Small Transport; Low Wing; 2 Recip Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 11
Situational Awareness
ATC Issue All Types; Deviation - Altitude Undershoot; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Flight Deck / Cabin / Aircraft Event Illness / Injury; Inflight Event / Encounter Other / Unknown
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Provided Assistance
Human Factors
I took over the sector from an R-Side training team with Aircraft Y level at 140 and Aircraft X climbing slowly to 130. After sitting down and evaluating the traffic; I cleared Aircraft X to climb to 160 on his current heading; and direct BERYL intersection reaching 150. When I came back to reevaluate whether the plan was still working; Aircraft X was approaching 150 and turning toward BERYL; so it had worked out fine. I went on to do some more things; including issue a departure clearance off SGU when I noticed Aircraft X's altitude was down at 130 or so and climbing; with Aircraft Y opposite direction at 140. Given the close proximity and aircraft performance; I decided to descend Aircraft Y below to 130 and expedite Aircraft X's climb through 140 to 160. Aircraft X said he was showing his altitude out of 14-something; I think. I told him that it was not what I was showing and issued traffic alerts; further descended Aircraft Y and turned Aircraft Y because I didn't know what was happening with Aircraft X. Since Aircraft X had been radar identified prior to my sitting down; I decided to make sure the correct target was tagged up and reconfirming the altitude Aircraft X showed versus what I showed. At this point; Aircraft X sounded confused to me; and I began to suspect hypoxia; but figured it was probably his confusion between what I was showing and what he was reading in his aircraft. To be safe; I decided to turn him 10 degrees right to put him on V21 and descend him to 100. When I had a second; I checked my MIA and realized I could go to 100 without him being on the airway; so I descended him with a firm voice and cleared him on route. He read back 100 but his altitude eventually started to climb. When I again asked his altitude he said he was level at 160; his responses were slower and I really believed he might be hypoxic; so I started asking him questions to try to gauge whether he was responsive enough. Eventually I asked if he had oxygen on board and asked him to use it. His responses were getting clearer and he finally descended to 100. In the middle of all of this; I was trying to find the CIC who was apparently in another area talking to another CIC. I was able to get some help from a Co-worker at Sector 08 next to me. After confirming the oxygen on board and descending the aircraft; the pilot sounded different and maintained the correct altitude. I issued him our OMIC's [Operation Manager In Charge] phone number to call when he had time to clear up what may have happened. Pilots; don't forget oxygen! It's vital.
Reporter stated the QA department had another problem with this aircraft later in the day and that problem was attributed to an equipment problem.  Reporter stated that in reporter's opinion; this was definitely a hypoxia related problem; but could not prove it.
LA Center Controller reported an aircraft conflict likely due to pilot hypoxia.