American Eagle Flight 4184

The flight was en route from Indianapolis International Airport, Indiana, to O'Hare International Airport, Chicago, Illinois. Bad weather in Chicago caused delays, prompting air traffic control to hold Flight 4184 over the nearby LUCIT intersection at 10,000 ft (3,000 m). While holding, the plane encountered freezing rain, a dangerous icing condition where supercooled droplets rapidly cause intense ice buildup. Soon after, they were cleared to descend to 8,000 ft (2,400 m). After this descent the pilots were ordered to enter another hold. During the descent, a sound indicating an overspeed warning due to the extended flaps was heard in the cockpit. After the pilot took action by retracting the flaps, a strange noise was heard on the cockpit voice recorder, followed by a sharp, uncommanded roll excursion that disengaged the autopilot. Flight recorder data showed that the aircraft subsequently went through at least one full roll, after which Aguilar and Gagliano regained control of the rapidly descending aircraft. However, another roll occurred shortly after as they engaged the autopilot again. This second occurrence was unrecoverable, and fewer than thirty seconds later, at 3:59 p.m., contact with Flight 4184 was lost as the plane crashed nose-down into a soybean field near Roselawn, Indiana, killing all 64 passengers and 4 crew on board.

The disintegration of the plane indicated extreme velocity, and data recovered from the flight data recorder showed that the plane had an indicated airspeed of 375 kn (694 km/h; 432 mph) at impact.[3] There was no fire. The bodies of all on board were fragmented by the impact forces; therefore, the crash site was declared a biohazard.[1]^:73

Flight 4184 was the first hull loss, and also the deadliest aviation accident, involving an ATR 72 aircraft.[3] Sixteen years later, Aero Caribbean Flight 883 crashed into a high range of terrain, also due to icing conditions, resulting in the same number of fatalities.[7]

As is customary in these reports, the National Transportation Safety Board (NTSB) issued a statement of "probable cause" near the end of its report. The probable cause statement in this report was considerably longer than is typical. The NTSB found that ATR (the manufacturer of that aircraft), the French Directorate General for Civil Aviation (the French counterpart of the American FAA), and the FAA itself had each contributed to that accident, because each had failed in its duty to ensure the highest possible level of safety to the traveling public.

The unabridged NTSB "probable cause" statement reads:

3.2 Probable Cause

The National Transportation Safety Board determines that the probable causes of this accident were 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 because: 1) ATR failed to completely disclose to operators, and incorporate in the ATR 72 airplane flight manual, flightcrew operating manual and flightcrew training programs, adequate information concerning previously known effects of freezing precipitation on the stability and control characteristics, autopilot and related operational procedures when the ATR 72 was operated in such conditions; 2) 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; and 3) 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: 1) 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 icing conditions not specified in 14 Code of Federal Regulations (CFR) Part 25, Appendix C; and 2) the FAA's inadequate oversight of the ATR 42 and 72 to ensure continued airworthiness in icing conditions.[1]^:210

The Bureau d'Enquêtes et d'Analyses pour la Sécurité de l'Aviation Civile (BEA) made its own separate investigation and agreed with the NTSB's cause of the accident being aileron deflection leading to loss of control. However the BEA stated in its response to the NTSB's report that the aileron deflection was caused by pilot error instead of ice, citing several off-topic conversations made by the crew during the holding phase, as well as the flight crew extending the flaps to 15 degrees while at a high speed, which can create large axial loads. The BEA also stated that the air traffic controller was not adequately monitoring the flight. However the NTSB refuted the BEA's arguments in their final report and in a separate detailed response article. In addition, there was no factual evidence to support the BEA's claims. The NTSB stated that the crews conversation took place in a non-critical moment of the flight and that the pilots were aware of the presence of ice on the wings. Therefore, the NTSB concluded that the non-pertinent conversation didn't contribute to the accident.[8]

In March 1995, some families of the victims discovered remains of their loved ones at the accident site, giving rise to a suspicion that cleanup efforts were not thorough. In a statement, the Newton County coroner - referring to other comments made - said he was not surprised there were remains left, given how serious the accident was.[9]

In April 1996 the FAA issued 18 Airworthiness Directives (ADs) affecting 29 turboprop aircraft which had the combination of unpowered flight controls, pneumatic deicing boots and NACA "five-digit sharp-stall" airfoils. They included significant revisions of pilot operating procedures in icing conditions (higher minimum speeds, non-use of the autopilot, different upset recovery procedures) as well as physical changes to the coverage area of the de-icing boots on the airfoils.

In the years following this accident, AMR Corporation stopped using its American Eagle ATRs out of its northern hubs and moved them to its southern and Caribbean hubs at Dallas-Fort Worth International Airport, Miami, Florida, and San Juan, Puerto Rico, to reduce potential icing problems in the future. Other U.S. former ATR operators, particularly the SkyWest, Inc., subsidiary and Delta Connection operator Atlantic Southeast Airlines, operated ATR 72 aircraft in areas where icing conditions were not common.

While the ATR 42 and ATR 72 aircraft are now compliant with all icing condition requirements imposed by those 18 ADs, the de-icing boots still only reach back to 12.5% of the chord. Prior to the accident, they had extended only to 5% and 7%, respectively. Robert Boser believes the ADs still fail to deal with the findings of tests conducted by British regulators at MoD Boscombe Down, which demonstrated that ice could form as far back on the wing as 23% of the chord, and on the tail at 30% of the chord. Both percentages are well beyond the reach of the extended deicing boots installed in compliance with the FAA ADs. Those tests limited the size of the droplets to 40 micrometres, near the maximum limit of the FAA design certification rules for Transport Category aircraft (Part 25, Appendix C), still in effect at that time of the Roselawn crash. "Extensive airborne testing" following the accident revealed that airliners can encounter water droplets exceeding 200 micrometers in average diameter.[10] It is likely that the lack of further ATR icing accidents during the 1990s is attributable to the changes in pilot operating procedures, as well as the moving of those aircraft to operating areas where severe icing is not a problem, rather than to the modest extension of the de-icer boots to 12.5% of the chord.

Three ATR 72's have since crashed due to icing: TransAsia Airways Flight 791 crashed on 21 December 2002, killing both pilots. Aero Caribbean Flight 883 crashed on 4 November 2010, killing all 68 people on board.[11] UTair Flight 120 crashed on 2 April 2012, due to a lack of de-icing of the aircraft prior to takeoff. 33 of the 43 people on board were killed. The crash of TransAsia Airways Flight 791 was attributed to water droplet size that was beyond Part 25, Appendix C of the FAA design certification.[12]^{:ii,131,156–157,178}

• This crash was featured in the Discovery Channel program The New Detectives in an episode titled "Witness to Terror". (The episode incorrectly gives the number of victims killed on the flight as 72.)[13]
• The crash was featured in the theatrical production Charlie Victor Romeo.
• The crash was featured in Season 7 of the Canadian-made documentary series Mayday, in an episode titled "Frozen in Flight".
• The crash was briefly mentioned in an episode of Modern Marvels ("Sub Zero") on The History Channel.

• Aero Caribbean Flight 883
• Air France Flight 447
• Air Ontario Flight 1363
• Arrow Air Flight 1285
• Colgan Air Flight 3407
• Comair Flight 3272
• Ice protection system
• Scandinavian Airlines Flight 751
• TransAsia Airways Flight 791
• USAir Flight 405
• UTair Flight 120
• West Caribbean Airways Flight 708
• West Wind Aviation Flight 280

• Accident description at the Aviation Safety Network
• "NTSB AAR-96/01 – NTSB Aircraft Accident Report" (PDF). (3.58 MiB), 340 pages)
• "NTSB AAR-96/02 – Comments of Bureau Enquête-Accidents" (PDF). (4.42 MiB), 341 pages)
• PlaneCrashInfo.Com entry on Flight 4184
• Tv.Com – New Detectives: Witness to Terror (Details Flight 4184 investigation)
• Site for Families and Friends of 4184