NTSB Identification: CHI92IA030.
The docket is stored on NTSB microfiche number 46628.
Scheduled 14 CFR operation of SIMMONS AIRLINES INC. (D.B.A. AMERICAN EAGLE )
Incident occurred Wednesday, November 20, 1991 in CHICAGO, IL
Probable Cause Approval Date: 05/05/1993
Aircraft: AEROSPATIALE ATR-42-300, registration: N235RM
Injuries: 1 Minor,21 Uninjured.

NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.

THE INCIDENT AIRPLANE ENCOUNTERED VORTEX TURBULENCE WHILE FOLLOWING RADAR VECTORS FOR LANDING SEQUENCE. THE AIRPLANE WAS NOT DAMAGED. A FLIGHT ATTENDANT SUSTAINED MINOR INJURIES DURING THE ENCOUNTER. THE AIRPLANE WAS OPERATING IN A TERMINAL CONTROL AREA UNDER POSITIVE RADAR CONTROL. FAA RECORDS AND RADAR TRACK DATA SHOW A LIKE CATEGORY AIRPLANE WAS APPROXIMATELY 4 NAUTICAL MILES IN FRONT OF THE INCIDENT AIRPLANE WHEN IT DESCENDED TO AND MAINTAINED THE SAME ALTITUDE AS THE INCIDENT AIRPLANE. THE INCIDENT AIRPLANE WAS PROCEEDING ALONG APPROXIMATELY THE SAME GROUND TRACK AS THE LEAD AIRPLANE AND ENCOUNTERED THE TURBULENCE NEAR THE POINT THE LEAD AIRPLANE LEVELED AT THE SAME ALTITUDE. AT THE TIME OF THE ENCOUNTER THE LATERAL SEPARATION OF THE TWO AIRPLANES WAS 4.6 NAUTICAL MILES. THE MINIMUM ATC SEPARATION IN A RADAR ENVIRONMENT WHEN THE AIRCRAFT ARE WITHIN 40 NAUTICAL MILES OF THE RADAR ANTENNA IS THREE NAUTICAL MILES.

The National Transportation Safety Board determines the probable cause(s) of this incident to be:

AN INFLIGHT ENCOUNTER WITH WAKE TURBULENCE DURING CRUISE FLIGHT.

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