Narrative:

Phase of flight was the descent to sat on the braun 2 RNAV arrival. Initial check-in with approach was uneventful. ATIS stated 13R in use. The brief and plan was for the RNAV (rnp) Z 13R from criss. I was pilot monitoring (pm) and requested the approach upon check-in with sat approach. I retrieved the [new] ATIS via ACARS. The runway in use was now 31L. The pilot flying (PF) began loading the approach in the FMS and changing approach plates. I revised the approach request for the RNAV (rnp) Z runway 31L from troop. Sat approach cleared us for the approach to cross troop at or above 6000 ft. I read back the clearance. I do not recall exactly the readback if I included the restriction. The PF asked if we were cleared the approach; I stated 'yes.' the PF was verbalizing something about the vertical path. The PF put 5000 ft. In the altitude window initially and then 2200 ft. When cleared for the approach. The waypoint altitude in the FMS shows troop as an at or below altitude. The ATC clearance was for at or above. ATC restated the restriction when we descended below 6000 ft. Prior to troop. A climb was initiated back to 6000 ft. To cross troop. The remaining part of the approach and flight were uneventful. Troop is an if on both the rnp Z 13R and on rnp Z 31L; but the altitude requirements are different. On the 13R approach; troop is at or above 6000 ft. On the 31L approach; troop is at or below 6000 ft. Since the airport had just switched runways; I suspect that the controller was expecting us to cross troop at or above 6000 ft. Yet once he cleared us for the 31R approach without any altitude instructions; we continued our VNAV descent below 6000 ft. I suspect he was surprised to see our aircraft below 6000 ft. Prior to troop.

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Original NASA ASRS Text

Title: B737-700 Captain reported an altitude excursion occurred on the BRAUN2 RNAV Arrival into SAT when TRACON issued a late runway change.

Narrative: Phase of flight was the descent to SAT on the BRAUN 2 RNAV Arrival. Initial check-in with approach was uneventful. ATIS stated 13R in use. The brief and plan was for the RNAV (RNP) Z 13R from CRISS. I was Pilot Monitoring (PM) and requested the approach upon check-in with SAT Approach. I retrieved the [new] ATIS via ACARS. The runway in use was now 31L. The Pilot Flying (PF) began loading the approach in the FMS and changing approach plates. I revised the approach request for the RNAV (RNP) Z Runway 31L from TROOP. SAT Approach cleared us for the approach to cross TROOP at or above 6000 ft. I read back the clearance. I do not recall exactly the readback if I included the restriction. The PF asked if we were cleared the approach; I stated 'yes.' The PF was verbalizing something about the vertical path. The PF put 5000 ft. in the altitude window initially and then 2200 ft. when cleared for the approach. The waypoint altitude in the FMS shows TROOP as an at or below altitude. The ATC clearance was for at or above. ATC restated the restriction when we descended below 6000 ft. prior to TROOP. A climb was initiated back to 6000 ft. to cross TROOP. The remaining part of the approach and flight were uneventful. TROOP is an IF on both the RNP Z 13R and on RNP Z 31L; but the altitude requirements are different. On the 13R Approach; TROOP is at or above 6000 ft. On the 31L approach; TROOP is at or below 6000 ft. Since the airport had just switched runways; I suspect that the controller was expecting us to cross TROOP at or above 6000 ft. yet once he cleared us for the 31R Approach without any altitude instructions; we continued our VNAV descent below 6000 ft. I suspect he was surprised to see our aircraft below 6000 ft. prior to TROOP.

Data retrieved from NASA's ASRS site 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.