WPR17FA185

The pilot's failure to maintain adequate airspeed while maneuvering in the airport traffic pattern for landing, which resulted in an exceedance of the airplane's critical angle of attack, a subsequent aerodynamic stall at an altitude too low to recover.

The private pilot, who was the builder of the experimental, amateur-built airplane, was approaching the airport for landing on a 3-mile left base leg at the conclusion of a cross-country flight. Shortly after being cleared for landing, witnesses observed the airplane make a steep left turn and dive toward the terrain. Ground scar analysis and wreckage fragmentation revealed that the airplane collided with terrain in a near-vertical, nose-low attitude; the wreckage was consumed by a postcrash fire. Examination revealed no mechanical malfunctions or anomalies that would have precluded normal operation of the airplane or engine, although a thorough examination of the flight control system could not be completed due to the extent of the thermal damage. The airplane was equipped with a 'cruciform' (mid-mounted horizontal stabilizer) tail. Several years before the pilot completed the airplane, the kit manufacturer disseminated a report to owners regarding the design and aerodynamic characteristics of the cruciform tail. It stated that the airplane's tail "may stall on approach and turbulent conditions or upon crossing another airplane's wake (or its own wake in a turn)" and that "rapid action must be taken to avoid diving straight into the ground." A report partially sponsored by the Federal Aviation Administration detailing the airplane's aerodynamic characteristics stated that, "during post stall and high yaw maneuvering the horizontal/elevators seemed to have inadequate effect" which "seemed to be caused by blanking of the horizontal tail and elevators by the vertical stabilizer at the highly yawed condition." Given these reported characteristics and the lack of mechanical anomalies found, it is likely that the pilot exceeded the airplane's critical angle of attack while turning from the base leg to final approach and the airplane entered an aerodynamic stall; the pilot was unable to recover from the near-vertical descent at low altitude.

The 1355 weather observation at S33 included: clear sky, 10 miles visibility, temperature 75°F, dew point 41°F, and altimeter setting of 30.15 inches of mercury. The pilot, age 58, held a private pilot certificate issued in January 2012 with ratings for airplane single-engine land and instrument airplane. He held a third-class Federal Aviation Administration (FAA) airman medical certificate issued in October 2016, with the limitation that he must have glasses available for near vision. On the application for that medical certificate, he reported a total flight time of 612 hours. The pilot's personal flight records were not recovered. The accident site was located on the north-facing slope of a canyon comprised of soft dirt and trees. The wreckage was about 1.1 nautical miles from the approach end of runway 34 on a bearing of 025° (see figure 2). Figure 02: Wreckage Location The first identified points of contact were freshly-severed tree limbs adjacent to the main wreckage. The tree was located in a slight ravine in the ebb of the canyon wall. The debris field was primarily contained in the area of the main wreckage on a 60-70° slope. The main wreckage sustained severe thermal damage and comprised the outboard right wing, empennage, engine, and the charred remnants of the fuselage. The cabin was completely consumed by fire. The inboard right wing was located upslope of the engine and was identified by the shape of the wingtip and a small blue/green shard of material embedded in the navigation light area. The left wing was downslope of the engine toward the ravine. Under the wing were parts of the cockpit area and instrument panel, identified by the seat frames, rudder pedals, and burned remnants of the radios. The engine came to rest on its left side with the Nos. 2, 4, and 6 cylinders completely imbedded in the dirt (see figure 3). A portion of one propeller blade was visible and the tip had melted. The propeller remained attached to the engine at the crankshaft flange. The spinner was bent around the hub and the bottom blade was broken free from the hub. The empennage was thermally destroyed and only portions of the flight controls were identifiable. The tail section was located in the ravine at the bottom of the debris field (about 15 ft from the engine). Figure 03: Engine at Accident Site Examination of the airframe revealed that the control materials constructed of steel were thermally damaged and a majority of the airplane that was composite material or aluminum was consumed by fire. Pitch control was accomplished by two control columns connected to a mixing assembly, which continued aft via a series of two steel rods and two steel tubes running centerline to the elevators (a total of six rod end bearings were used in the assembly). Four rod end bearings exhibited signs of damage from overload. Two rod end bearings, both of which connected to the same steel bellcrank, exhibited no signs of overload or elongation and were lacking attachment hardware. The aforementioned two rod ends were shipped to the NTSB Materials Laboratory for analysis. No evidence or markings consistent with the presence of fastening hardware were observed in either bearing bore. Because of the severe thermal damage to the wreckage, it could not be determined if the absence of material was due to it not being fastened at the time of impact or if the material was thermally consumed after impact. Rotation of the engine was not possible due to binding of various components as a result of impact damage; compression could not be obtained. The cylinder combustion chambers were examined upon removal of the Nos. 1, 3, and 5 cylinders and borescope examination of the remaining cylinders. The combustion chambers remained mechanically undamaged and there was no evidence of foreign object ingestion or detonation. The valves were intact and undamaged. There was no evidence of valve-to-piston-face contact. The gas path and combustion signatures observed at the spark plugs, combustion chambers, and exhaust system components displayed coloration consistent with normal operation. Ignition system continuity could not be established due to thermal damage. The engine oil sump was crushed upward against the internal engine components. Examination of the McCauley propeller, model B3D32C417-C, serial number 900150, revealed that all blades sustained thermal damage. Blade 1 was broken free with leading edge polishing, chordwise scratches, and twisting. Blade 2 was loose in the hub with its tip melted away. Blade 3 was bent aft at its tip. Examination of the airframe and engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation. A detailed examination report is contained in the public docket for this accident. The Wheeler Express CT is an experimental, amateur-built airplane that is sold as a kit. The composite construction airplane was a four-place, low-wing, tricycle landing gear monoplane equipped with a 'cruciform' (mid-mounted horizontal stabilizer) tail. The pilot completed the build in 2002 as the original design. The accident airplane, serial number 145, received a special airworthiness certificate in the experimental category in August 2002. The airplane was equipped with a Lycoming IO-540-KIG5 engine, serial number L-17422-48A, rated at 235 shaft horsepower. The maintenance logbooks were not recovered. The airplane was last fueled before departure from San Carlos with the addition of 58.9 gallons of 100LL aviation fuel. Airplane Design In 1991, Wheeler Express sent a letter to owners addressing the possibility of deep stalls occurring in the tail; the letter stated that if the builder installs the tail correctly there would not be abnormal stall characteristics. Wheeler Express was then purchased by Express Design Inc (EDI), who sent a letter in 1995 to builders and owners detailing a report about the design and aerodynamic characteristics of the cruciform tail. In pertinent part, it stated: At the forward CG, flaps down, approach condition, a larger tail requires much less up elevator to trim. Very small (and therefore heavily loaded) tails may stall on approach and turbulent conditions or upon crossing another airplane's wake (or its own wake in a turn). If the tail stalls in the approach condition, rapid action must be taken to avoid diving straight into the ground… We believe therefore that because the Express CT can experience uncommitted pitch down motion under certain flight conditions, that certain actions are required by EDI and by Express CT builders and owners specifically… Because certain flight conditions such as turbulent air can cause the uncommitted pitch down without pilot involvement, EDI believes that each owner of an Express CT kit for aircraft should seriously consider updating their kit or airplane with the S-90 tail update available from EDI. A Wheeler Express performance report, sponsored and funded by the FAA and the Experimental Aircraft Association (EAA), was completed in 1997. The performance report identified that the 'cruciform' tail (mid-mounted horizontal stabilizer), wide tapering fuselage, and highly swept vertical stabilizer combined to produce low yaw stability. In part, it stated: The tail power seemed to be adequate for most regimes of flight, however, during post stall and high yaw maneuvering the horizontal/elevators seemed to have inadequate effect. This seemed to be caused by blanking of the horizontal tail and elevators by the vertical stabilizer at the highly yawed condition. More testing is definitely in order to study the flow field around the tail in this situation and to develop improved tail effectiveness. On August 19, 2017, about 1352 Pacific daylight time, a Rich Wheeler Express CT, N246TM, impacted the wall of a canyon while on approach to land at Madras Municipal Airport (S33), Madras, Oregon. The private pil