20060504013189I

(-23) On may 4, 2006, at approximately 1758 local CDT, a Hawker BAE-125-800A, N71MT, experienced an inflight loss of control during a maintenance test flight northwest of Lincoln, Nebraska. The aircraft had just come out of extensive maintenance and refurbishment. On board the flight were two Raytheon test pilots and four passengers that Raytheon considered crucial for the test flight. The first maneuver to be performed was a clean stall. Prior to the flight the crew calculated the stick shaker activation speed to be 115 knots, pusher speed at 107.5 knots, and where aerodynamic buffet speed to occur at 105.5 knots. The aircraft was level at 17,000 feet MSL with the autopilot engaged in altitude hold and heading hold modes. As the aircraft slowed to approximately 126 knots, verified by FDR date, the right wing suddenly stalled and the nose dropped through the horizon and the aircraft continued to roll to the right in a near vertical descent. The aircraft entered a cloud layer below them and due to the attitude of the aircraft, the gyros tumbled. The crew was unable to determine the attitude of the aircraft until they exited the cloud layer. The aircraft continued to roll to the right, about three turns total when it experienced a rapid roll reversal to the left. The aircraft rolled about 2 to three turns to the left. When they exited below the base of the cloud layer, the captain saw only ground through the windshield and immediately pulled back on the yoke and regained control at approximately 7,000 feet MSL. The crew returned to the Lincoln airport, declared an emergency and made a no-flap landing on runway 36. No further complications were noted. The captain stated that there were no uncommanded control inputs from the auto flight system. The aileron and rudder controls were neutral when the stall occurred. He also stated that there was not any aerodynamic buffet prior to the right wing stall. The aircraft spent two and a half months at duncan aviation for trouble shooting. Raytheon sent an aerodynamicist engineer to measure the wing to check for conformity and damage. The aerodynamicist found nothing remarkable. Duncan aviation performed a symmetry check of the aircraft. The symmetry check found nothing remarkable. Raytheon developed a structural inspections checklist for duncan aviation to perform to check for damage. Duncan aviation performed this checkand found no damage. Duncan aviation performed a pitot static system leak test and calibration check, a fuel quantity system check, a stall warning system check (long check), and an auto pilot system check. There were no anomalies found. Duncan aviation also checked cable tensions and control throws and all checked normal. The aircraft was determined to be airworthy by duncan aviation and Raytheon aircraft. A test flight was scheduled to be conducted by Raytheon aircraft using their experimental test pilots. Raytheon aircraft departed the Lincoln airport for wichita, Kansas to perform the test flight close to their headquarters because that is where all their expertise is located. The pilots first test flight tried to duplicate the stall that the maintenance test pilot encountered. During this test flight, the pilots were unable to duplicate the stall or discover any abnormal flight characteristics. All of the stalls occurred at the calculated speeds and the stall warning system functioned correctly and activated at the correct indicated airspeeds. A second test flight was performed to try to aggravate the stall. The pilots used half rudder deflections during the stall to try to get a wing to drop. They were unable to get the airplane to behave in any abnormal way. During the investigation of the air traffic control voice tapes revealed that the clouds in the area had bases of 9,000 feet and tops of 15,000 feet. The crew of N71MT also was level at 12,000 feet MSL for several minutes performing some kind of checks. The temperature at the surface was 15 degrees celsius which would make the temperature at 12,000 feet MSL -9 degrees celsius, assuming a standard lapse rate of 2 degrees celsius per 1,000 feet of altitude. The crew then requested an altitude of 17,000 feet MSL. The voice tapes also revealed that the crew had difficulty in locating an area that was VMC to perform their stall tests. Inspector privacy data omitted ^ recognized the voice of another pilot on the frequency that was in the same area and altitudes as N71MT. This pilot provided a statement to inspector privacy data omitted ^ that he encountered light icing conditions during his climb to 16,000 feet. This pilot stated that he accumulated about 1/2 inch of mixed ice, clear and rime. Two passenger statements mentioned that they saw ice on the leading edge of the wings. One stated that the ice seemed to dissipate by either warmer temps or the crew activation of the tks anti-ice system. During the interview with the crew they stated that they never activated the tks system. During this same interview the captain stated that the ice detection light illuminated at gear retraction. They knew that this was a malfunction because they had not entered any visible moisture yet. They elected to continue the flight because they felt that they would not enter any icing conditions. During both test flights this light illuminated again and malfunctioned. The first test flight had a chase plane and the crew verified that they did not have any ice with the chase plane. The flight also never entered any visible moisture during either test flight. Due to the absence of any malfunction with the aircraft systems, any abnormal flight characteristics after flight test, and the addition of statements from passengers and another pilot, it is possible that the wing of this aircraft was contaminated with ice during the stall. The aircraft flight manual states that the clouds should be at least 10,000 feet below the aircraft prior to stalls, the autopilot disengaged, and not to be made in icing conditions. Due to the evidence of the voice tapes, it is doubtful that the clouds were 10,000 feet below the aircraft. Since this incident, Raytheon aircraft has developed a training syllabus for all of their pilots on stalls. Raytheon is also going to have all company pilots be trained in and perform the stalls during their recurrent training. Raytheon is also making the airplane flight manual more specific to pilots that the autopilot is not to be engaged during the stall. The autopilot is to be disengaged prior to the actual stall.