NTSB Identification: CHI02IA058.
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Scheduled 14 CFR (D.B.A. United Airlines)
Incident occurred Friday, December 28, 2001 in Chicago, IL
Probable Cause Approval Date: 03/30/2004
Aircraft: Boeing 737-522, registration: N941UA
Injuries: 104 Uninjured.
NTSB investigators used data provided by various sources and may have traveled in support of this investigation to prepare this aircraft incident report.
The Boeing 737 airplane encountered wake turbulence during cruise descent. The captain reported the aircraft was descending from 10,000 feet mean sea level (msl) when he noticed a "subtle vibration" that was followed by the airplane entering an uncommanded roll to the right. The captain reported that he disconnected the autopilot system at approximately 20 degrees right roll, and he regained control of the aircraft using manual control inputs of left rudder and left aileron. The captain stated that he had "complete usage of the rudder" during the recovery. The flightcrew declared an emergency and made an uneventful landing. Aircraft radar track data indicated the incident airplane (UAL Flt 1260) was following a Boeing 777, operated by American Airlines as flight 154 (AAL Flt 154). Both aircraft were en route to Chicago O'Hare International Airport and were flying the same arrival procedure. Both airplanes were maintaining a lateral separation of approximately 8.3 nm or about 97 seconds. AAL Flt 154 began its descent 12.56 nm from KRENA intersection, and 23 seconds later UAL Flt 1260 began its descent 6.23 nm from the intersection. During the uncommanded roll and subsequent recovery, all rudder control surface movements correlated with the position of the rudder pedals. No flight control anomalies were found during the post-incident examination and verification flight test.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: The wake turbulence encounter. Full narrative available
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