CEN15FA136

The pilot's failure to maintain adequate airspeed during the instrument approach in night instrument meteorological conditions, which resulted in the airplane exceeding its critical angle of attack and an aerodynamic stall/spin at a low altitude.

The instrument-rated commercial pilot was approaching the destination airport after a cross-country flight in night instrument meteorological conditions. According to radar track data and air traffic control communications, while receiving radar vectors to the final approach course, the pilot did not always immediately comply with assigned headings and, on several occasions, allowed the airplane to descend below assigned altitudes. According to airplane performance calculations based on radar track and GPS data, the pilot made an engine power reduction about 2.5 minutes before the accident as he maneuvered toward the final approach fix. Following the engine power reduction, the airplane's airspeed decreased from 162 to 75 knots calibrated airspeed, and the angle of attack increased from 2.7° to 14°. About 4 miles from the final approach fix, the airplane descended below the specified minimum altitude for that segment of the instrument approach. The tower controller subsequently alerted the pilot of the airplane's low altitude, and the pilot replied that he would climb. At the time of the altitude alert, the airplane was 500 ft below the specified minimum altitude of 2,000 ft mean sea level. According to airplane performance calculations, 5 seconds after the tower controller told the pilot to check his altitude, the pilot made an abrupt elevator-up input that further decreased airspeed, and the airplane entered an aerodynamic stall. A witness saw the airplane abruptly transition from a straight-and-level flight attitude to a nose-down, steep left bank, vertical descent toward the ground, consistent with the stall. Additionally, a review of security camera footage established that the airplane had transitioned from a wings-level descent to a near-vertical spiraling descent. A postaccident examination of the airplane did not reveal any anomalies that would have precluded normal operation during the accident flight. Although the pilot had monocular vision following a childhood injury that resulted in very limited vision in his left eye, he had passed a medical flight test and received a Statement of Demonstrated Ability. The pilot had flown for several decades with monocular vision and, as such, his lack of binocular depth perception likely did not impede his ability to monitor the cockpit instrumentation during the accident flight. The pilot had recently purchased the airplane, and records indicated that he had obtained make and model specific training about 1 month before the accident and had flown the airplane about 10 hours before the accident flight. The pilot's instrument proficiency and night currency could not be determined from the available records; therefore, it could not be determined whether a lack of recent instrument or night experience contributed to the pilot's difficulty in maintaining control of the airplane.

The published inbound course for the GPS runway 36 approach was 357° magnetic; the crossing altitude for the final approach fix (NULUX) was 2,000 ft msl; and the distance between NULUX and the runway threshold was 4.2 nautical miles (nm). After crossing NULUX, lateral-navigation (LNAV)-equipped aircraft descended to 1,300 ft msl until crossing the stepdown fix (SHIEV) that is located 2 nm from the end of runway 36. After crossing SHIEV, LNAV-equipped aircraft descended to the minimum descent altitude of 1,040 ft msl (413 feet agl). The instrument approach minimums required a 1-mile inflight visibility at the missed approach point to continue the landing. The missed approach instructions were to climb on runway heading to 4,000 ft msl, proceed direct to the ZITAG waypoint, then turn left and proceed direct to the CRAFF waypoint and hold. A review of available air traffic control information indicated that the accident flight received normal services and handling. Transcripts of the voice communications recorded between the accident pilot, the approach controller, and the tower controller are included in the docket materials associated with the investigation. Denton Municipal Airport (DTO), a public airport located about 3 miles west-southwest of Denton, Texas, was owned and operated by the City of Denton. The airport field elevation was 642 ft msl. The airport had a single asphalt runway, runway 18/36 (7,002 ft by 150 ft). Runway 36 had a displaced threshold that reduced the available runway landing length by 100 ft. Runway 36 was equipped with medium intensity runway lights and a four-light precision approach path indicator. The airport was equipped with an air traffic control tower that was operational at the time of the accident. The airplane was equipped with a Honeywell KMH-820 Multi-Hazard Awareness System, serial number 1340. With the assistance of the manufacturer, the non-volatile memory was downloaded from the damaged device. The recovered data identified two alerts that had been issued during the accident flight. The first alert was issued when the airplane's flight path came near a tower during the final seconds of the flight. The airplane was located about 361 ft south of the accident site and about 700 ft west of the tower when the obstacle pull-up (OBPU) alert was issued. The airplane was at a GPS altitude of 1,030 ft (about 330 ft agl) and 46.7 knots groundspeed. The OBPU would have resulted in an audible alert "Obstacle, Obstacle, Pull-Up." The second alert was for an excessive sink rate; however, additional data was not recorded to non-volatile memory before there was a loss of electrical power to the device during impact. The pilot's Apple iPhone, Apple iPad, and Appareo Stratus II were recovered at the accident site and sent to the National Transportation Safety Board (NTSB) Vehicle Recorders Laboratory to be examined. The content of the Apple iPhone and Apple iPad were examined using forensic software, and there was no data found that was associated with the accident flight. The Appareo Stratus II was an automatic dependent surveillance broadcast (ADS-B) device with GPS capability. The device had been configured to interface with the pilot's iPad ForeFlight application. An external examination of the device revealed minor impact damage; however, an internal examination revealed additional damage to the Wi-Fi module. The device was repaired and examined using laboratory hardware and software. The device contained flight parameter data for the accident flight. The recovered Appareo Stratus II flight parameters and recorded ATC radar track data were used to develop an aircraft performance study. According to the study, at 2106:38, during the approach, the pilot made an engine power reduction that resulted in a 1,500 ft per minute descent and a 25 knot per minute airspeed deceleration. Between 2106:38 and 2109:00, the airplane's airspeed decreased from 162 to 75 knots calibrated airspeed (KCAS), and the angle of attack increased from 2.7° to 14°. At 2108:52, 5 seconds after the tower controller told the pilot to check his altitude, the pilot made an abrupt elevator-up input that increased the airspeed deceleration to 168 knots per minute. At 2109:00, the airplane entered an aerodynamic stall after it decelerated to 75 KCAS. According to first responders with the Argyle Fire Department, upon their arrival at the accident site, there was no evidence of ice or frost accumulation on the airplane's fuselage, wings, or tail. Additionally, the first responders reported that there was a substantial smell of Jet-A fuel at the accident site; however, there was no evidence of an explosion or postimpact fire. The pilot was seated in the left cockpit seat and was secured by a lap belt. The available shoulder harness did not appear to have been used. According to the FAA Airplane Flying Handbook (FAA-H-8083-3B), "Night flying is very different from day flying and demands more attention of the pilot. The most noticeable difference is the limited availability of outside visual references. Therefore, flight instruments should be used to a greater degree in controlling the airplane." The handbook further states, "Distance may be deceptive at night due to limited lighting conditions. A lack of intervening references on the ground and the inability to compare the size and location of different ground objects cause this. This also applies to the estimation of altitude and speed. Consequently, more dependence must be placed on flight instruments, particularly the altimeter and the airspeed indicator." The Tarrant County Medical Examiner's Office, located in Fort Worth, Texas, performed an autopsy on the pilot. The cause of death was attributed to multiple blunt-force injuries sustained during the accident. The FAA's Bioaeronautical Sciences Research Laboratory located in Oklahoma City, Oklahoma, performed toxicology tests on samples obtained during the autopsy. The test results were negative for carbon monoxide, ethanol, and all tested drugs and medications. The pilot had monocular vision following a childhood injury that resulted in very limited vision in his left eye. On June 18, 1991, after a review by an ophthalmologist and passing a medical flight test, the pilot was issued a Statement of Demonstrated Ability (SODA) that authorized a third-class medical certificate. On June 24, 1995, the pilot was issued an updated SODA after he passed another medical flight test and was authorized for a second-class medical certificate. The pilot continued to routinely receive second-class medical certificates with a limitation for corrective lenses. The airplane wreckage was found in a grass-covered industrial storage yard located about 6.4 nm south of the runway 36 threshold. The accident site was about 400 ft northeast of the final radar return and about 207 ft right of the final approach course. The main wreckage consisted of the entire airplane, which was orientated on a west-northwest heading. The elevation of the accident site was 679 ft msl. The wreckage was in an upright position, and there was no appreciable wreckage debris path. There was no evidence of an inflight or postimpact fire. All observed airframe structural separations were consistent with impact-related damage. The entire lower fuselage surface was crushed upward, consistent with a vertical impact while in a near level pitch attitude. The airplane's tail section was found partially separated immediately aft of the aft pressure bulkhead. The vertical stabilizer, rudder, horizontal stabilizers, and elevators remained relatively undamaged. The leading edges of both wings, the propeller spinners, and the airframe radome did not exhibit evidence of a ground impact. Aileron control cable continuity was established through an overstress separation of the aileron sector drive cable in the mid-cabin area and a separation of the balance cable near the right wing root. All other flight