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.
Synopsis
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.
Second reporter narrative
I was performing supervisory duties when I turned to face the area; Conflict Alert activated at Sector 35; as I went to the sector I observed Air Carrier X leaving FL284 descending approximately 3 miles behind another Air Carrier level at FL280; the following update Air Carrier X was at FL277. The Controller had passed the the traffic and then inquired to Air Carrier X his assigned altitude which was FL250 not FL290 as depicted in the data block. The previous Controller stated in the relief briefing that he had been given a restriction to cross 25 miles north of NAVHO at FL290; upon review of the tapes Air Carrier X read back FL250. Make sure you get a proper read back from pilot; if in doubt reiterate clearance and always pass traffic.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.