Large Transport
Flight Crew Last 90 Days 210; Flight Crew Total 13000; Flight Crew Type 8000
Distraction; Communication Breakdown; Confusion
Conflict NMAC; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Human Factors; Procedure; Aircraft
The captain was the flying pilot as we were approaching SAT runway 22 from the east.  We were slightly left of course and intercepting final with about a ten degree angle.  On about a ten mile final we received a TCAS traffic alert showing an aircraft at our right 3 o'clock about 700 feet above us and descending.  Looking out to the right I saw a aircraft; approaching form the west; on a right base turn to final also to runway 22.  His trajectory would have had him intercepting final at the same place and time that we would intercept final. Being that he was slightly higher and belly up to us I don't think he could see us.  As we gently turned away from him I noticed he had made a sharp turn away from us; turning back toward the west.  We had already switched to tower frequency; he was still on approach frequency so there was no direct communication between us.  Querying the tower controller we were informed the other aircraft was supposed to be following us.  We continued the approach to landing having never received an RA.  At the closest point I estimate the distance between the two aircraft was less than 800 feet laterally and 100 vertically.
Pilot reports of being on final and another aircraft coming very close to their aircraft and then it is turned away from them.  The aircraft was supposed to follow their aircraft.