Union Star, MO, USA
N910ME
BOEING 717-200
After departure, the incident airplane was climbing toward its cruising altitude in a stratified region of precipitation within a convective system, and in conditions which were favorable for the accumulation of structural icing. At some point, the pitot/static system began accumulating ice because the air data heat system had not been activated or was not functioning. The condition first manifested itself as a "RUDDER LIMIT FAULT" warning due to icing of the rudder limiting system pitot tube. The icing continued to accumulate on the other probes of the air data system, degrading its ability to reliably determine the airplane's airspeed. About 19,000 feet, the flight crew disengaged the autopilot and pushed the pitch control column forward, and the airplane entered a descent. The flight crew initially applied uncoordinated control inputs, in the process reaching nearly 100 pounds of differential force on the pitch control column, while attempting to recover the airplane. During this period, airplane's pitch continued to oscillate through 5 cycles, for duration of 8 minutes, reaching altitudes as low as 10,600 feet and as high as 23,300 feet. During the oscillations the airplane's indicated airspeed varied greatly, between 54 and 460 knots; however, the airplane systems tests and aircraft performance data show that the recorded, as well as the displayed, airspeed indications were adversely affected by the icing conditions. Once regaining control of the airplane, the crew diverted and made an uneventful landing. Post-incident testing of the airplane's mechanical and electronic systems revealed no abnormalities that would have accounted for the unreliable airspeed indications or the loss of control reported by the flight crew. Post-incident computer modeling also confirmed that the airplane performed in a manner consistent with all deviations from normal flight having been initiated or exacerbated by the control inputs of the flight crew. Review of flight data recorder, cockpit voice recorder, and flight crew interviews revealed that the flight crew's actions during the event were in part contradictory with operator's training and operational procedures. Specifically, the crew initially failed to properly identify and respond to the erroneous airspeed indications that were presented and failed to coordinate their recovery of the airplane to controlled flight.
HISTORY OF FLIGHT On May 12, 2005, about 2316 central daylight time, a Boeing 717-200, N910ME, operated by Midwest Airlines, Inc., as flight 490, experienced a series of pitch oscillations while climbing to cruise altitude over Union Star, Missouri. The flight crew declared an emergency, and diverted to Kirksville Regional Airport (IRK), Kirksville, Missouri. There were no injuries to the 2 certificated airline transport pilots, 2 flight attendants, and 76 passengers. Night instrument meteorological conditions (IMC) prevailed for the flight, which was operating on an instrument flight rules flight plan from Kansas City International Airport (MCI), Kansas City, Missouri, to Ronald Reagan Washington National Airport (DCA), Washington, D.C. The scheduled passenger flight was conducted under 14 Code of Federal Regulations Part 121. According to flight crew statements, the flight had initially originated in Los Angeles, California, and landed at Omaha, Nebraska, due to weather, before continuing on to Kansas City. Once in Kansas City, the captain was briefed by company dispatch personnel regarding significant weather in the area, and noted the weather on a display screen that was located in the operations area. As the airplane was taxied for departure at 2231, the captain elected to delay the takeoff, and wait for the weather to pass further to the east. Once the weather passed, and the flight crew received a "ride report" from a departing flight, the incident flight departed from runway 1L. The captain performed the takeoff, and afterwards engaged the autopilot with a configuration selected, vertical profile guidance was provided by the flight management system. The crew requested a northeasterly departure, and flew a 010-degree heading, taking the airplane away from weather cells that were about 25 miles away. The crew did not observe any weather cells or lightning on the departure path, and as the airplane climbed, the captain observed only green areas on the airborne radar, despite adjusting radar pitch and tilt. After departure, the airplane entered IMC, but no turbulence was noted. The captain turned to a heading of 060 degrees, and about flight level (FL) 230, the crew was advised to change radio frequencies and was cleared to climb to FL 270. At that point in the flight the airplane was under the control of the autopilot, and was climbing at airspeeds between 280 and 300 knots. The closest weather cell was about 20 to 25 miles away, and the crew felt they did not need to utilize airplane anti-icing because the outside temperature was still too warm to require it. The first indication of something abnormal was when the captain noticed the master caution light was illuminated. After commenting to the first officer, he looked down at the center pedestal and notice that the CONFIG cue switch was illuminated. He then pushed the CONFIG key, located on the center instrument panel, and noted the "RUDDER LIM FAIL" alert on the engine and alert display unit. He looked at the alert for about 10 seconds, and was about to ask the first officer to retrieve the quick reference handbook (QRH) when the event began. The airplane initially pitched down, to what the first officer recalled was in excess of 20 degrees. The captain remembered hearing the autopilot disconnect aural signal. When the pitch down occurred, the captain was still the pilot flying, but the first officer then began assisting him on the controls. The airplane continued in a steep dive, which the first officer felt was "almost beyond recovery." Both pilots recalled saying, "up, up, up" during the initial descent, and noted that the airplane did not respond to control inputs at first and that the flight controls felt very heavy. The first officer thought that the airplane lost at least 5,000 feet of altitude during the first descent. The captain recalled that the elevator response was "not normal," and that he was not getting the amount of response he expected from the flight control inputs. At times he would get little response from the elevator control inputs, but then it would quickly change to "a lot" of response, unlike any training scenario or airplane flight characteristics he had previously experienced. The airplane then pitched up, and the first officer stated that he told the captain to push forward on the control wheel, and assisted him in pushing forward. As the airplane pitched up the airspeed decreased, and slowed to about 190 knots. At that point the autothrottles were not engaged, and the first officer increased the engine power to about 3/4 of the "throttle throw," which equated to about 80 percent N1. The first officer stated that the power levers felt "normal." The captain stated that while he was trying to recover the airplane, he attempted to maintain a level pitch attitude by placing the pitch of the airplane in a fixed position, and tried to level the wings of the airplane, but altitude control was unobtainable. The airplane entered another dive, and the first officer stated to the captain, "I have the airplane," and said that the captain responded, "Okay." The first officer continued manipulating the flight controls with inputs from the captain, and was able to bring the airplane under control with his help. The captain, however, stated that he heard the first officer state, "I'm flying," and the captain took that to mean, "He was with me on the flight controls." The captain continued operating the flight controls until he later began to communicate with ATC, and it was at that point that he relinquished control of the airplane to the first officer. He did not recall "formally" transferring control of the airplane to the first officer, but stated that it was "understood." During the event, the airplane went through a series of climbs and descents before the flight crew was able to recover. The first officer stated that during the recovery he was trying to keep the airspeed away from the stall speed and away from the overspeed red zone. The captain stated that the airspeed changed instantaneously from low to high, at points becoming greater than 400 knots with an overspeed warning in literally seconds. The airspeed went from the bottom of the airspeed tape to the top so quickly, that the captain could not visualize the airplane doing this. When the airspeed was indicating over 400 knots on his Primary Flight Display (PFD), the captain stated that he glanced at the integrated standby instrument system (ISIS), and noted that there was a difference in the two displayed speeds. He could not recall the difference in the two speeds. The captain did not report any fluctuations in the instruments prior to the event, but that during the event there was a "blinking" or flashing on and off of the PFD. He would notice a very high airspeed during a pitch down, and then would see a "flicker" or "flash," and when the display came back, it would indicate a reversal of information, with the airspeed low and the pitch high. He only recalled seeing the PFD flash, and was not sure if the navigation display (ND) did so as well. He was not using the flight director bars during the recovery, nor did he recall seeing them. As the flight crew began to recover from the event, the captain felt that they needed to get the airplane on the ground, and elected to divert to IRK. As there were many tasks to be accomplished, the captain began making arrangements with ATC and briefed the passengers, while the first officer continued to fly the airplane. The first officer had the captain re-engage the autopilot, and they continued using the autopilot until they were on the approach to IRK. Following the recovery, the crew noted a "COLUMN DISC" alert on the engine and alert display. The entire event and recovery occurred in IMC. During the descent the captain performed the in-range checklist silently. The flight crew then performed the before landing procedures and entered a right traffic pattern for runway 18 at IRK. The first officer flew the airplane during the landing, while the captain operated the speedbrakes and the throttles. The crew did not re-engage the autothrottles. The first officer stated that while on approach, the elevator response seemed "normal." The airplane subsequently landed uneventfully. PERSONNEL INFORMATION The captain held an airline transport pilot certificate with ratings for airplane single and multiengine land, which was issued on December 7, 1988, as well as a type rating for DC-9 aircraft. His most recent Federal Aviation Administration (FAA) first-class medical certificate was issued on January 17, 2005, with the limitation, "Holder shall wear lenses that correct for distant vision and possess glasses that correct for near vision." According to company records, and interviews, the captain had accumulated 18,000 total hours of flight experience, including 203 hours in the Boeing 717 (B-717). A review of FAA records found no accident, incident, or enforcement actions pertaining to the captain. The captain was hired by Midwest Airlines in January 1987, and subsequently upgraded to captain in December 1988. While working at Midwest Airlines he had flown as captain on DC-9, MD-80, and B-717 airplanes. He worked in the Flight Standards and Training department at the airline for about 12 years in various positions, including check captain, ground instructor, simulator instructor, proficiency check airman, and aircraft instructor. He returned to line flying in 2001, and at the time of the event he was no longer a check airman. He transitioned to captain in the B-717 on February 11, 2005, and the requirements for his most recent proficiency check were also completed on that date. The first officer held an airline transport pilot certificate with a rating for airplane single and multiengine land, which was issued on September 14, 2004, as well as a type rating for BA-3100 aircraft. His most recent FAA first-class medical certificate was issued on January 25, 2005, and was issued without any limitations. According to company records, and interviews, the first officer had accumulated 12,000 total hours of flight experience, including 1,313.5 hours in the B-717. The first officer was hired by Midwest Airlines in January 1993. His most recent proficiency check was completed on October 13, 2004, in the B-717. Review of the first officer's FAA airman file revealed that he had received several notices of disapproval for previous pilot proficiency check flights between 1978 and 1991. These included checks for the instrument airplane rating, flight instructor airplane single engine, flight instructor instrument airplane, flight instructor multiengine airplane, airline transport pilot, and a type rating. He additionally received notices of disapproval during a proficiency checks for the DC-9 type rating and an upgrade to captain in 1997 and 2000. AIRCRAFT INFORMATION Crew Warning and Alerting System The incident airplane was equipped with a Crew Warning and Alert System, an integrated system that provided the crew information about the status of the airplane with alerts of varying severity. The crew was notified of these alerts via several components of the system, including the Engine and Alert Display (EAD), System Synoptic Pages, Status Page, System Control Panel (SCP), Master Warning and Master Caution Lights, and an Aural Warning. The EAD was located on Display Unit (DU) number 3 (a display screen located near the center of the instrument panel). The System Synoptic Pages, Status Page was located on DU 4 (immediately right of DU 3), and the SCP was located on the center pedestal. There were four levels of alerts (0, 1, 2, and 3), which could appear on the EAD, System Synoptic Pages, Status Page, or the SCP. The system cue switches (on the SCP) illuminated to identify the associated system that was generating an alert or warning. Pushing the associated cue switch would extinguish the master Caution (if illuminated), and display the appropriate configuration synoptic in the lower left hand corner on the Secondary Engine Page on DU 4. Level 1 alerts indicated abnormal operational or airplane system conditions, which required crew awareness and may have required subsequent compensatory action. However, they normally did not affect safety of flight. Level 1 alerts were amber, and not enclosed in a rectangular box. They may have been displayed on the EAD or only on the system synoptic display. The two MASTER CAUTION lights may have illuminated simultaneously with the display of a Level 1 alert on the EAD. Other Level 1 alerts would be annunciated by a flashing reminder message in the lower right-hand corner of the EAD and illumination of a Systems Display (SD) cue switch. Level 1 alerts that appeared on the EAD were generally resettable. Level 2 alerts indicated abnormal operational or airplane system conditions that required immediate crew awareness and subsequent compensatory action. Level 2 alerts were displayed in amber on the EAD, and enclosed within an amber rectangular box accompanied by two amber MASTER CAUTION lights, as well as an illuminated cue light on the SCP. Pressing the cue switch extinguished the Master Caution, displayed the Synoptic Page, and removed the alert from the EAD. At that time, the "MISC" reminder message appeared on the EAD. Level 3 alerts indicated abnormal operational or airplane system conditions which required immediate crew response. Level 3 alerts were displayed on the EAD in red and enclosed within a red rectangular box. The two red MASTER WARNING lights would illuminate simultaneously, an aural warning sounded (for some alerts), and the appropriate cue light on the SCP illuminated. The Master Warning was extinguished by pressing either Master Warning light or the SCP Cue Light. The alerts remained on the EAD until the condition no longer existed. Air Data System The incident airplane's air data system was comprised of a pitot-static system and an air data sensor heating system. The pitot-static system provided dynamic (pitot) and ambient (static) pressure information to various flight instruments and airplane systems via eleven air data system sensors. Seven of those sensors provided air data information that was used by the captain's, first officer's and auxiliary airspeed indicators. These sensors were: three pitot probes, one left (captain), one center (auxiliary) and one right (first officer), which were located just below the cockpit windshield; and two main static plates along with two alternate static ports. Each of the three pitot probes was connected to a condensation sump via drain lines. The drains allowed for the removal of moisture which may have accumulated in the pitot system. Each pitot probe and static port supplied a pressure input to its related Air Data Module (ADM), which converted the air pressure into digital pressure data. Once converted, the ADM supplied the digital air pressure data to its respective Air Data Inertial Reference Unit (ADIRU), which provided input signals to the Flight Control Computer (FCC) and the flight instruments by the Versatile Integrated Avionics (VIA) unit. The digital air data from the auxiliary ADM was sent directly to the Integrated Standby Instrument System (ISIS). There was a static plate located symmetrically on each side of the forward fuselage. Each plate had four static ports that were part of the plate; only two ports provided pressure, the other two were capped off at the plate. There was an alternate static port aft of and below each static plate. There were three ADMs that sensed and supplied static air pressure to the ADIRUs. Each static port was cross-connected to a port on the opposite side of the airplane. An ADM would convert the average of the left and right static port air pressure into ARINC 429 digital pressure data. The data was then sent to its respective ADIRU, or to the ISIS system. The air data sensor heating system operated electrically to prevent the formation of ice on each of the eleven air data system sensors. Ice protection for the pitot probes (including the captain's, first officer's, auxiliary, and rudder limiter), static plates, static ports, Ram Air Temperature (RAT), and angle of attack transducer
A loss of reliable airspeed indication due to an accumulation of ice on the air data/pitot sensors. Contributing to the incident was the flight crew's improper response to the erroneous airspeed indications, their lack of coordination during the initial recovery of the airplane to controlled flight, and icing conditions.
Source: NTSB Aviation Accident Database
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