American Eagle Flight 4184

American Eagle Flight 4184 was an American Eagle ATR 72 that crashed after flying into unknown icing conditions on October 31, 1994. Control was lost and all aboard were killed.

History
The aircraft, N401AM, was an ATR 72-212 operated by Simmons Airlines on behalf of American Eagle, the banner carrier regional airline branding program of AMR Corporation's regional system, prior to the formation of the fully certificated carrier named American Eagle Airlines. 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 make another hold. While in the holding pattern, a warning 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 an uncommanded roll excursion which disengaged the autopilot. Flight recorder data showed that the ATR subsequently went through at least one full roll with the crew able to regain control of the rapidly descending aircraft. However, another roll occurred shortly thereafter. Fewer than thirty seconds later, contact was lost as the plane crashed into a soybean field near Roselawn, Indiana (coordinates: 41.0944; -87.3222), killing all 64 passengers and 4 crew on board. The disintegration of the plane indicated an extreme velocity, and data recovered from the flight data recorder verified that the plane was traveling 375 knots (695 km/h) indicated airspeed at impact.

Flight 4184 was the first loss of an ATR 72 aircraft and was also the highest death toll of any aviation accident involving an ATR 72 anywhere in the world. Robert A. Clifford, a Chicago airplane accident attorney, represented 16 of the victims. As the trial was ready to begin, the defendants agreed to a record $110 million settlement and an apology in open court.

Probable Cause
Following the crash near Roselawn, longstanding problems with the ATR aircraft operating in some icing conditions were revealed by Stephen Fredrick (a whistleblower later fired by the airline).

The National Transportation Safety Board (NTSB) issued its usual statement of "Probable Cause" near the end of the report. However, that statement was considerably longer than is the norm for most other NTSB accident reports. The statement clearly blames ATR (the manufacturer of that aircraft), the French DGAC (the French equivalent of the American FAA), and the FAA itself, as contributing to that accident, because they all failed in their duty to ensure the highest possible level of safety to the traveling public. The unabridged NTSB Probable Cause statement:


 * 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.

Aftermath
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 alleviate 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.

In April, 1996 the American FAA issued 18 Airworthiness Directives (ADs), in an apparent attempt to prevent further icing accidents in ATR aircraft. 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.

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. They still fail to deal with the findings of the Boscombe Down tests, conducted by British regulators, which demonstrated that ice could form as far back on the wing as 23% of the chord, and on the tail at 30% of chord. Both percentages remain well beyond the limits of the extended deicing boots, installed in compliance with those 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 that accident, revealed it is possible for airliners to encounter water droplets exceeding 200 micrometers in average diameter.

It is likely that the lack of further ATR icing accidents 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.

Dramatization
This crash was featured on the Discovery Channel program The New Detectives and in the theatrical production, Charlie Victor Romeo.

The crash was also featured in Season 6 of Discovery Channel's Mayday / National Geographic's Air Crash Investigation in the episode "Frozen In Flight"