Pewaukee, WI, USA
N247DH
FAIRCHILD SA227-AT
The flight crew reported that while on an instrument landing system (ILS) approach with the autopilot system engaged in approach mode, they noticed that the airplane flight director indicated a climbing right turn but the airplane was still tracking the localizer and glideslope. The airplane’s ice protection was on, and no visible ice had accumulated. They reported that they disconnected the autopilot, and the airplane suddenly rolled to the right. They attempted to regain control by increasing engine power and applying counteractive control inputs, but the airplane impacted the ground in a near-wings-level attitude. Examination of the airplane’s primary flight control system and engines after the accident did not reveal any defects. The rudder trim was neutral, and the pitch trim was airplane nose up. Aileron trim could not be determined. Examination of the airplane’s autopilot components revealed deficiencies in the yaw damper system that rendered it inoperative; however, on the accident airplane the yaw damper system was an optional component and was not necessary for airplane operation. Testing of the remaining autopilot components revealed some deficiencies that could have degraded performance but would not have resulted in a complete failure of the automatic flight control system. A performance study based on ADS-B data showed that the airplane intercepted the localizer and glideslope for the ILS approach and was descending in a level attitude. While maintaining the ILS approach guidance, the airplane slowed below the 130 knots (kts) airspeed that the crew stated was the desired approach speed. The airspeed continued to slow to about 102 kts when the ADS-B data indicated that the airplane rolled slightly to the right, likely corresponding to the flight crew’s description of events after they disconnected the autopilot. The airplane continued to slow below 100 kts and the airplane was at a bank angle of 27° right wing down. Subsequently, the descent rate increased to over 4,000 ft/min and airspeed increased while groundspeed remained between 80 and 90 kts. The airplane rolled sharply to the left. The sudden roll and loss of altitude after reaching a low airspeed was consistent with an aerodynamic stall. Based on the available evidence, the airplane entered an inadvertent aerodynamic stall due to exceedance of the critical angle of attack after the flight crew allowed the airspeed to decay during the instrument approach. Although an unknown anomaly in the flight director system could have resulted in the crew becoming fixated on an errant flight director indication at the expense of airspeed control, the postaccident component examination was not able to explain the errant flight director indication that the flight crew described.
On November 15, 2022, about 1500 central standard time, a Fairchild SA227-AT airplane, N247DH, was substantially damaged when it impacted the ground near Pewaukee, Wisconsin. Both flight crew members received minor injuries, and the passenger was not injured. The flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 135 as an on-demand cargo flight. The flight crew reported that during the climb after departure, and during the cruise portion of the flight they occasionally experienced a momentary right yaw of the airplane. They determined that even though they experienced an occasional momentary yaw that the yaw damper system was otherwise operating and not interfering with other systems, so they elected to leave the yaw damper engaged. The airplane was on an ILS approach to runway 10 at the Waukesha County Airport (UES) when the accident occurred. After air traffic control (ATC) had cleared them for the approach the airplane intercepted the localizer and glideslope. During the approach, the flight crew noticed that the command bars on the airplane’s flight director were indicating a climbing right turn. According to the flight crew, at that time, the autopilot was in approach mode and was still tracking the localizer and glideslope, airspeed was about 135 kts and not trending up or down significantly, and everything except the flight director looked to be normal. The flight crew decided to disconnect the autopilot and continue the approach, and manually fly the airplane for the remainder of the flight. The crew reported that once the autopilot was disconnected, the airplane immediately rolled to the right. Both pilots got on the controls and increased engine power to attempt to regain control of the airplane. They were able to level the airplane; however, it impacted the ground in a near-wings-level attitude. During the accident sequence the airplane contacted trees, separating the wings from the airplane and resulting in substantial damage to the wings and fuselage. Figure 1: The airplane at the accident site. (Photo courtesy of FAA) Examination of the airplane after the accident showed that the airplane’s flight control system was intact except for breaks attributable to the impact sequence. The airplane was equipped with manual trim only for yaw and roll. Electric trim was available for pitch, which moved the leading edge of the horizontal stabilizer up or down as necessary. The rudder trim tab was found in a neutral position. The horizontal stabilizer’s leading edge was down, near the maximum trim marking on the vertical stabilizer, indicating nose up trim. The aileron trim was not determined due to the damage to the airplane’s wings. The airplane’s engines were free to rotate and no preimpact anomalies were detected. Further engine examination was not performed, and the flight crew did not claim any engine operation issues during interviews. The airplane’s autopilot components and navigation receivers were removed from the airplane for testing. Although the accident airplane was equipped with a yaw damper system, the system was optional and was not necessary for aircraft operation. Examination of the yaw damper servo revealed that the clutch would not engage, yielding the yaw servo inoperable. Additionally, the yaw damper computer was not receiving signals from its internal accelerometer. This would result in the yaw damper computer interpreting that the airplane was always in coordinated flight. As a result, the output signal from the yaw damper computer remained constant in the neutral position. Testing of the remaining avionics components revealed some parameters that were outside of test specifications that could have degraded performance but would not have resulted in a failure of the automatic flight control system. The airplane did not have a cockpit voice recorder or flight data recorder. None of the avionics had capability for recording flight parameters. ADS-B data showed that the airplane took off from New Orleans Lakefront Airport (NEW) at 1204, climbed to near 20,000 ft, and flew north. About 1440, the airplane began to descend toward UES. A performance study of the ADS-B data showed that at 1458:15, while at an altitude of 2,160 ft and an airspeed of 192 kts, the airplane turned inbound on the localizer for runway 10 at UES. At 1459:00 the airplane slowed below 160 kts airspeed, which the crew reported in interviews was the minimum speed for flight in icing conditions. The flight crew reported that ice protection was on and that no visible ice had accumulated. At 1459:07, the airplane began descending along the ILS glideslope for runway 10 to UES. At 1459:14, the airplane slowed through 130 kts, the selected approach speed according to crew statements. The airplane continued to descend along the glideslope and localizer while slowing, reaching an airspeed of 102 kts by 1459:28. The performance study showed that after capturing the runway heading and glideslope, the airplane was descending in a level attitude. At 14:59:28 the airplane descended below the glideslope while beginning a slight turn to the right, likely corresponding with the flight crew’s description of events that occurred after disconnecting the autopilot. Initially, the airplane’s descent rate was a steady 1,500 ft/min while the airspeed continued to slow. By 14:59:34, the airspeed was below 100 kts and the airplane was at a bank angle of 27° right wing down. Subsequently, the descent rate increased to over 4,000 ft/min and airspeed increased while groundspeed remained between 80 and 90 kts. The airplane rolled sharply to the left. The sudden roll and loss of altitude after reaching a low airspeed was consistent with aerodynamic stall. At 1459:46, the airplane altitude was 1,260 ft, which was about 400 ft above ground level. The final two data points indicate that the descent rate had been slowed and the wings were near level, which was consistent with the airplane’s orientation when it impacted the ground as well as the description from the flight crew.
The flight crew’s failure to maintain adequate airspeed and their exceedance of the airplane’s critical angle of attack during the instrument approach, which resulted in an aerodynamic stall. Contributing to the accident was an undetermined anomaly in the airplane flight director system which resulted in the flight crew likely becoming fixated on the anomaly at the expense of airspeed control.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
