Background
American Eagle Flight 4184, a scheduled passenger flight from Indianapolis, Indiana, to Chicago-O'Hare International Airport, experienced a loss of control while holding near the LUCIT intersection. The flight departed the gate at 14:14, delayed due to changing traffic flow from deteriorating weather at the destination. After a 42-minute ground hold, the flight received an IFR clearance and took off at 14:55. The aircraft climbed to 16,300 feet before beginning a descent to 10,000 feet at 15:13.
Sequence of Events
During the descent, the flight data recorder (FDR) indicated activation of the Level III airframe de-icing system. At 15:18, the crew was cleared to enter a holding pattern at 10,000 feet. The de-icing system activated again three minutes later. At 15:56, the controller instructed the flight to descend to 8,000 feet. The crew reduced engine power to flight idle, set propeller speed to 86 percent, and engaged the autopilot in vertical speed and heading select modes.
At 15:57:21, while descending in a 15-degree right-wing-down attitude at 186 knots indicated airspeed (KIAS), the cockpit voice recorder (CVR) captured the flap overspeed warning. The crew retracted flaps from 15 to 0 degrees, increasing angle of attack (AOA) and pitch attitude. At 15:57:33, as the aircraft passed through 9,130 feet, AOA exceeded 5 degrees, and ailerons deflected right-wing-down. Half a second later, ailerons rapidly deflected to 13.43 degrees right-wing-down, the autopilot disconnected, and the aircraft rolled rapidly right. Pitch and AOA decreased, then the ailerons returned nearly neutral as the roll stopped at 77 degrees right-wing-down. The aircraft then rolled left toward wings level, with elevator moving nose-up and AOA increasing.
At 15:57:38, as the roll passed 59 degrees right-wing-down, AOA again exceeded 5 degrees and ailerons deflected right-wing-down. The captain's nose-up control column force exceeded 22 pounds, and the aircraft rolled rapidly right at over 50 degrees per second. The captain's force decreased below 22 pounds at 120 degrees, and the first officer's force exceeded 22 pounds after the aircraft passed inverted (180 degrees). Nose-up elevator inputs were recorded throughout, with AOA increasing accordingly. At 15:57:45, the aircraft completed a full roll to wings level, then nose-up elevator and AOA decreased, ailerons deflected left-wing-down briefly, and the roll stopped at 144 degrees right-wing-down.
At 15:57:48, as the aircraft rolled left toward wings level, airspeed increased through 260 KIAS, pitch decreased through 60 degrees nose-down, normal acceleration fluctuated between 2.0 and 2.5 G, and altitude descended through 6,000 feet. At 15:57:51, the captain applied over 22 pounds nose-up column force, elevator moved to 3 degrees nose-up, pitch stopped decreasing at 73 degrees nose-down, airspeed exceeded 300 KIAS, and acceleration remained above 2 G. At 15:57:53, as captain's force decreased, first officer's force exceeded 22 pounds again, and the captain said "nice and easy." At 15:57:55, normal acceleration increased above 3.0 G. Approximately 1.7 seconds later, at 1,700 feet, elevator position and vertical acceleration increased rapidly. The last FDR data at 1,682 feet indicated 375 KIAS, 38 degrees nose-down pitch, 5 degrees nose-up elevator, and 3.6 G vertical acceleration. The aircraft impacted a wet soybean field partially inverted, nose-down, left-wing-low.
Investigation Findings
The National Transportation Safety Board (NTSB) determined the probable cause and contributing factors, updating its findings in September 2002 based on petitions for reconsideration.