Description

On 12 May 2005, a Boeing 717-200 (N910ME) being operated by Midwest Airlines on a scheduled passenger flight from Kansas City to Washington National as flight 490 and climbing in night Instrument Meteorological Conditions (IMC) experienced a sudden loss of control from which recovery was only achieved after a prolonged period of pitch oscillation involving considerable height variation. An en-route diversion to Kirksville MO was then made without further event. None of the 80 occupants were injured and the aircraft was not damaged.

Investigation

An Investigation was carried out by the National Transportation Safety Board (USA) (NTSB). Data from the 2-hour CVR and the FDR were downloaded and provided useful information for the Investigation.

It was found that the 54 year-old Captain had been with the operator since 1987 and after being promoted to command in 1988 had served as a Training Captain before returning to line flying in 2001. He had 18000 total flying hours mainly on the DC9 and MD80 and just 203 hours on the Boeing 717. The 52 year-old First Officer had 12,000 total flying hours including 1313 hours on the Boeing 717.

It was established that the onset of the loss of control just above FL190 had occurred with the aircraft commander as PF and had followed a progressive loss of airspeed, attributed to failure to select air data system anti icing on, an omission which had not been noticed by the flight crew. IMC had prevailed prior to the loss of control, and throughout the recovery, The recovery took some 8 minutes and although engine ice protection had been on, airframe ice protection had not been selected on until some 4 minutes after the event began. The initial steep dive led to an overspeed which the First Officer felt was "almost beyond recovery." Both pilots stated that the aircraft had not responded normally or consistently to their pitch control inputs and Flight Data Recorder (FDR) data for the event showed a series of large pitch oscillations within altitude range of 10,600 feet and 23,300 feet and with ground speed varying between 290 knots and 552 knots.

The aircraft commander was unable to level the aircraft from the initial recovery climb and it entered a second dive, at which point the First Officer called “I’m Flying” and took control and was eventually able to bring the aircraft under control. It was apparent afterwards, however, that the aircraft commander had taken this call and subsequent action as advice that the First Officer` was merely assisting on the controls and so had continued to make inputs and only positively relinquished control of the aircraft to the First Officer (without a formal handover) when he began to communicate with ATC once recovery had been achieved. FDR data showed that this confusion had led to the two control columns being split due to opposite forces being applied.

In respect of the risk of airframe icing, the investigation noted that the actual freezing level was likely to have been in the region of 12000 feet with conditions conducive to icing prevailing in the area up to 25,000 feet. It was estimated that a 40% probability of severe clear icing conditions around 19000 feet had existed and noted that convective activity would have increased both the probability of icing conditions and of their severity if encountered.

In respect of the report from the flight crew that there had not been any significant returns on the weather radar, the investigation noted that the low power output and wavelength of these radars makes them prone to attenuation which is further aggravated in precipitation and / or icing conditions. It was calculated that in the prevailing circumstances, sufficient attenuation would have existed to create the reported ‘green only’ returns.

The Conclusions of the Investigation included that:

“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 modelling 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.”

And that:

“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.”

The Probable Cause of the event was formally determined as “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.”

The Final Report of the Investigation was adopted and published on 14 January 2009. No Safety Recommendations were made.

Further Reading

• Ice Formation on Aircraft
• In-Flight Icing
• Recovery from Unusual Aircraft Attitudes
• Aerodynamic Effects of In-Flight Icing
• Aircraft and In Flight Icing Risks
• Normal Checklists and Crew Coordination (OGHFA BN)