Introduction
A twin-engine airplane, flown by an airline transport pilot, was on a cross-country flight and approaching its destination airport in night instrument meteorological conditions (IMC). The destination airport weather about 1 minute before the accident included an overcast ceiling at 200 ft and 1/2-mile visibility with light rain and fog. Air traffic control (ATC) provided radar vectors to the final approach course for an instrument landing system (ILS) approach to runway 20.
Accident Sequence
A post-accident simulation study, based on radar data and data from the airplane's electronic horizontal situation indicator (EHSI), revealed that the airplane's flight path did not properly intercept or track the localizer or glideslope during the instrument approach. The airplane crossed the final approach fix about 360 ft below the glideslope and then maintained a descent profile below the glideslope until it leveled briefly near the minimum descent altitude, likely for a localizer-only instrument approach. The lateral flight path from the final approach fix inbound was one or more dots to the right of the localizer centerline until the airplane was about 1 nautical mile from the runway 20 threshold, at which point it turned 90° left to an east course. This turn was initiated before the airplane had reached the missed approach point and was not in accordance with the published missed approach instructions, which specified a climb on runway heading before making a right turn to a 270° magnetic heading. The airplane then made a series of pitch excursions as it flew away from the localizer.
The simulation study determined that dual engine power was required to match the recorded flight trajectory and ground speeds, indicating both engines were operating throughout the approach. Based on calculated angle of attack and lift coefficient data, the airplane likely encountered an aerodynamic stall during its course deviation to the east. The airplane impacted the ground about 2.2 miles east-northeast of the runway 20 threshold and about 1.75 miles east of the localizer centerline.
Investigation Findings
FAA documentation indicated that all components of the airport's ILS were functional at the time of the accident, with no recorded errors, and the localizer was radiating a front-course to the correct runway. A post-accident flight check found no anomalies with the instrument approach. An onsite examination established that the airplane impacted the ground upright and in a nose-low attitude; the lack of an appreciable debris path was consistent with an aerodynamic stall/spin. Wreckage examinations revealed no anomalies with the airplane's flight control systems, engines, or propellers.
The glideslope antenna was found disconnected from its associated cable circuit. Laboratory examination determined that the glideslope antenna cable was likely inadequately connected or secured during the flight, resulting in an unusable glideslope signal to the cockpit avionics. No history of recent maintenance on the glideslope antenna was found, and the reason for the inadequate connection could not be determined.
Data from the airplane's EHSI established that the device was in ILS mode during the instrument approach and had achieved a valid localizer state on both navigation channels; however, it never achieved a valid glideslope state on either channel. Replay of the EHSI data confirmed that during the approach, the device displayed a large "X" through the glideslope scale and did not display a deviation pointer, both indications of an invalid glideslope state.
There was no evidence of cumulative sleep loss, acute sleep loss, or medical conditions indicating poor sleep quality for the pilot. However, the accident occurred more than 2 hours after the pilot's routine bedtime, suggesting the pilot's circadian system would not have been promoting alertness. At the time of the accident, the pilot had likely been awake for 18 hours, leading to the development of fatigue. The presence of low cloud ceilings and the lack of glideslope guidance would have been stresses, increasing the pilot's workload and situational stress as he flew the localizer approach—a procedure likely unanticipated.
Weight and balance calculations indicated the airplane's center of gravity (CG) was aft of the allowable limit. The series of pitch excursions that began shortly after the airplane turned left suggests the pilot had difficulty controlling airplane pitch, likely due to adverse handling characteristics associated with the aft CG. These adverse handling characteristics further increased workload and provided distractions from maintaining control.
Probable Cause
The pilot's failure to maintain control of the airplane 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. Contributing to the accident were pilot fatigue, the pilot's increased workload during the instrument approach resulting from the lack of glide slope guidance due to an inadequately connected/secured glide slope antenna cable, and the airplane being loaded aft of its balance limit.