PHOENIX, AZ, USA
N86BP
PRAKER THORP T18C
AFTER THE FIRST TEST FLIGHT FOR THE HOME BUILT AIRPLANE, THE TEST PILOT TOLD THE OWNER/BUILDER THAT HE HAD TO HOLD 35 POUNDS OF FORWARD STICK PRESSURE TO MAINTAIN LEVEL FLIGHT IN CRUISE. THE BUILDER AND TEST PILOT REMOVED THE STABILATOR ANTI-SERVO TAB CONTROL ARMS AND BENT THEM IN A VICE WITH A MALLET. THE ARMS WERE SUBSEQUENTLY REINSTALLED AND THE AIRPLANE TOOK OFF ON A SECOND TEST FLIGHT. VIDEO TAPE SHOWED THE SECOND TAKEOFF INITIAL CLIMB TO BE STEEPER THAN THE FIRST. SHORTLY AFTER TAKEOFF, THE TOWER ASKED THE PILOT WHY HE WAS TURNING RIGHT INSTEAD OF THE REQUIRED LEFT TURN. HE STATED 'I GOT PROBLEMS HERE.' THE AIRPLANE CONTINUED TO CLIMB AND WAS SEEN TO ENTER A RIGHT SPIN FROM A NOSE HIGH, STEEP RIGHT BANK ATTITUDE. THE TEST PILOT HAD SEVERAL YEARS TEST EXPERIENCE AND 1,000 HOURS IN THE ACCIDENT MAKE AND MODEL. THE REASON FOR THE STABILATOR CONTROL MALFUNCTION WAS NOT DETERMINED. HOWEVER, A MAINTENANCE INSTRUCTION ADJUSTMENT RIGGING CHECK WAS NOT PERFORMED AFTER THE REINSTALLATION OF THE ARMS.
HISTORY OF FLIGHT On May 24, 1994, at 1329 mountain standard time, a Praker home built Thorp T18C, N86BP, was destroyed when it impacted the ground about 1/2 mile southeast of the Deer Valley Municipal Airport, in Phoenix, Arizona. The airplane, flown by a commercial pilot, had just departed on a local test flight. There was no flight plan filed and visual meteorological conditions prevailed. The pilot, the sole occupant, received fatal injuries. The pilot, who was an acquaintance of the owner/builder, offered to test fly the airplane which had recently been completed. The first test flight took place earlier in the day, after which the pilot told the owner that he needed to maintain about 35 pounds of forward pressure on the stick to maintain level flight. The pilot and the owner subsequently removed the stabilator anti-servo tab control arms and modified them by bending, then reinstalled the arms. The pilot then initiated the second test flight. The tower had cleared the flight for a left turn out after takeoff; however, the controllers noticed the airplane turning right and queried the pilot. He replied "I got problems here." The airplane was observed entering a steep right bank, nose high attitude at between 1,000 and 1,500 feet AGL and then enter a two turn spin to the right. The witnesses stated that the spin stopped, but immediately reversed itself and the airplane was spinning left until they lost sight of it. The airplane impacted in a land fill adjacent to the airport. The initial start and taxi and both takeoffs were recorded on video tape. The video of the taxi out showed that when the stabilator was parallel to the ground or about 10 degrees stabilator nose down relative to the fuselage water line, the stabilator anti-servo tab was in the trail or faired position. The tape showed that as the stabilator was moved further nose down, the tab went up; however, the amount could not be determined. The first takeoff was characterized by a shallow climb angle and some wing oscillations. During the second takeoff, the climb angle was steeper than the first. There was no video tape of the maneuvers immediately prior to or during the loss of control and descent. WITNESSES In addition to the tower operator, and the airplane owner and his wife, there were several ground witnesses. Their statements are summarized in the Phoenix Police Department report contained as a separate attachment to this report. All of the witness statements were consistent, with the exception that some ground witnesses stated that they thought they heard the engine sputtering during the descent. PERSONNEL INFORMATION In addition to holding commercial and instructor ratings, the pilot also held an airframe and powerplant mechanic's license. He had received a Class II medical certificate on August 3, 1993, with a limitation for the possession of corrective lenses for near vision. A pair of corrective glasses were found in the wreckage. The pilot's personal logs were not found during the investigation; however, on his last medical application, he had indicated a total time logged of 5,300 hours, 50 of which had been in the preceding six months. In addition, the pilot built the first Thorp T18C, and had flown over 35 others during his career. Fellow pilots estimated that he had accumulated over 1,000 hours total time in the accident make and model. Acquaintances of the pilot stated that he had conducted test programs on numerous other Thorp airplanes. AIRCRAFT INFORMATION The airplane had been under construction by the owner for a period of over 11 years and had been issued a special airworthiness certificate to operate in the experimental category on April 2, 1994. According to the owner, the airplane had undergone an engine test run and leak check on May 22, 1994, for about 15 minutes and an engine run and taxi tests on May 23, 1994, for another 15 minutes. On the day of the accident, the first test flight had taken place at 0930 and lasted 20 minutes. It was after the first test flight that the pilot stated the airplane required 35 pounds of forward stick pressure to maintain level flight at cruise power. The owner and pilot decided to remove the anti-servo trim tab arms and adjust them. The tab arms were placed in a vice and bent by means of a rubber mallet. While the exact measurements could not be determined, either before or after the accident, the owner stated that it appeared that they were bent about 1/2 inch at the link end. The trim tab arms were identical on each side and drove the anti- servo tab in a direction opposite of the leading edge of the horizontal stabilator. During construction, the tab arms were initially installed and adjusted by cutting the arms to the desired length prior to welding on the attachment clevis. The Thorp T18 Newsletter #34, dated November 19, 1971, stated in part that prior to a first flight, the arm adjustment should be checked by a procedure of aligning the tabs in the trail position with the horizontal stabilator parallel to a rivet line at water line 42, and once so positioned, ensuring that the stick was in a position 7.5 degrees forward of a line perpendicular to water line 42. The owner/builder stated that he complied with all of the Thorp plans and had copies of all the newsletters, but, that he did not recall performing the rigging check specified in Newsletter #34. The airplane had last been refueled with 8 gallons of 100LL aviation gasoline on the day prior to the accident and departed with approximately 12 gallons of fuel on board. Estimates indicated that the airplane was within prescribed limits for weight and center of gravity at the time of the accident. COMMUNICATIONS A rerecording of the air traffic control tower tapes revealed the tower cleared the flight to taxi to runway 7L and asked the pilot to say his direction of flight. The pilot responded that he wanted to turn out to the north. Some time later, the pilot called the tower and stated "we got lost down here," and the tower issued alternate taxi instructions. The flight was subsequently cleared for takeoff on 7L. About one minute later, the tower called the airplane stating "86BP verify you want to go northbound." This was when the tower controller noticed the airplane was turning right, southbound. The pilot responded immediately with "I got problems here." The tower then advised the pilot that he was cleared to land any runway and asked if he needed any assistance. There was no response until about 10 seconds later when the voice of the pilot was heard in a garbled transmission which lasted approximately one second. WRECKAGE AND IMPACT INFORMATION The primary impact crater was found on a compacted road in a land fill 1/4 mile southeast of the departure end of runway 7L, on a measured heading of 300 degrees. The main impact point consisted of a six inch deep indentation in the road which corresponded to the propeller hub, there were no other ground scars. The main wreckage came to rest six feet southeast of the impact point. Remnants of one propeller blade were found immediately adjacent to the crater and the remaining blade was undamaged and remained attached to the hub. The engine, fire wall, and instrument panel were displaced aft and upwards. Both wings exhibited compression damage aft to about mid-chord, and the fuselage was accordioned to a point just aft of the baggage compartment. The right horizontal stabilator separated from the attachment yoke and rotated forward into the fuselage skin. The stabilator was found in the wreckage area. The right wing had rotated forward and broken the wing folding mechanism. Control continuity was established from the rudder to the pedals. The stabilator push/pull tube was found separated in compression in two places, both were overstress type fractures. Continuity was established to the stabilator anti-servo tabs, with the exception of the right tab arm, which had separated in tension when the stabilator separated from the airframe. Stabilator trim was measured as being 7.5 degrees nose up. Control continuity could not be established to the ailerons or flaps; however, all of the separations appeared to be overstress and impact related. It was established that the flaps were up at impact. The majority of the cockpit instrumentation was destroyed; however, the airspeed indicator needle was found stuck on 113 miles per hour and the lift reserve indicator needle was stuck at the top end of the red arc, above stall angle of attack. Post-accident examination of the flight control construction and rigging revealed minor construction deficiencies, none of which would have rendered the airplane uncontrollable. The pre-impact rigging of the anti-servo tabs system could not be determined due to separations and impact related distortions in the components. Following recovery, the engine was disassembled and examined. The engine had sustained damage due to frontal and bottom impact. Rotational continuity and compression to all cylinders was established once the rear accessory case was removed. The oil pump driving impeller was found jammed into the side of the case under rotation and the pump could not be rotated by hand. No evidence of oil starvation was found in the engine and the recovered oil appeared normal. All of the spark plugs had normal burn patterns and there was no evidence of soot. The carburetor was equipped with a metal float and clear, blue fuel was found in the accelerator pump cavity and the bowl. The left magneto was rotated and fired on all four leads. The right magneto was not tested due to impact damage. MEDICAL AND PATHOLOGICAL INFORMATION An autopsy and toxicology testing was ordered and performed on the pilot. The autopsy was performed by the Maricopa County Medical Examiner's Office and the toxicology testing was performed by the FAA Civil Aeromedical Institute. There were no significant findings. ADDITIONAL INFORMATION Wreckage Release: The wreckage was released to the owner on November 17, 1994. All of the retained components were returned with the release. No original aircraft records were retained.
THE PILOT'S INABILITY TO PREVENT AN INADVERTENT STALL AND HIS INABILITY TO RECOVER DUE TO A RESTRICTION IN THE MOVEMENT OF THE STABILATOR FOR UNDETERMINED REASONS. A FACTOR WAS THE FAILURE OF THE OWNER/BUILDER AND THE PILOT TO ACCOMPLISH A CONTROL RIGGING CHECK PRIOR TO THE FLIGHT.
Source: NTSB Aviation Accident Database
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