WPR24FA083

The flight crew’s decision to conduct a post-maintenance stall test in an area of icing conditions, which resulted in wing contamination that significantly decreased the airplane’s critical angle of attack. Also causal was the airplane manufacturer’s lack of training and experience requirements for the flight crew to safely conduct the stall test, which resulted in an attempted remedial action that aggravated the aerodynamic stall and led to a loss of control from which they were unable to recover. Contributing to the accident was the flight crew’s failure to follow the test conditions regarding cloud clearance, altitude limit, visual meteorological conditions, and ensuring all external surfaces were free from ice.

The flight crew, comprised of the pilot-in-command (PIC) and second-in-command (SIC), was conducting a stall test in the airplane following the recent removal, inspection, and reinstallation of the wing leading edges and de-ice panels as part of routine maintenance. The airplane departed normally, entered a climbing right turn to the northeast, and leveled off about 20,000 ft mean sea level (msl). In its final minute of flight, the airplane entered a rapid vertical descent consistent with a flat spin and never recovered. The airplane was mostly consumed by postcrash fire and was highly fragmented, which precluded a complete and thorough wreckage examination of the airframe and engines. A review of the flight data showed no anomalies with the flight controls or engines, as the flight control surface movements were consistent with the flight control inputs and engine performance matched the power lever movement. Performance data indicated that before the airplane entered the spin, it decelerated and its pitch attitude increased, consistent with the flight crew preparing to perform their planned post-maintenance stall warning and identification system checks. The airspeed then slowed further, with flaps retracted for the first system check. Performance calculations showed that the stick shaker activated about 117.5 kts, one knot below the activation speed for the stick shaker at flaps zero. The stick pusher then activated at 113.5 kts, 3 kts above the stick pusher activation speed. Given the airplane’s weight and load factor, the shaker and pusher both activated at appropriate speeds; however, the airplane entered the stall at the same time the stick shaker activated, which provided no warning to the flight crew. The correct stall warning sequence should be stick shaker, stick pusher, then, if the pilot does not attempt a stall recovery, stall entry. The stall warning sequence for the accident flight, with the stall occurring at the same time the stick shaker activated, was likely due to a degradation in the relationship between lift and angle of attack (AOA) from wing contamination, either by icing or the airplane’s recent maintenance. A weather study showed that the airplane was in instrument meteorological conditions (IMC) during some or all of its climb from 5,000 ft through 16,700 ft msl and could have accreted up to 1 mm of ice on the wings during this time. An icing study determined that even a 2-minute icing encounter could have reduced the maximum coefficient of lift by up to 40% and reduced the stall AOA by up to 6°. Wing Contamination Two AIRMETs were issued about 2.5 hours before the accident flight for moderate icing throughout the airplane’s climb and cruise altitude for the stall test. In addition, video showed that the departure airport was surrounded by obscuration and precipitation during takeoff. Evidence showed that the flight crew was aware of the presence of sleet and “storm” conditions before departure during their interactions with maintenance personnel. The presence of these conditions should have given the flight crew pause before they executed the stall test, which required visual meteorological conditions and no icing according to the airplane’s Pilot’s Operating Manual (POM). The airplane recently underwent routine maintenance to inspect the wing leading edges for signs of cracks and corrosion. This inspection included an extensive inspection of the flight controls and wing ribs that required a removal of the leading edges, numerous inspection panels, and control surfaces. After maintenance, the wings were subject to multiple inspections for the reinstallation of the TKS panels and wing leading edges and a final post-maintenance inspection of the leading edges. While the postcrash fire prevented the investigation from determining if there was any wing contamination that was introduced by the maintenance team during reassembly, the number of post-maintenance and preflight inspections decreases the likelihood of improper maintenance. Stall Test Conditions During the flight, the crew complied with most of the stall test conditions required by the POM: an empty ventral tank, an operative stall identification system, and autopilot OFF. However, the pilots did not follow the cloud clearance and height limitations prescribed by the stall test procedure. Audio from the cockpit voice recorder also indicated that the flight crew knowingly chose to execute the stall test above clouds about 2,000 ft above the prescribed maximum altitude. There is also no evidence they attempted to verify that the external surfaces were free of ice after flying through IMC and icing conditions despite the airplane being equipped with an ice detection spotlight system that could have been used to illuminate the wing fairings. The airplane was also equipped with an ice detector that had to be manually activated by the flight crew as it was not directly connected to the airframe ice protection system. Flight recorder data and cockpit voice recorder audio indicated that the flight crew intentionally departed without engine ice protection and likely without airframe ice protection. The reason for this decision is unknown but may have been to prevent the de-icing fluid from contaminating the airflow over the wing during the stall test. After the airplane entered the stall, the flight crew input full left-wing-down aileron when the airplane abruptly banked right and applied full power and full aft control column, which aggravated the aerodynamic stall/spin. The flight crew’s attempted remedial action suggested that they were insufficiently trained for the flight and the brief guidance from the POM provided no clear instructions for the possible consequences of “unacceptable stall characteristics” referenced as a cautionary note or a proper recovery. Stall Test Pilot Qualifications The structural repair manual (SRM) offers only a broad definition of those qualified to perform the stall test flight, requiring that the pilot be “familiar with the stall identification system and stall characteristics” of the airplane. Further, the stall section of the POM states that “pilots conducting stall checks should have prior experience in performing stalls in the Hawker and must be prepared for unacceptable stall behavior at any point leading up to and throughout the maneuver.” In the previous year, the flight crewmembers attended separate simulator training sessions at a commercial facility for the SIC’s initial training and the PIC’s recurrent training. These courses covered the operation of the stall warning and identification system (shaker and pusher), which is focused on recognizing and avoiding stalls. The simulator training was not designed to teach full stalls, including stall entry, or to prepare the crew for a possible uncommanded roll as described in the stall section of the POM. Although the PIC participated in a stall test flight 4 years before the accident, it was likely with limited involvement in the test as he was SIC at the time. The SIC for the accident flight had not participated in a stall test before the accident flight. Therefore, it is unlikely that the flight crewmembers’ simulator training on the stall warning and identification system and the PIC’s previous participation in a stall test flight adequately prepared them to safely conduct a stall flight test or address any unacceptable stall behavior. The accident was the result of the flight crew’s decision to conduct a post-maintenance stall test in an area of icing conditions, which resulted in wing contamination that significantly degraded the airplane’s critical angle of attack. The airplane manufacturer’s lack of training and experience requirements to ensure flight crew preparedness to safely conduct the stall test resulted in an attempted remedial action that aggravated the aerodynamic stall and led to a loss of control from whi

The PIC and SIC completed their most recent training at a commercial simulator training facility. According to one of the facility instructors, “we will do a demonstration especially for the initial class of…stick pusher on the airplane. That’s up a simulator high altitude like, I don’t know, 30,000 ft or so, and we’ll have them slow right down and we demonstrate the features of the stall protection system. So, once they get into the caution regime. We do this on autopilot, because we demonstrate that once the stick shaker goes off, your autopilot turns off. So now you’ve got to fly the airplane, but then we have them wait because if you haven’t recovered by then, you’re going to get a stick pusher, and that’s hydraulically actuated. So it’s dramatic and they just descend down, and it’s a demonstration. There’s no proficiency required for that.” An instructor at the facility reported that they do not teach full stalls. The PIC completed his most recent recurrent training in a Hawker 800XPi simulator at the same facility in October 2023, about 9 months before the accident. He was rated satisfactory in all categories including stall prevention with flaps, without flaps, on landing, and unusual attitudes. The SIC completed initial training in the Hawker 800XPi 4 months before the accident, and also received a satisfactory rating in every category, including the stall prevention categories referenced above. The initial training included 60 hours of ground training and 31.4 hours in the simulator; 17 hours as the pilot flying and 14.4 hours as the pilot monitoring. One simulator session dated October 24, 2023, noted that the pilot received a rating of “1” under the inflight maneuver category “Stall Prev[evention] Part/Flap,” which indicated “insufficient progress in acquiring the knowledge and skills to achieve proficiency.” The SIC also received a rating of “1” in six other areas during the same simulator session. The following day the SIC repeated the same simulator sessions and achieved a grade of “3” in each area, which is considered “proficient. The exercise has been mastered and performance is consistently repeatable.” During the checkout on October 28, 2023, the pilot received a grade of “S” (satisfactory) for all events including the “Stall Prev” areas. The operator’s records indicated that the PIC conducted an inflight stall test once before, on December 7, 2019, and was the second-in-command on that flight. There was no record that the SIC had conducted an inflight stall test before the accident flight. According to the type certificate data sheet (TC), the Hawker 900 XP was launched in 2006 by Raytheon Aircraft Company. The type certificate was transferred to Hawker Beechcraft Corporation on March 26, 2007, and then Beechcraft Corporation on April 12, 2013, and on October 12, 2016, to Textron Aviation, Inc., the TC holder at the time of the accident. Before the accident, the airplane had undergone several phase inspections in addition to numerous required task inspections as a part of routine maintenance. The maintenance facility released the airplane on the day of the accident. Stall Test Requirement The recent maintenance that required the stall test was outlined in the structural repair manual (SRM), which required the removal of the wing leading edges every 4 years to access and remove the de-icing panels for a visual inspection of cracks and signs of corrosion (SRM task code No. 570026). According to Section 57-41-00 of the repair manual, “WINGS LEADING EDGE AND LEADING EDGE DEVICES WING LEADING EDGE GENERAL REPAIR,” “In accordance with the Flight Manual procedures the airplane must be test flown by a pilot familiar with the stall identification system and stall characteristics of the 750, 800, 800XP, 850XP, and 900XP series if: (1) The leading edge assembly was removed as a whole for any reason (2) Two or more TKS [de-icing] wing distribution panels on one side are removed or installed” This section of the SRM also refers the operator of the Series 900XP airplane to Section V of the Pilot’s Operating Manual for the procedure and technique to complete the stall test.The manual requires that, before taking the test flight, the operator verify that the Stall Warning and Identification System is functioning properly while on the ground. Stall Test Procedure from Pilot’s Operating Manual (POM) The airplane’s POM contained instructions on operating limitations, system descriptions, flight planning, flight handling, and ground information. Section V Sub-section 1 (Page 1-16) of the POM contains the required conditions and techniques to complete the stall test, separate from the SRM. The required conditions for the stall test included an altitude above 10,000 ft above ground level, 10,000 ft above clouds and below 18,000 ft mean sea level. In addition, this check flight should only be conducted during day visual meteorological conditions with a good visual horizon, with the autopilot disengaged, an operative stall identification system, external surfaces free of ice, the ventral tank empty, and weather radar on standby. The POM also noted that stalls should be made in “wings level” flight with thrust set and maintained throughout the approach to stall and recovery. In addition, the airplane should be trimmed to 1.4 Vs1 and airspeed should be reduced at no more than 1 kt/second. The flight crew is also required to avoid any rapid or violent control movements during the approach to the stall, particularly at airspeeds below the activation of the stick shaker. Accident Flight Stall Test According to the flight data recorder, the flight was flown with an empty ventral tank, and the autopilot was switched off before the flight crew began the stall test. The stall identification system was operative at the time. The flight was flown in VMC conditions about 5,000 ft above clouds and about 2,000 ft above the maximum prescribed altitude of 18,000 ft msl. The flight crew’s visual horizon at the time and weather radar status are unknown. The crew decelerated the airplane at 1kt/second as prescribed by the published stall technique while maintaining level flight and no rapid movements were observed in the FDR data until the stall break. The flight crew’s control inputs are captured in the FDR section of this report. Stall Characteristics The stall test section of the POM also contained a description of the stall characteristics with a “Caution” advisory. The section noted a moderate roll was acceptable provided that the use of the ailerons limits the roll angle to no more than 20°. In addition, the advisory discussed the potential for experiencing “aileron snatch,” which should not be considered acceptable. In the case of aileron snatch, the POM advised that elevator control should be moved forward to decrease angle of attack and to allow the return of normal aileron control. The section also advised the pilot to be prepared to recover from an unusual attitude. According to the POM’s “Caution” advisory: PILOTS CONDUCTING STALL CHECKS SHOULD HAVE PRIOR EXPERIENCE IN PERFORMING STALLS IN THE HAWKER AND MUST BE PREPARED FOR UNACCEPTABLE STALL BEHAVIOR AT ANY POINT LEADING UP TO AND THROUGHOUT THE MANEUVER. The POM also warned that there is no natural stall warning or aerodynamic buffet before the stall. The airplane type certificate holder (Textron Aviation) provided the following definition of aileron snatch: Roll upset can be caused by airflow separation (aerodynamic stall), which induces self-deflection of the ailerons and loss of, or degraded, roll handling characteristics. For example, on an airplane with a fully functioning ice protection system, a roll upset can occur when the icing conditions are severe enough to result in water droplets that flow back past the wing’s ice-protected leading edge. When these