pattern and that the engine sounded normal. Examination of the engine revealed continuity of the valve and drive train and compression in each of the cylinders. Removal of the cylinders revealed light scratches and corrosion of the combustion chambers and barrels. The intake and exhaust valves were intact and undamaged; the No. 3 exhaust valve was consistent in appearance with exposure to high temperatures. Severe spalling was noted on the faces of the camshaft intake lifters and several exhaust lifters; the camshaft lobes did not appear rounded. Although visual examination of the oil filter media did not reveal any metallic debris, microscopic inspection revealed metal particles, and there were several small pieces of metal in the oil suction screen. It is unlikely that any of these findings would result in a total loss of engine power. There was evidence of fuel in the fuel system. A video recorded about five months before the accident was recovered from an iPad owned by one of the pilots. The video indicated that the airplane had recently exhibited problems with the trim system leading to the airplane oscillating and “throwing the elevator up and down.” A modification was made to the trim system sometime after the pilots purchased the airplane about seven months before the accident; however, there was no record of this modification in the available maintenance records. At the accident site, the right trim tab was sheared off at the piano hinge and came to rest under the elevator, yet remained connected to the bellcranks and was continuous to the fuselage bellcrank. When attempting to move to nose-down trim, the control cable could not move to that position because the cable would bind on the sleeve and not move over the attach fitting. Whether the modified trim system caused a problem inflight and what effect it may have had on the control surface and airplane’s controllability could not be determined. The circumstances of the pilots’ decision to land at the accident airport could not be determined; however, based on the available information, it is likely that they exceeded the airplane’s critical angle of attack while maneuvering for landing, which resulted in an aerodynamic stall and loss of control at an altitude too low for recovery.

Information gathered from local weather stations indicated an average wind from about 255° at 10 kts gusting to 15 kts. The accident airplane was a two-seat biplane equipped with tricycle landing gear. The airplane had dual controls installed and could be flown from both the forward and aft seats. The pilots purchased the airplane from the builder on November 2, 2022. The maintenance records from the prior owner were located in the pilots’ hangar; there were no records from after the purchase. The last annual inspection was recorded as being completed on March 13, 2022, at a tachometer time of 511.2 hours. Thereafter, a pre-buy inspection was performed on June 28, 2022, at 515.4 hours. Through the records and tachometer reading, it was determined that the airplane had accrued a total time of 543.9 hours at the time of the accident. The build manuals depicted a trim tab system incorporating a McKenzie Vernier control, which was not installed on the accident airplane. Photographs of the installed trim system were shared with the airplane’s builder, who confirmed that the system shown did not match the configuration he originally installed. The timing and origin of the modification could not be determined since the airplane’s logbooks were only maintained up to the point of sale and contained no subsequent maintenance entries. A 40-second video recovered from the pilot’s iPad depicted one of the pilots, presumably having made a recording for the previous owner. The video, recorded on January 18, 2023, showed the pilot narrating, “here is what we found,” while manipulating the right trim tab up and down. He remarked, “see how much that moves,” before inspecting the left trim tab in the same manner, noting, “that’s stiff.” He then returned to the right trim tab, again demonstrating its movement, and stated his belief that this was “what the problem was,” describing how it “just started oscillating … throwing the elevator up and down.” He concluded the video by stating that he had found the issue and planned to fix it. Autopsy of the pilots was performed by the Solano Coroner’s Office, Fairfield, California. According to the pilots’ autopsy reports, their cause of death was blunt force injuries. The accident site was located about 2,615 ft from the approach end of runway 25. The wreckage was distributed over about 45 ft on a true heading of 350°. The nose was pointed on a median true heading of about 20°. The first identified pieces of debris were paint chips and an air scoop from the left side fuselage. The furthest wreckage pieces were large splinters of wood from the two-bladed propeller. The fuselage came to rest upright with both lower wings attached at their respective roots. The upper wings remained attached but had partially collapsed onto the lower wings. The lower right wing sustained crush damage over the inboard 44 inches from resting on the landing gear, with additional aft damage to the outboard leading edge. The upper right wing’s outboard leading edge and underside showed aft and upward crushing. Internal inspection revealed fractured and loose ribs. The lower right aileron remained attached at the outboard fitting; the inboard fitting was broken, but the control rods stayed connected. The upper aileron was attached at the outboard end and disconnected inboard. The aileron interconnect tube was buckled but remained attached at both ends. Control tubes continued through the outboard lower bellcrank and remained continuous to the cockpit. The lower left wing showed crush damage over the inboard 56 inches from resting on the gear. The aft section near the aileron was bent downward about five inches, and the outboard two feet of the lower wing had additional damage. The underside of the upper left wing displayed aft deformation, and the internal ribs were fractured and splintered. Both upper and lower ailerons remained attached, and the control rods and tubes were intact through to the cockpit interconnect tube. The fuselage exhibited crush damage aft, right-aft, and upward from the firewall to the aft bulkhead behind the pilot’s seat, compromising cockpit space. The empennage, vertical stabilizer, and rudder remained largely intact. Both horizontal stabilizers were severely deformed downward and aft at the inboard ends and bent slightly upward outboard. The left stabilizer fractured at the aft tube near a wire attach fitting. The left elevator was attached and bent downward about 22 inches outboard; its balance weight remained in place. The right elevator was attached, but its balance weight hung downward. Rudder and elevator cables were continuous from the cockpit to their fittings. The rudder horn moved freely with no damage to the stops. Elevator movement was restricted, which appeared to be the result of forward structure deformation. Slight rubbing was noted on the rudder’s left side fabric, with a corresponding mark on the adjacent left elevator, consistent with contact. The left trim tab remained attached at its surfaces, and the control rod was continuous from the fuselage bellcrank. The right trim tab sheared off the piano hinge and came to rest under the elevator, yet remained connected to the bellcranks and was continuous to the fuselage bellcrank (see figure 2 below). Figure 2: Trim tabs on the horizontal stabilizers Neither trim tab moved freely. The cockpit trim control, folded into the cabin, was still attached to the deformed airframe, with its selector found in the neutral position. Moving the control to nose-down shifted the surface upward, but the control would not move toward the nose-up position. When attempting to move the control aft, the control cable remained continuous to the aft bellcrank; however, a sleeve on the end attachment floated loosely and would bind against the sleeve housing (see figure 3). Figure 3: Trim control cable showing the sleeve binding An external examination of the engine revealed multiple fractures in the crankcase, with the oil sump and intake manifold broken apart. The spark plug electrodes were undamaged and, according to the Champion Spark Plugs Check-A-Plug chart AV-27, were consistent with normal operation but in worn condition. Valve and gear train continuity was established by rotating the crankshaft via the crankshaft flange (exerting force on the splintered propeller blades); compression was detected in all cylinders. Rust residue and oil were observed at all four rocker box areas. Removal of the cylinders revealed that the combustion chambers and barrels showed light scratches and corrosion. There was no evidence of foreign object ingestion or detonation. The valves were intact and undamaged; the No. 3 exhaust valve was red in color, consistent with high-temperature operation. There was no evidence of valve-to-piston face contact observed. The pistons and their respective rings were intact. The piston skirts were dark in color and exhibited numerous scuffs/grooves. Removal of the pistons revealed that the cam lobes and lifter faces showed evidence of wear. There was severe spalling on the faces of all the intake lifters and several exhaust lifters; there was also evidence of galling noted. The camshaft lobes did not appear rounded and no material deformation was observed on the lobes. Both magnetos were broken away from the engine and timing could not be ascertained. The magneto lead on the No. 3 cylinder was severed at the spark plug. Both produced spark upon rotation. There was no evidence of pre-mishap mechanical malfunctions observed during the examination of the engine. There was fluid consistent in odor and appearance with 100LL aviation fuel found throughout the fuel system. Corrosion was found in the fuel servo at the mixture arm in the air cavity diaphragm; an oily substance was also located in the air cavity. There was no fuel in the fuel distributor. The oil filte