Date: 201502
Local time of day: 1201-1800
Location: ZOA.ARTCC, CA
Light: Daylight
Aircraft: Large Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZOA
ATC / advisory: Center ZOA
Second aircraft: Large Transport
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Workload; Situational Awareness; Distraction; Confusion; Communication Breakdown; Fatigue
Second reporter function: Enroute; Handoff / Assist
Anomaly: ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Separated Traffic
Contributing factors: Procedure; Human Factors; Weather
Primary problem: Procedure

Narrative: I was the R-side working sector 30/33 combined.  I descended Aircraft X to FL330.  Aircraft X read back what I thought was FL 330; and then Aircraft X asked if that was 'Pilot discretion.'   I said 'Negative'; but did not restate the altitude because I thought the read back was correct.  Aircraft X then reported leaving FL350 for FL230.  I heard FL330.  I put Aircraft X on ZLA's frequency.  Aircraft X was descended to FL330 for procedures going into ZLA.   I was working 11 aircrafts in the sector to the north of this incident.  I had a D-side. I did observe Aircraft X at FL330 just before crossing the ZLA boundary.  It would have showed this for about 600 feet while descending.  At the ZLA boundary the Conflict Alert (CA) activated between Aircraft X and Aircraft Y; northwest bound at FL320.  I made a comment to my D-side questioning if maybe I issued FL310 instead of FL330.  My D-side thought that we issued the correct altitude.  It appeared that ZLA turned Aircraft X to the east; but it was a small turn.My 1st mistake was not hearing the readback.  My 2nd mistake was not reissuing FL330 after the pilot asked for 'Pilot's discretion.'  It did cross my mind.  I missed the 2nd read back; also. I should have called in sick.  I don't hear as well as I used to and don't deal with distractions as well; either.  The new control rooms for Display System Replacement (DSR) are very noisy compared to the old Main Display Monitor (MDM) control room.  I transferred to ZOA just over 2 years ago.  The DSR control room in ZOA is designed differently than the DSR control room in ZLC.  The 4 most used sectors are all up against the wall.  When training; there can be as many as 16 people in that small area.  I mentioned this in a previous report.  You can hear every clearance and conversation.  It is very noisy for an old controller.  Also; we have had radio problems with the MVA; specific transmitter site location; transmitter for days.  The radios are not very clear.  It sounds much clearer when listening to the playback than at the sector.  I had 2 breaks early in the shift; but did not eat anything because I thought I would get another break soon.  That was a mistake because I was feeling weak and unreadable towards the end of that session.  We were on the southeast plan for bay area arrivals and almost every aircraft needed to be re-cleared via a new arrival.  We also had some complex traffic situations earlier and I was feeling tired.  This is not an excuse.  I think it is time to retire.


Second narrative: Aircraft X was a Las Vegas arrival at FL350 and was required by ZLA to descend to FL330 as per LOA. The sector was unusually busy that day being on a southeast arrival plan to SFO and the Bay Area; which meant we were issuing a different arrival to every single plane that checks in on frequency; and updating each one's route; while maintaining separation; sequence; and avoiding active military airspace in the sector. We also had turbulent rides which made flights change altitudes repeatedly.  My R-side issued Aircraft X a descent clearance to FL330; in which I did not catch the read back because I was trying to keep up with checking routes and making helping the R-side separate incoming traffic. The R-side switched Aircraft X to ZLA; then shortly we noticed Aircraft X descending through FL330 while Aircraft Y was opposite direction at FL320. I wanted to call ZLA but we saw the targets vectored so I assumed they had made a mistake issuing a descent and turned both aircraft. I found out later from my supervisor that me and my R-side had missed the readback on Aircraft X and had taken a descent to FL230.The sector was busy at the time with the R-side sequencing; separating traffic; changing altitudes for the bad rides; and issuing routes while the D-side had to keep up with updating the routes and making coordination. ZLC is required to have all bay area arrivals on a west arrival route as default. I think if the proper routes were issued ahead of time instead of depending on ZOA to issue all of it and do the sequencing it may alleviate and lift some of the workload from both the R-side and D-side which in turn can focus on making sure everything is separated and be more focused on catching read backs.


Synopsis: ZOA Controller describes a read back hear back issue resulting in an aircraft descending lower than the Controller wanted. Controller complains about the noise level in the control room as a distraction. Descending aircraft was traffic for another aircraft 1000 feet below what controllers thought was the altitude assigned; resulting in a loss of separation.