CEN15FA190

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 glideslope guidance due to an inadequately connected/secured glideslope antenna cable, and the airplane being loaded aft of its balance limit.

The twin-engine airplane, flown by an airline transport pilot, was approaching the destination airport after a cross-country flight in night instrument meteorological conditions. The destination airport weather conditions about 1 minute before the accident included an overcast ceiling at 200 ft and 1/2-mile visibility with light rain and fog. According to air traffic control (ATC) data, the flight received radar vectors to the final approach course for an instrument landing system (ILS) approach to runway 20. As shown by a postaccident simulation study based on radar data and data recovered from the airplane's electronic horizontal situation indicator (EHSI), the airplane's flight path did not properly intercept and track either the localizer or the 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. However, 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 when it turned 90° left to an east course. The turn was initiated before the airplane had reached the missed approach point; additionally, the left turn 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 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, which indicated that both engines were operating throughout the approach. The simulation results also indicated that, 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. According to FAA documentation, at the time of the accident, all components of the airport's ILS were functional, with no recorded errors, and the localizer was radiating a front-course to the correct runway. Additionally, a postaccident 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, and the lack of an appreciable debris path was consistent with an aerodynamic stall/spin. Wreckage examinations did not reveal any anomalies with the airplane's flight control systems, engines, or propellers. The glideslope antenna was found disconnected from its associated cable circuit. Laboratory examination and testing determined that the glideslope antenna cable was likely inadequately connected/secured during the flight, which resulted in an unusable glideslope signal to the cockpit avionics. There was no history of recent maintenance on the glideslope antenna, and the reason for the inadequate connection could not be determined. Data downloaded from the airplane's EHSI established that the device was in the ILS mode during the instrument approach phase and that it had achieved a valid localizer state on both navigation channels; however, the device never achieved a valid glideslope state on either channel during the flight. Further, a replay of the recorded 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 of which were indications of an invalid glideslope state. There was no evidence of cumulative sleep loss, acute sleep loss, or medical conditions that indicated poor sleep quality for the pilot. However, the accident occurred more than 2 hours after the pilot routinely went to sleep, which suggests that the pilot's circadian system would not have been promoting alertness during the flight. Further, at the time of the accident, the pilot likely had been awake for 18 hours. Thus, the time at which the accident occurred and the extended hours of continuous wakefulness likely led to the development of fatigue. The presence of low cloud ceilings and the lack of glideslope guidance would have been stresses to the pilot during a critical phase of flight. This would have increased the pilot's workload and situational stress as he flew the localizer approach, a procedure that he likely did not anticipate or plan to conduct. In addition, weight and balance calculations indicated that the airplane's center of gravity (CG) was aft of the allowable limit, and the series of pitch excursions that began shortly after the airplane turned left and flew away from the localizer suggests that the pilot had difficulty controlling airplane pitch. This difficulty was likely due to the adverse handling characteristics associated with the aft CG. These adverse handling characteristics would have further increased the pilot's workload and provided another distraction from maintaining control of the airplane. Therefore, it is likely that the higher workload caused by the pilot's attempt to fly an unanticipated localizer approach at night in low ceilings and his difficulty maintaining pitch control of the airplane with an aft CG contributed to his degraded task performance in the minutes preceding the accident.

During an ILS approach, the localizer provides lateral guidance for the final approach course, and the glideslope provides vertical guidance as the aircraft descends toward the runway. For a precision approach, such as an ILS approach, the missed approach point (MAP) is where the aircraft reaches the decision altitude while on the glideslope. If a pilot observes an invalid glideslope indication, such as an "X" displayed through the glideslope scale of an electronic horizontal situation indicator or a warning flag on an analog course deviation indicator, the pilot may continue the instrument approach using the lateral guidance of the localizer; however, without the vertical guidance of a glideslope, a higher minimum descent altitude (MDA) is stipulated for the non-precision localizer instrument approach. Further, the location of the MAP for a non-precision approach will be a specified distance from a navigational aid, a fixed distance (from the final approach fix to the MAP) with an associated elapsed time that is based on the groundspeed of the aircraft, or a specific intersection/waypoint. The published inbound course for the ILS runway 20 approach at BMI was 198° magnetic, the crossing altitude for the final approach fix (EGROW) was 2,459 ft msl, and the distance between EGROW and the runway 20 threshold was 4.8 nm. The touchdown zone elevation was 871 ft msl. The decision altitude was 1,071 ft msl (200 ft agl) and required 1,800 ft RVR. The missed approach procedure was to climb on runway heading to 1,500 ft msl, then make a right turn to a 270° magnetic heading and climb to 3,000 ft msl, then join the 214° radial from the Pontiac VOR, and hold at MCLEN intersection. In the event of a loss of vertical guidance from the glideslope during an approach, or when performing the non-precision localizer approach, the MAP was located 4.8 nm from EGROW on the localizer. The non-precision localizer approach MDA was 1,260 ft msl (389 ft agl) and required 2,400 ft RVR. The MDA for a circling approach was 1,340 ft msl (468 ft agl) and required 1 mile surface visibility. According to air traffic control documentation, at the time of the accident, all components of the ILS were functional, with no recorded errors, and the localizer was radiating a front-course to runway 20. A postaccident flight check found no anomalies with the instrument approach. The airplane was not equipped, nor was it required to be equipped, with a cockpit voice recorder or flight data recorder. Central Illinois Regional Airport (BMI), a public airport located about 3 miles east of Bloomington, Illinois, was owned and operated by the Bloomington-Normal Airport Authority. The airport field elevation was 871 ft msl. The airport had two runways: runway 2/20 (8,000 ft by 150 ft, concrete) and runway 11/29 (6,525 ft by 150 ft, asphalt/concrete). Although the airport was equipped with an air traffic control tower, the control tower was closed at the time of the accident. Runway 20 incorporated a dual-mode Approach Lighting System II (ALSF-2) and Simplified Short Approach Lighting System with Runway Alignment Indicator Lights (SSALR). The SSALR system was active when the control tower was closed. The runway was also equipped with runway touchdown zone and centerline lighting and high intensity runway edge lighting. Glideslope Validity A laboratory examination of the airplane's Garmin GNS 530W NAV/COM/GPS receiver, serial number 78410737, established that the active communication (COM) frequency was set to the BMI control tower frequency (124.6 MHz), which also served as the airport's CTAF when the control tower was closed. The standby COM frequency was set to Peoria Approach Control (128.725 MHz). The active navigation (NAV) frequency was set to the BMI ILS runway 20 instrument approach (111.9 MHz). The standby NAV frequency was set to the BMI VOR/DME frequency (108.2 MHz). The course deviation indicator (CDI) mode was selected to VOR/Localizer (VLOC). The Garmin GNS 530W did not record any historical flight parameter or navigational data. A laboratory examination of the airplane's Garmin GNS 430W NAV/COM/GPS receiver, serial number 97103703, established that the active COM frequency was set to the BMI control tower frequency (124.6 MHz). The standby COM frequency was set to the BMI automatic terminal information service (ATIS) frequency (135.35 MHz). The active NAV frequency was set to the BMI ILS runway 20 approach (111.9 MHz). The standby NAV frequency was set to the BMI VOR/DME frequency (108.2 MHz). The CDI mode was selected to VLOC. The Garmin GNS 430W did not record any historical flight parameter or navigational data. The airplane was equipped with a Sandel Avionics SN3500 electronic horizontal situation indicator (EHSI), serial number 1058. The device performs the basic functions of a traditional horizontal situation indicator and radio magnetic indicator. Additionally, depending on installation, the device can provide RMI navigation to GPS waypoints, weather information, and traffic information. The device was configured to receive navigational data from the Garmin 530W and Garmin 430W as NAV Channels 1 and 2, respectively. The device recorded the incoming navigation data once per second to a 24-megabyte circular buffer. The intended purpose of the recorded data was for diagnostic purposes by the manufacturer. The device was sent to the manufacturer to be downloaded and decoded. The recovered dataset included, but was not limited to, the following historic flight parameters: latitude, longitude, ground speed, magnetic heading, ground track, VOR/ILS mode status, localizer and glideslope validity, and localizer and glideslope deviation. The device did not record an altitude data parameter. A review of the data recorded by the Sandel Avionics SN3500 during the previous flight leg (BMI to IND) established that, despite being in ILS mode during the approach phase and having achieved a valid localizer state on both NAV channels, the device did not achieve a valid glideslope state until about 0.6 nm from the approach end of runway 23L at IND. A postaccident review of available weather documentation established that the airplane had landed at IND in day visual meteorological conditions that included a surface visibility of 10 miles and an overcast cloud ceiling at 2,400 ft agl (about 3,200 ft msl). A review of the recovered data for the accident flight revealed that the Sandel Avionics SN3500 was in the ILS mode during the instrument approach phase and that it had achieved a valid localizer state on both NAV channels; however, the device never achieved a valid glideslope state on either NAV channel during the accident flight. With the assistance of the manufacturer, the recorded data for the accident flight was replayed back through the Sandel Avionics SN3500 to document the navigational information that was displayed by the device. The replay confirmed that the glideslope did not achieve a valid state on either NAV channel during the accident flight. The device displayed a large "X" through the glideslope scale and did not display a glideslope deviation pointer. According to the Sandel Avionics SN3500 pilot's guide, an "X" through the glideslope scale and the absence of a glideslope pointer indicated a lack of valid glideslope data. According to the manufacturer, the glideslope deviation and validity state are independently determined by the NAV/COM/GPS devices (Garmin 530W and Garmin 430W) before being transmitted, along with other navigational data, to the SN3500 device as NAV Channel 1 and NAV Channel 2 data via a standard avionics data transfer protocol. According to the FAA Instrument Flying Handbook, a glideslope signal consists of two intersecting radio signals that are modulated at 90 Hz and 150 Hz. According to Garmin, the operating conditions that would result in an invalid glideslope state include any of the following conditions: (a) In the a