BOEING 717-200Midwest Airlines
Union Star, MO, USA · 2005-05-13·N910ME
What happened
NTSB summary, verbatimAfter 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.
Probable cause
the Board's determinationA 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.
Safety recommendations
2 issuedwhat changed because of this accidentTO THE FEDERAL AVIATION ADMINISTRATION: Require, on all new airplanes certificated under 14 Code of Federal Regulations Part 25, that the air data sensor heating system automatically activate after engine start.
TO THE FEDERAL AVIATION ADMINISTRATION: Require modification of existing airplanes certificated under 14 Code of Federal Regulations Part 25 that currently require manual activation of the air data sensor heating system to incorporate, to the extent technically practical, the automatic activation of this system; for airplanes that cannot be modified, require that cockpit warnings provide an upgraded warning associated with the failure to activate the heating system.
Sequence of events
12 timed eventsfrom the FDR factual report · claude-sonnet-5- 04:08:09Takeoffp.4Aircraft lifted off from Kansas City, Missouri
- Shortly after takeoffClimbp.4Autopilot engaged and aircraft was in the climb thrust vertical mode
- 04:16:26Climbp.4While climbing through 19,000 feet, DU4 parameter changed from EICAS Secondary Engine page to Configuration page; captain and first officer computed airspeed values begin to diverge
- 04:16:36Climb/Upsetp.4Autothrottle system (ATS) Vert Accel parameter indicated system no longer active
- ~04:16:39Upsetp.5Autopilot system registered disengaged, three seconds after ATS deactivation; many discrete parameters toggled around this time
- Following 04:16:39Upsetp.5Aircraft pitched down, groundspeed began to increase, computed airspeed initially decreased, captain and first officer values split
- Next eight minutesUpsetp.5Aircraft went through a series of altitude excursions with five cycles of pitch down and up oscillations; airspeed values fluctuated; altitude reached minimum of 10,600 feet and maximum of 23,300 feet; airspeeds varied 52-460 knots; groundspeed varied 290-552 knots
- 04:18:33Upsetp.6Difference between two column positions first occurs, lasting over 20 seconds, coinciding with difference in right and left elevator values
- 04:20:18Upsetp.7Wing and tail anti-ice systems switched on, remained active for about 10 minutes
- 04:24Recoveryp.7Aircraft achieved and maintained level pitch attitude, shortly after last airspeed discrepancy between crewmembers recorded; aircraft started descending through 23,300 feet
- ~04:29Descentp.7Autopilot engaged (5 minutes after level pitch attitude achieved)
- 04:35:26Landingp.7Aircraft touched down in Kirksville, Missouri
NTSB analysis
from the final reportAfter 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.