Narrative:

Route of flight was san-las. Incident occurred during radar vectors for visual approach to runway 25. Tower advised us of departing aircraft small transport. We reported 'no joy' and requested further advisories. Having reported runway 'in sight' tower cleared us for visual approach with stipulation to remain above 6000' MSL until past las 100 degree radial. We interpreted this to stay within TCA arc from 10-15 DME (which has floor of 6000' MSL). This request was similar to the normal clearance of staying above 8000' MSL until past las 110 degree radial. But since our route brought us within 10 NM of the field we continued our descent to 5000' MSL (which was floor of TCA within 10 DME until just north of henderson airfield). Passing 5600' tower advised us that we had descended past 6000' MSL without being north of the 100 degree radial. I replied that the TCA floor was 5000' MSL within 10 NM. Tower finally admitted that the restriction had nothing to do with the TCA but to provide separation from the departing traffic. This connection was never specified in any of the initial tower communication so we proceeded in accordance with normal TCA flight rules and airspace adherence. Confusion would never have existed if altitude restriction was linked to traffic avoidance.

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Original NASA ASRS Text

Title: ACR FLT CREW CLEARED TO REMAIN ABOVE 6000' UNTIL PAST A REFERENCED RADIAL. CONTINUED DESCENT BECAUSE FLOOR OF TCA CHANGED TO 5000'.

Narrative: ROUTE OF FLT WAS SAN-LAS. INCIDENT OCCURRED DURING RADAR VECTORS FOR VISUAL APCH TO RWY 25. TWR ADVISED US OF DEPARTING ACFT SMT. WE REPORTED 'NO JOY' AND REQUESTED FURTHER ADVISORIES. HAVING REPORTED RWY 'IN SIGHT' TWR CLRED US FOR VISUAL APCH WITH STIPULATION TO REMAIN ABOVE 6000' MSL UNTIL PAST LAS 100 DEG RADIAL. WE INTERPRETED THIS TO STAY WITHIN TCA ARC FROM 10-15 DME (WHICH HAS FLOOR OF 6000' MSL). THIS REQUEST WAS SIMILAR TO THE NORMAL CLRNC OF STAYING ABOVE 8000' MSL UNTIL PAST LAS 110 DEG RADIAL. BUT SINCE OUR ROUTE BROUGHT US WITHIN 10 NM OF THE FIELD WE CONTINUED OUR DSCNT TO 5000' MSL (WHICH WAS FLOOR OF TCA WITHIN 10 DME UNTIL JUST N OF HENDERSON AIRFIELD). PASSING 5600' TWR ADVISED US THAT WE HAD DESCENDED PAST 6000' MSL WITHOUT BEING N OF THE 100 DEG RADIAL. I REPLIED THAT THE TCA FLOOR WAS 5000' MSL WITHIN 10 NM. TWR FINALLY ADMITTED THAT THE RESTRICTION HAD NOTHING TO DO WITH THE TCA BUT TO PROVIDE SEPARATION FROM THE DEPARTING TFC. THIS CONNECTION WAS NEVER SPECIFIED IN ANY OF THE INITIAL TWR COM SO WE PROCEEDED IN ACCORDANCE WITH NORMAL TCA FLT RULES AND AIRSPACE ADHERENCE. CONFUSION WOULD NEVER HAVE EXISTED IF ALT RESTRICTION WAS LINKED TO TFC AVOIDANCE.

Data retrieved from NASA's ASRS site as of July 2007 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.