An A320 captain reported a 300 ft near midair collision with a light aircraft on a visual approach w...

Narrative:

Shortly after being cleared for a visual approach on a left base leg and out of 5;500 ft the controller came back with an instruction to maintain 4;000 ft. I immediately stopped my decent and leveled off at 4;000 ft. Then I got a TCAS RA to climb which I followed as the first officer pointed out the traffic at our 10 o'clock and slightly beneath us. Then I saw a small white t-tail private aircraft pass directly underneath us by about 300 ft. Then discovered he was at 4;000 ft by the controller reprimanding him for not being at his assigned 3;500 ft. The TCAS saved our lives because we were able to climb to about 4;500 ft before cpa. Had we of just stopped at 4;000 ft we would have been on collision course.

Original NASA ASRS Text

Title: An A320 Captain reported a 300 FT NMAC with a light aircraft on a visual approach when the other aircraft did not maintain its assigned altitude and ATC detected that aircraft's deviation until just prior to the conflict.

Narrative: Shortly after being cleared for a visual approach on a left base leg and out of 5;500 FT the Controller came back with an instruction to maintain 4;000 FT. I immediately stopped my decent and leveled off at 4;000 FT. Then I got a TCAS RA to CLIMB which I followed as the First Officer pointed out the traffic at our 10 o'clock and slightly beneath us. Then I saw a small white T-Tail private aircraft pass directly underneath us by about 300 FT. Then discovered he was at 4;000 FT by the Controller reprimanding him for not being at his assigned 3;500 FT. The TCAS saved our lives because we were able to climb to about 4;500 FT before CPA. Had we of just stopped at 4;000 FT we would have been on collision course.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	917489
Time
Date	201010
Aircraft 1
Make Model Name	A320
Operating Under FAR Part	Part 121
Flight Phase	Landing
Route In Use	Visual Approach
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Conflict NMAC
Miss Distance	Horizontal 0 Vertical 300

An A320 captain reported a 300 ft near midair collision with a light aircraft on a visual approach when the other aircraft did not maintain its assigned altitude and ATC detected that aircraft's deviation until just prior to the conflict.