Narrative:

I was working stadium arrival sector into los angeles airport. Citation came in on the sadde arrival; and had been assigned descent to 7;000 ft. He was direct smo; then heading 070 degrees. I instructed; 'after santa monica; maintain 2;500 ft.' he read back the altitude. There was an overflight being worked by another sector at 4;000 ft northwestbound over lax. I looked up; and the citation had descended through 5;900 ft. I told him to stop his descent; then to maintain 5;000 ft. I issued a traffic alert; and instructed him to climb back to 5;000 ft now since I saw that he was continuing descent as I was speaking. I again issued traffic and asked if he had it in sight. He did; and I told him to maintain visual. The other controller had also noticed the conflict; and obtained visual separation with his aircraft. Since the clearance was a 4 word instruction after his call sign; I had no reason to believe that if he had heard his call sign; heard the altitude; that he had not heard the 2 word instruction in the middle (after santa monica). I do not know if he is unfamiliar with lax; but the 7;000 ft altitude to smo; then lower 'after' smo is standard. The sector prior; zuma; issues maintain 7;000 ft when the aircraft reaches sadde intersection. This is a standard rate descent from 10;000 ft; which will remain above 8;000 ft in departure airspace; be at 9;000 ft by 1 1/2 mile west of smo; below 8;000 ft 1 1/2 mile east of smo; and at 7;000 ft by the time it enters stadium airspace. If the instruction was 'cross smo at or above 7;000 ft; maintain 2;500 ft;' then the aircraft would have descent pilot's discretion until smo; then standard descent; which could result in airspace deviation. If the aircraft is given 'after smo; descend and maintain 2;500 ft;' then the aircraft is expected to continue previous standard rate descent to 7;000 given by previous controller; then standard rate descent after smo to 2;500 ft. This would separate from other airspace; and be at 7;000 ft by stadium airspace; which would comply with the SOP. I think that if the sadde arrival were to be extended to include smo 278 degree radial to smo; then 068 degree radial; with a crossing restriction between 80 and 70 at smo; pilots may more clearly understand the restriction and altitude violations would be less likely to occur. This arrival also should have a more clear lost communication instruction to include if NORDO transmissions received by smo 068 15 DME; fly heading 220 degrees to join runway 24R localizer; maintain XXX altitude; and proceed with the ILS. I also am unclear what descent rate this aircraft was using; since he descended from about 10;000 ft to 4;400 ft between sadde intersection and smo. Normally; even if aircraft are low at smo; they are not below 6;800 ft. The problem we would more often have is that the aircraft would be too high at smo. It was curious that he descended so very quickly. Supplemental information from acn 822564: we were on a short trip to lax. Upon reaching ventura (vtu) VOR; ZLA had cleared us to descend to 5;000 ft MSL from 9;000 ft MSL and direct to smo VOR. We were also given a speed to maintain -- I believe it was 210 KTS but it may have been higher. After that we got handed over to another controller (which I believe was socal approach control but not 100% sure) and he/she told us to descend to 2;500. Unfortunately; that is exactly what I heard and so did my co-pilot so; he repeated back the exact instruction 'descend to 2;500 ft; citation X' as expected. However; descending from 5;000 ft; we got a traffic alert and at the same time; ATC contacted us requesting us to get back to 5;000 ft. I saw my altitude at that point and it was just above 4;640 ft. I immediately climbed back to 5;000 ft. At that point ATC told us that we were supposed to maintain 5;000 ft until smo VOR and then descend to 2;500 ft. However; we did not hear the first part of that communications and only thing we heard thus repeated back was; 'descend to

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Original NASA ASRS Text

Title: SCT controller described separation event at 4500 FT with LAX arrival and overflight aircraft; resulting from missed incorrect altitude assignment readback.

Narrative: I was working Stadium Arrival sector into Los Angeles Airport. Citation came in on the SADDE Arrival; and had been assigned descent to 7;000 FT. He was direct SMO; then heading 070 degrees. I instructed; 'after Santa Monica; maintain 2;500 FT.' He read back the altitude. There was an overflight being worked by another sector at 4;000 FT northwestbound over LAX. I looked up; and the Citation had descended through 5;900 FT. I told him to stop his descent; then to maintain 5;000 FT. I issued a traffic alert; and instructed him to climb back to 5;000 FT now since I saw that he was continuing descent as I was speaking. I again issued traffic and asked if he had it in sight. He did; and I told him to maintain visual. The other Controller had also noticed the conflict; and obtained visual separation with his aircraft. Since the clearance was a 4 word instruction after his call sign; I had no reason to believe that if he had heard his call sign; heard the altitude; that he had not heard the 2 word instruction in the middle (after Santa Monica). I do not know if he is unfamiliar with LAX; but the 7;000 FT altitude to SMO; then lower 'after' SMO is standard. The sector prior; ZUMA; issues maintain 7;000 FT when the aircraft reaches SADDE Intersection. This is a standard rate descent from 10;000 FT; which will remain above 8;000 FT in departure airspace; be at 9;000 FT by 1 1/2 mile west of SMO; below 8;000 FT 1 1/2 mile east of SMO; and at 7;000 FT by the time it enters Stadium Airspace. If the instruction was 'cross SMO at or above 7;000 FT; maintain 2;500 FT;' then the aircraft would have descent Pilot's Discretion until SMO; then standard descent; which could result in airspace deviation. If the aircraft is given 'after SMO; descend and maintain 2;500 FT;' then the aircraft is expected to continue previous standard rate descent to 7;000 given by previous Controller; then standard rate descent after SMO to 2;500 FT. This would separate from other airspace; and be at 7;000 FT by Stadium Airspace; which would comply with the SOP. I think that if the SADDE Arrival were to be extended to include SMO 278 degree radial to SMO; then 068 degree radial; with a crossing restriction between 80 and 70 at SMO; pilots may more clearly understand the restriction and altitude violations would be less likely to occur. This arrival also should have a more clear lost communication instruction to include if NORDO transmissions received by SMO 068 15 DME; fly heading 220 degrees to join Runway 24R LOC; maintain XXX altitude; and proceed with the ILS. I also am unclear what descent rate this aircraft was using; since he descended from about 10;000 FT to 4;400 FT between SADDE Intersection and SMO. Normally; even if aircraft are low at SMO; they are not below 6;800 FT. The problem we would more often have is that the aircraft would be too high at SMO. It was curious that he descended so very quickly. Supplemental information from ACN 822564: We were on a short trip to LAX. Upon reaching Ventura (VTU) VOR; ZLA had cleared us to descend to 5;000 FT MSL from 9;000 FT MSL and direct to SMO VOR. We were also given a speed to maintain -- I believe it was 210 KTS but it may have been higher. After that we got handed over to another Controller (which I believe was Socal Approach Control but not 100% sure) and he/she told us to descend to 2;500. Unfortunately; that is exactly what I heard and so did my co-Pilot so; he repeated back the exact instruction 'descend to 2;500 FT; Citation X' as expected. However; descending from 5;000 FT; we got a Traffic Alert and at the same time; ATC contacted us requesting us to get back to 5;000 FT. I saw my altitude at that point and it was just above 4;640 FT. I immediately climbed back to 5;000 FT. At that point ATC told us that we were supposed to maintain 5;000 FT until SMO VOR and then descend to 2;500 FT. However; we did not hear the first part of that communications and only thing we heard thus repeated back was; 'descend to

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.