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

Accident narrative

On October 31, 1994, Simmons Airlines Flight 4184, an ATR 72-212 operating as American Eagle, suffered a loss of control and rapid descent near Roselawn, Indiana, following an uncommanded roll excursion in icing conditions. The National Transportation Safety Board (NTSB) drafted an accident report, to which the French Bureau Enquetes-Accidents (BEA) submitted a formal response volume under Annex 13 to the Convention on International Civil Aviation. The BEA strongly disagreed with the NTSB’s draft analysis and probable cause, stating that the NTSB focused selectively on the aircraft manufacturer and French certification authorities while ignoring critical flight crew failures, Air Traffic Control actions, and the extreme meteorological conditions encountered.

**The flight**

Flight 4184 was a scheduled passenger flight from Indianapolis, Indiana, to Chicago O’Hare, Illinois. Aboard the aircraft were 64 passengers, two flight attendants, and two pilots.

The First Officer was the flying pilot for the leg. He had accumulated 5,176 hours of total flight time, with over 3,657 hours in the ATR. However, he was not type certificated for the ATR and did not hold an Airline Transport Pilot certificate. The Captain’s airman certification records showed that on March 10, 1993, he had failed an ATR-42 check ride, failing to competently demonstrate a single-engine non-precision approach.

**The sequence of events**

Prior to departure, the crew received a flight plan and weather package. American Eagle policy precluded the distribution of AIRMET Zulu Update 3 in the flight release. This AIRMET was applicable to the route and forecast light to occasional moderate rime icing in cloud and in precipitation.

Flight 4184 was delayed on the ground at Indianapolis for 42 minutes due to anticipated deteriorating weather in Chicago. The flight was cleared for takeoff at 14:55 Central Standard Time (CST) and became airborne at 14:56. The aircraft climbed to 16,300 feet, then descended toward 10,000 feet. During the descent, the crew increased propeller speed to 86% NP and activated the Level III airframe deicing system at 15:16:32.

At 15:18:07, Chicago TRACON cleared the flight to enter a holding pattern at the LUCIT intersection at 10,000 feet. The flight held for approximately 35 minutes. The first holding pattern was flown at 175 knots indicated airspeed (KIAS) with flaps retracted (0 degrees). At 15:23:12, the airframe deicing system was deactivated, and propeller speed was reduced to 77% NP. The Total Air Temperature (TAT) was +2.5°C. The ATR-72 Airplane Flight Manual (AFM) defined icing conditions as existing when TAT is below +7°C in visible moisture, requiring Level II anti-icing and 86% NP.

The Cockpit Voice Recorder (CVR) started at 15:27:59. A junior flight attendant was in the cockpit, and loud broadcast radio music was playing. During the second holding circuit, the Captain noted the aircraft was getting a high deck angle. The First Officer asked, "you want flaps fifteen?" and the Captain agreed. Flaps were extended to 15 degrees at 15:33:26. The ATR-72 Airplane Operating Manual (AOM) did not provide performance data for a Flaps 15 holding configuration.

At 15:33:56, a single tone similar to the Anti-icing Advisory System (AAS) caution chime sounded. The crew did not discuss it; the Captain was demonstrating the Ground Proximity Warning System to the flight attendant. A second caution chime sounded at 15:41:07. The crew selected Level III airframe de-icing and increased propeller speed to 86% NP. The flight attendant left the cockpit shortly after.

At 15:48:43, a pilot stated, "I'm showing some ice now." There was no further discussion of the icing. At 15:49:07, the Captain left the cockpit to use the restroom. He was absent for over five minutes. During this time, the First Officer flew the aircraft, operated the ACARS system, and engaged in intercom conversations with the cabin crew and the Captain.

The Captain returned to his seat at 15:54:47. At 15:55:42, the First Officer stated, "we still got ice." The Captain did not acknowledge the statement. At 15:56:24, a Traffic Alert and Collision Avoidance System (TCAS) warning sounded, which the crew did not acknowledge or discuss.

At 15:56:27, ATC cleared the flight to descend to 8,000 feet. Power was reduced to flight idle. The aircraft initiated a right turn, stabilizing at a 15-degree bank angle at 176 KIAS. At 15:57:22, the flap overspeed warning sounded at 186 KIAS. The flaps were retracted to 0 degrees. The Angle of Attack (AOA) increased from -1 degree to 6.5 degrees.

At 15:57:28.5, the autopilot disconnected. The left aileron abruptly deflected downwards, and the aircraft rapidly rolled to the right to a maximum bank angle of 77 degrees. The aircraft stopped rolling, then rolled back to the left to 59 degrees right wing down. At 15:57:33, the left aileron deflected downwards again, and the aircraft rolled rapidly to the right, continuing through an inverted position and pitching down to 55 degrees nose down. The airspeed increased to over 260 KIAS.

The Digital Flight Data Recorder (DFDR) indicated uncoordinated pitch inputs from both pilots, with erratic aileron and rudder deflections. The aircraft reached a maximum of 73 degrees nose down and 296 KIAS before the recording ended.

**What the investigation found: The BEA's analysis**

**Meteorological conditions and ice accretion** The BEA found the flight operated in freezing drizzle and freezing rain, with droplet diameters of 100 microns or more, well outside the 14 CFR Part 25 Appendix C certification envelope. The BEA calculated that the aircraft held for over 30 minutes in a cloudy atmosphere with liquid precipitation and a static air temperature varying between -2 and -4°C. Ice accreted at an estimated 1 to 2 millimeters per minute, resulting in a total thickness of 35 to 65 millimeters.

Post-accident testing, including flights behind an Air Force tanker at Edwards Air Force Base, revealed the specific mechanism of the upset. The crew's use of Flaps 15 in holding created a negative AOA on the outer wing. This caused ice to accrete in a ridge on the upper wing surface, aft of the active de-icing boots. When the crew retracted the flaps to 0 degrees, the AOA increased. The ice ridge caused airflow separation upstream of the ailerons, leading to an "aileron hinge moment reversal" that snatched the aileron to full deflection at an AOA far below the stall warning threshold.

**Flight crew performance** The BEA determined the crew failed to maintain a sterile cockpit during a critical phase of flight. The presence of the flight attendant, loud music, and non-pertinent conversations distracted the crew. The crew failed to follow icing procedures, initially leaving propeller speed at 77% NP instead of the required 86%, and holding with Flaps 15.

The Captain's departure from the cockpit left the First Officer with a high workload in severe icing conditions. The crew did not discuss the ice accumulation, did not provide a Pilot Weather Report (PIREP) to ATC, and did not request a different altitude to exit the icing. During the roll upset, the crew made uncoordinated control inputs and did not communicate regarding the aircraft's attitude or recovery actions. Post-accident testing showed the aircraft was fully recoverable with firm, coordinated aileron inputs.

**Air Traffic Control** The BEA noted that ATC improperly released the flight from a ground hold despite anticipated airborne delays. ATC extended the holding time multiple times without notifying the Central Flow Control Facility that delays exceeded 15 minutes. Furthermore, ATC failed to solicit PIREPs for icing conditions, as required by FAA orders. Had ATC solicited a PIREP, precautionary action could have been taken to exit the icing area.

**Previous ATR incidents and manufacturer warnings** The NTSB draft report claimed ATR knew about the aileron hinge moment reversal phenomenon from previous ATR-42 incidents, specifically a 1988 incident in Mosinee, Wisconsin. The BEA strongly disputed this. The BEA stated that DFDR data from previous incidents in Mosinee, Mauritius, Wales (Ryanair), Newark, and Burlington showed standard aerodynamic stalls caused by crew failures to maintain speed or use de-icing equipment. The specific "ice-induced aileron hinge moment reversal" phenomenon was discovered for the very first time during the post-Roselawn flight tests.

The BEA noted ATR had previously provided operators with warnings about freezing rain. Following the Mosinee incident, ATR issued an All Operators Information Message advising that freezing rain could affect aileron control forces, disconnect the autopilot, and cause a large bank angle. ATR provided an All Weather Operations brochure to Simmons Airlines in 1991, which quoted FAA Advisory Circular 20-117, warning that freezing rain exceeds the capability of ice protection equipment and should be avoided. The brochure instructed crews to exit freezing rain immediately and provided recovery procedures for roll anomalies.

**Certification and oversight** The NTSB draft criticized the French Direction Générale de l'Aviation Civile (DGAC) for inadequate oversight and failure to share continuing airworthiness information under the Bilateral Airworthiness Agreement (BAA). The BEA rejected this, citing a joint FAA/DGAC Special Certification Review that confirmed the ATR 42 and 72 were properly certificated. The BEA stated the DGAC fully complied with the BAA, sharing all relevant data from prior incidents that raised airworthiness questions.

**Probable cause**

The BEA proposed the following probable cause statement:

The Probable Cause of this accident is the loss of control of the aircraft by the flight crew, caused by the accretion of a ridge of ice aft of the de-icing boots, upstream of the ailerons, due to a prolonged operation of Flight 4184 in a freezing drizzle environment, well beyond the aircraft's certification envelope, close to VFE, and utilizing a 15 degree flap holding configuration not provided for by the Aircraft Operating Manuals, which led to a sudden roll upset following an unexpected Aileron Hinge Moment Reversal when the crew retracted the flaps during the descent.

The contributing factors to this highly unusual chain of events are: 1. The failure of the flight crew to comply with basic procedures, to exercise proper situational awareness, cockpit resource management, and sterile cockpit procedures, in a known icing environment, which prevented them from exiting these conditions prior to the ice-induced roll event, and their lack of appropriate control inputs to recover the aircraft when the event occurred; 2. The insufficient recognition, by Airworthiness Authorities and the aviation industry worldwide, of freezing drizzle characteristics and their potential effect on aircraft performance and controllability; 3. The failure of Western Airworthiness Authorities to ensure that aircraft icing certification conditions adequately account for the hazards that can result from flight in conditions outside 14 CFR Part 25, Appendix C, and to adequately account for such hazards in their published aircraft icing information; 4. The lack of anticipation by the Manufacturer as well as by Airworthiness and Investigative Authorities in Europe and in the USA, prior to the post accident Edwards AFB testing program, that the ice-induced Aileron Hinge moment reversal phenomenon could occur. 5. The ATC's improper release, control, and monitoring of Flight 4184.