NTSB Identification: CHI97LA162.
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Accident occurred Sunday, June 08, 1997 in VALPARAISO, IN
Probable Cause Approval Date: 01/30/1998
Aircraft: Boeing 737-300, registration: N321UA
Injuries: 1 Serious,1 Minor,79 Uninjured.

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

While on initial descent into O'Hare International Airport, the flight crew of United 1517 received a Traffic Advisory (T/A) followed by a Resolution Advisory (R/A) from the Traffic Alert and Collision Avoidance System (TCAS). The flight crew reported their altitude as being about 11,500' when the T/A was received. The first radar hit on the target airplane was at 11,200'. The Captain executed an abrupt pull-up in order to avoid colliding with the other airplane. Two flight attendants were injured during the evasive maneuver. The pilot of the other airplane reported that when passing through 10,000' he noticed his Mode C transponder was not selected; at that time, he changed the selector position. Investigation of the event revealed the closest separation between the two airplanes was .1nm horizontally and 50' vertically.

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

a delay by the pilot of the other aircraft in turning on his mode C transponder.

Full narrative available

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