On 2 January 2006, a Saab 340B (N390AE) being operated by American Eagle on a scheduled domestic passenger flight from San Luis Obispo to Los Angeles and climbing through 11700 feet in day Instrument Meteorological Conditions (IMC) stalled and control was not recovered until after 5200 feet of altitude had been lost. After recovery the flight was completed to destination. None of the 28 occupants were injured and the aircraft did not sustain substantial damage.
An Investigation was carried out by the NTSB. FDR data was successfully downloaded. It was established that the 34 year-old Captain had 6765 hours total flying experience including 3981 hours on type. The 32 year-old First Officer had 1367 hours total flying experience including 132 hours on type.
It was found that the flight had departed into icing conditions with the continuous mode of the pneumatic boot airframe de-ice system inoperative in accordance with the MEL which would have required manual operation of the system. The Captain initially acted as PF with the intention of transferring this role to the First Officer after completing the Climb Checklist. The applicable minimum clean speed in icing conditions for the flight was calculated as 141 KIAS.
Shortly after taking over as PF at 2,500 feet, the First Officer selected VS mode for the AP. Although the Captain reported having noted "light rime ice accumulating on the windshield wiper blades and about a 1/2-inch-wide area of ice on the left wing" he did not notice a concurrent progressive decrease in indicated airspeed as airframe ice accumulated. At 11,700 feet "as he began to activate the manual device boot system", the Captain reported feeling a "heavy vibration in the airframe and the windscreen immediately turned white with ice". The aircraft nose and left wing dropped and the AP disconnected, then the Stall Protection System (SPS) was activated and the EGPWS annunciated a 'Bank Angle' alert.
FDR data showed that in the 26 seconds prior to this upset, speed reduced from 144 KIAS to 130 KIAS and did so at an increased rate in the final 10 seconds. The aircraft was found to have departed controlled flight from a pitch attitude of 14º up prior to the SPS activation and thereafter performed "a series of roll and pitch excursions, reaching maximum values of 86 degrees left wing down, 140 degrees right wing down, 23 degrees nose up and 40 degrees nose down" before the flight crew recovered control at 6500 feet QNH. The data also showed that "14 seconds after the initial stall, both ailerons simultaneously traveled to the full up position for approximately 14 seconds". It also recorded that the minimum airspeed of 105 knots occurred shortly before controlled flight was regained after the aircraft had descended at an average rate of around 4000 fpm.
It was noted that although the IAS mode was the only authorised FD mode for climb in the icing conditions which had prevailed, the VS mode had been used which allowed the AP to reduce airspeed to maintain the selected rate of climb. It was also noted that the fact that the AP was engaged had masked the correction of a slight rolling motion which had occurred 26 seconds prior to the loss of control which might otherwise have alerted the crew to the speed decay which was occurring.
The Operator's recurrent simulator training syllabus was found to include "approach to stalls in the takeoff, clean, and landing configurations and unusual attitude recoveries from nose low and nose high positions" with one section dealing specifically with normal and abnormal emergency situations in relation to anti-icing and de-icing systems, SPS and Stick Pusher operation. In addition, it was noted that the Operator's Advanced Aircraft Maneuvering Program (AAMP) included "a review of phenomena that cause upset events and unusual attitude recovery procedures".
However, when a number of American Eagle pilots, including the incident Captain and First Officer, were interviewed during the Investigation, they all stated that they had "never practiced encountering a stall in icing conditions as part of their simulator training" and "could not recall ever having the opportunity to practice a complete stall in the simulator, as they were always instructed to recover at the first indication of an impending stall".
The Conclusion of the Investigation was that the onset of a stall ahead of SPS activation, the upset and the uncommanded upward deflection of the ailerons had been caused by ice accretion on the wing in Supercooled Large Droplet (SLD) conditions. It was considered that the cause of the simultaneous upward deflection of the ailerons was airflow separation over them, as was known to have occurred in similar accidents and incidents involving the modification of aileron forces by SLD ice accretion.
Four Safety Recommendations were issued as a result of the Investigation as follows:
- that the FAA should require all operators of Saab SF340 series airplanes to instruct pilots to maintain a minimum operating airspeed of 1.45 x Vs during icing encounters and before entering known or forecast icing conditions and to exit icing conditions as soon as performance degradations prevent the airplane from maintaining 1.45 x Vs. (A-06-48)
- that the FAA should require the installation of modified stall protection logic in Saab SF340 series airplanes certified for flight into known icing conditions. (A-06-49)
- that the FAA should require the installation of an icing detection system on Saab SF340 series airplanes. (A-06-50)
- that the FAA should require all operators of turbo propeller-driven airplanes to instruct pilots to disengage the autopilot and fly the airplane manually when operating in icing conditions. (A-06-51)
Two earlier Safety Recommendations were also reiterated:
- that the FAA should convene a panel of airplane design, operations, and aviation human factors specialists to determine whether a requirement for the installation of low-airspeed alert systems in airplanes engaged in commercial operations under 14 Code of Federal Regulations Parts 121 and 135 would be feasible. (A-03-53)
- that the FAA should, if the panel (as recommended in A-03-53) determines that a requirement as investigated would be feasible, establish requirements for low-airspeed alert systems. (A-03-54)
The Probable Cause of the investigated event was determined as:
"An in-flight loss of control due to ice accreted in Supercooled Liquid Droplet (SLD) conditions, and the flight crew's failure to maintain the specified minimum airspeed in icing conditions. Contributing to the accident was the flight crew's decision to climb the airplane in known and forecast icing conditions using an autopilot mode contrary to that specified in the operator's Airplane Operations Manual for climbs during periods of ice accretion or when ice was present on the airframe."
The Final Report was adopted by the NTSB on 30 April 2009.