Narrative:

In order to meet altitude restriction requirements for ZAB; aircraft landing phx need to be at or below FL290 by their airspace. This is a confliction point for aircraft westbound to las. In order to miss aircraft at FL300; I gave a crossing restriction to air carrier X to cross 20 miles north of navjo at and maintain FL290. Air carrier X didn't read the restriction back and questioned the clearance. I then instructed air carrier X to cross 25 miles north of navjo at and maintained FL290. The pilot read back 25 north of nqavjo at 250. I was satisfied with the read back because I thought he said 25 north of navjo at 290. After I gave this clearance another controller plugged in to get a relief briefing. He was not plugged in to hear the read back. The incident did not happen until I left the area. The other aircraft was an air carrier westbound at FL280. It is my understanding that the controller issued a traffic call to air carrier X. Air carrier X said that he was in a slow descent and then responded to an RA. I was on break for a few minutes when I found out about the incident. The numbers 5 and 9 can sound similar when they are not spoken correctly for air traffic. If a pilot does not say 'niner' for the number nine then we should get the read back again. Calling traffic for the descending aircraft was done in this situation; and could have prevented this occurrence; however the pilot did not question the traffic call.

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Original NASA ASRS Text

Title: ZLA Controller described a loss of separation event when an incorrect altitude assignment read back was missed by both the controller working and the controller in the process of relieving the position.

Narrative: In order to meet altitude restriction requirements for ZAB; aircraft landing PHX need to be at or below FL290 by their airspace. This is a confliction point for aircraft westbound to LAS. In order to miss aircraft at FL300; I gave a crossing restriction to Air Carrier X to cross 20 miles North of NAVJO at and maintain FL290. Air Carrier X didn't read the restriction back and questioned the clearance. I then instructed Air Carrier X to cross 25 miles north of NAVJO at and maintained FL290. The pilot read back 25 north of NQAVJO at 250. I was satisfied with the read back because I thought he said 25 north of NAVJO at 290. After I gave this clearance another controller plugged in to get a relief briefing. He was not plugged in to hear the read back. The incident did not happen until I left the area. The other aircraft was an Air Carrier westbound at FL280. It is my understanding that the controller issued a traffic call to Air Carrier X. Air Carrier X said that he was in a slow descent and then responded to an RA. I was on break for a few minutes when I found out about the incident. The numbers 5 and 9 can sound similar when they are not spoken correctly for air traffic. If a pilot does not say 'niner' for the number nine then we should get the read back again. Calling traffic for the descending aircraft was done in this situation; and could have prevented this occurrence; however the pilot did not question the traffic call.

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.