Narrative:

We were approaching the jatax IAF at 5;000 ft MSL for the RNAV (GPS) for runway 12R at sat. Approach control had just called out 11 o'clock traffic; which was now in sight but had not been acknowledged to ATC because of radio chatter. The first officer and I were talking about configuring for the approach. ATC issued instructions for the approach; which I took to be 'cleared RNAV (GPS) 12R. Fly the ground track from jatax.'because we were talking about the three-degree glide path configuration on the approach; the first officer missed the call so I read back the clearance; which I believed to be as stated above. The first officer questioned me as to our clearance when I set the MCP altitude to 3;500 ft for the hasdo crossing; and I said to him; 'yes; we are cleared for the approach.' VNAV began to take us down the glidepath. ATC was talking to our previously issued traffic and told us to maintain 4;000 ft MSL. ATC asked if we had the traffic in sight and we acknowledged we did. ATC told us; 'follow traffic to runway 12R; cleared for the RNAV (GPS) runway 12R.' we acknowledged that call and flew the approach uneventfully.on final just before the yabup the final approach fix; the controller asked something like; 'company flight number; it's not a problem but I might have led you to believe you were cleared for the approach back there. Is that how you took it?' I answered; 'yes. I'm sorry. I did think I was cleared for the approach.' the controller was very polite and professional at all times.given the question in the first officer's mind; we should have cleared the confusion with ATC at that time. If the ATC call was only to fly the RNAV ground track and not clearance for the approach; it would be much clearer if we were also given a maintain 5;000 ft in the instructions also.

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Original NASA ASRS Text

Title: A B737 left their assigned altitude following a hearback readback error with ATC.

Narrative: We were approaching the JATAX IAF at 5;000 FT MSL for the RNAV (GPS) for Runway 12R at SAT. Approach Control had just called out 11 o'clock traffic; which was now in sight but had not been acknowledged to ATC because of radio chatter. The First Officer and I were talking about configuring for the approach. ATC issued instructions for the approach; which I took to be 'Cleared RNAV (GPS) 12R. Fly the ground track from JATAX.'Because we were talking about the three-degree glide path configuration on the approach; the First Officer missed the call so I read back the clearance; which I believed to be as stated above. The First Officer questioned me as to our clearance when I set the MCP altitude to 3;500 FT for the HASDO crossing; and I said to him; 'Yes; we are cleared for the approach.' VNAV began to take us down the glidepath. ATC was talking to our previously issued traffic and told us to maintain 4;000 FT MSL. ATC asked if we had the traffic in sight and we acknowledged we did. ATC told us; 'Follow traffic to Runway 12R; cleared for the RNAV (GPS) Runway 12R.' We acknowledged that call and flew the approach uneventfully.On final just before the YABUP the final approach fix; the Controller asked something like; 'Company flight number; it's not a problem but I might have led you to believe you were cleared for the approach back there. Is that how you took it?' I answered; 'Yes. I'm sorry. I did think I was cleared for the approach.' The Controller was very polite and professional at all times.Given the question in the First Officer's mind; we should have cleared the confusion with ATC at that time. If the ATC call was only to fly the RNAV ground track and not clearance for the approach; it would be much clearer if we were also given a maintain 5;000 FT in the instructions also.

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.