ATR 72-212 accident at Roselawn, Indiana, 31 Oct 1994 (N401AM)

Accident narrative

On October 31, 1994, at 1559 Central Standard Time, an Avions de Transport Regional ATR 72-212, registration N401AM, crashed near Roselawn, Indiana. The airplane, operated by Simmons Airlines, Inc., doing business as American Eagle flight 4184, was on a regularly scheduled passenger flight from Indianapolis, Indiana, to Chicago, Illinois. While descending in a holding pattern to a newly assigned altitude of 8,000 feet, the airplane experienced an uncommanded roll excursion and rapid descent. The airplane was destroyed by impact forces. The captain, first officer, two flight attendants, and 64 passengers received fatal injuries.

### The flight

Flight 4184 was dispatched from Indianapolis at a gross takeoff weight of 45,338 pounds. The flightcrew consisted of a 29-year-old captain who had accumulated 7,867 hours of total flight time, including 1,548 hours as pilot-in-command in the ATR, and a 30-year-old first officer with 5,176 total flight hours, including 3,657 hours in the ATR.

Prior to departure, the flightcrew received a weather package that did not contain airman's meteorological information (AIRMETs) or forecasts of light-to-moderate turbulence or in-flight icing conditions along the intended route. The flight was delayed on the ground for 42 minutes due to deteriorating weather conditions at Chicago's O'Hare International Airport before departing at 1455.

### Sequence of events

The flight climbed to a cruising altitude of 16,000 feet. At 1513, flight 4184 began a descent to 10,000 feet. During the descent, the flight data recorder (FDR) recorded the activation of the Level III airframe deicing system and the propeller speed at 86 percent.

At 1518, the flight was cleared to hold at the LUCIT intersection at 10,000 feet and given an expect further clearance (EFC) time that was subsequently revised multiple times due to traffic delays. The flight entered the holding pattern at 1524. The first holding pattern was flown at approximately 175 knots indicated airspeed (KIAS) with the wing flaps retracted. The airframe deice system was deactivated, and the propeller speed was reduced to 77 percent.

At 1533, the captain noted that the airplane had a high deck angle in the turns, stating they were "just wallowing in the air right now." The flaps were then extended to the 15-degree position, which decreased the angle-of-attack (AOA). At 1541, following a caution alert chime, the Level III airframe deicing systems were activated, and the propeller speed was increased to 86 percent.

At 1548, the first officer commented, "that's much nicer, flaps fifteen." Shortly after, one of the pilots stated, "I'm showing some ice now." The captain subsequently left the cockpit to use the restroom, returning at 1554. At 1555, the first officer noted, "we still got ice."

At 1556, air traffic control instructed flight 4184 to descend and maintain 8,000 feet. The airplane began to descend, engine power was reduced to flight idle, and the autopilot remained engaged. At 1557:21, as the airplane descended at 186 KIAS, the flap overspeed warning sounded. The flightcrew began retracting the flaps to the zero-degree position. As the flaps transitioned, the AOA and pitch attitude began to increase.

At 1557:33, descending through 9,130 feet, the AOA increased through 5 degrees, and the ailerons began deflecting to a right-wing-down position. Half a second later, the ailerons rapidly deflected to 13.43 degrees right-wing-down, the autopilot disconnected, and the autopilot disconnect warning sounded. The airplane rolled rapidly to the right.

The airplane stopped rolling at 77 degrees right-wing-down, then began to roll left toward a wings-level attitude as the elevator moved in a nose-up direction. At 1557:38, as the airplane rolled left through 59 degrees right-wing-down, the AOA again increased through 5 degrees, and the ailerons rapidly deflected right-wing-down again. The captain applied more than 22 pounds of nose-up control column force, and the airplane rolled rapidly to the right at a rate exceeding 50 degrees per second, rolling past the inverted position.

The airplane completed a full roll and passed through a wings-level attitude at 1557:45. The ailerons then stabilized near neutral, and the airplane began rolling left back toward wings level. The airspeed increased through 260 knots, and the pitch attitude decreased through 60 degrees nose down. The flightcrew applied nose-up elevator inputs, and the normal acceleration increased to over 3.0 G. At 1557:55, the ground proximity warning system sounded. The last recorded FDR data occurred at 1,682 feet, indicating an airspeed of 375 KIAS, a pitch attitude of 38 degrees nose down, and a vertical acceleration of 3.6 G. The airplane impacted a wet soybean field.

### What the investigation found

**The wreckage** The airplane was destroyed by impact forces. The outboard portions of both wings separated approximately 10 feet inboard from their respective tips, and the horizontal stabilizer separated. The fractures bore evidence of tensile overload, indicating the structural separation occurred in flight due to excessive aerodynamic loads. The engines and propellers sustained damage consistent with rotation under power at the time of impact.

**Weather analysis** The Board concluded that flight 4184 encountered a mixture of rime and clear airframe icing in supercooled cloud and drizzle/rain drops while in the holding pattern. The supercooled drops were estimated to be greater than 100 microns in diameter, with some as large as 2,000 microns. These conditions constituted freezing drizzle and freezing rain, which are outside the 14 CFR Part 25, Appendix C icing certification envelope that specifies a maximum mean effective drop diameter of 50 microns.

**Aircraft flight controls and icing tests** The ATR 72 lateral flight control system consists of unpowered, aerodynamically balanced ailerons. Following the accident, icing tanker tests were conducted at Edwards Air Force Base. The tests revealed that in freezing drizzle conditions (drop sizes of 100 to 140 microns), the ATR 72 accreted a jagged, spanwise ridge of ice on the upper wing surface just aft of the active portion of the deice boots.

**Aileron hinge moment reversal** The investigation determined that as the flaps were retracted during the descent, the airplane's AOA increased. When the AOA increased through 5 degrees, the ridge of ice caused the airflow over the right aileron to separate. This airflow separation caused a reversal of the aileron hinge moment characteristics, forcing the ailerons to deflect rapidly uncommanded. The deflection rate exceeded the autopilot's capabilities, causing it to disconnect. The unpowered ailerons required approximately 60 pounds of control wheel force to maintain a wings-level attitude, but the sudden, uncommanded roll and high control forces precluded a timely recovery.

**Prior incidents and manufacturer action** The Board examined the service history of the ATR 42 and 72 and found 24 roll control incidents reported since 1986, 13 of which were icing-related. Five of these occurred in weather conditions consistent with freezing drizzle or freezing rain, including a 1988 incident involving a Simmons Airlines ATR 42 in Mosinee, Wisconsin.

The Board concluded that before the Roselawn accident, ATR recognized the reason for the aileron behavior in the previous incidents and demonstrated that ice accumulation behind the deice boots, at an AOA sufficient to cause an airflow separation, would cause the ailerons to become unstable. Therefore, ATR had sufficient basis to modify the airplane and/or provide operators and pilots with adequate, detailed information regarding this phenomenon. However, ATR's subsequent actions, including a 1992 All Weather Operations brochure and simulator training packages, did not adequately communicate the known catastrophic potential of ATR operations in freezing rain, nor did they provide specific guidance on recovering from sudden autopilot disconnects and rapid, uncommanded aileron deflections.

**Oversight** The French Directorate General for Civil Aviation (DGAC) failed to require ATR to examine the combinations of icing conditions and airplane configurations that could produce the performance, stability, and control characteristics exhibited in prior incidents. The DGAC did not require ATR to take additional corrective actions, such as performing additional icing tests or developing additional airplane modifications. Furthermore, the FAA's Aircraft Evaluation Group did not receive pertinent documentation regarding the previous ATR icing incidents, hampering the FAA's ability to monitor the airplane's continued airworthiness.

**Flightcrew actions** The Board concluded that the flightcrew's actions were consistent with their training and knowledge. They were not provided adequate information by the manufacturer or regulatory authorities to recognize and cope with the problems experienced during an encounter with freezing rain. Neither the flight attendant's presence in the cockpit nor the captain's restroom break contributed to the accident.

### Probable cause

The National Transportation Safety Board determines that the probable cause of this accident was the loss of control, attributed to a sudden and unexpected aileron hinge moment reversal, that occurred after a ridge of ice accreted beyond the deice boots while the airplane was in a holding pattern during which it intermittently encountered supercooled cloud and drizzle/rain drops, the size and water content of which exceeded those described in the icing certification envelope. The airplane was susceptible to this loss of control, and the crew was unable to recover.

Contributing to the accident were 1) the French Directorate General for Civil Aviation’s (DGAC’s) inadequate oversight of the ATR 42 and 72, and its failure to take the necessary corrective action to ensure continued airworthiness in icing conditions; 2) the DGAC’s failure to provide the FAA with timely airworthiness information developed from previous ATR incidents and accidents in icing conditions, as specified under the Bilateral Airworthiness Agreement and Annex 8 of the International Civil Aviation Organization.

Contributing to the accident were: 3) the Federal Aviation Administration’s (FAA’s) failure to ensure that aircraft icing certification requirements, operational requirements for flight into icing conditions, and FAA published aircraft icing information adequately accounted for the hazards that can result from flight in freezing rain and other conditions not specified in 14 Code of Federal Regulations (CFR) Part 25, Appendix C; 4) the FAA’s inadequate oversight of the ATR 42 and 72 to ensure continued airworthiness in icing conditions; and 5) ATR’s inadequate response to the continued occurrence of ATR 42 icing/roll upsets which, in conjunction with information learned about aileron control difficulties during the certification and development of the ATR 42 and 72, should have prompted additional research, and the creation of updated airplane flight manuals, flightcrew operating manuals and training programs related to operation of the ATR 42 and 72 in such icing conditions.