Date: 201407
Local time of day: 0001-0600
Location: ZZZ.ARTCC, US
Aircraft: Any Unknown or Unlisted Aircraft Manufacturer
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Personal
Flight phase: Descent
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Second aircraft: Small Transport; Low Wing; 2 Turbojet Eng
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Time Pressure; Troubleshooting; Situational Awareness; Distraction; Confusion; Communication Breakdown
Anomaly: Aircraft Equipment Problem Critical; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Took Evasive Action
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: I was working 51 manual positions. Aircraft X checked on at FL240 requesting lower because they were running low on oxygen. Directly below the aircraft's path was Approach at 15;000 and below. Aircraft X was vectored left to get around Approach; because they do not allow overflights; and the aircraft was cleared to 16;000. I used a shout line to Approach to point out Aircraft X. The response to my point out was a garbled 'thank-you.' I asked for control; but the Approach Controller had apparently hung up. I noticed the aircraft was descending below 16;000 and recognized it was an emergency to get the aircraft down. I called Approach again several times on the shout line saying 'break for control' a few times before they picked up. I asked for and got control for Aircraft X. In the descent; Aircraft X's Mode C showed XXX; indicating a steep descent. In the path of Aircraft X was Aircraft Y; level at 11;000; inbound to ZZZ on a STAR arrival. Aircraft X's Mode C began reporting again as the aircraft passed behind Aircraft Y at approximately 1 mile. The Mode C of Aircraft X showed 10;100. At that point; we realized there was a loss of separation; possible airspace violation and possible pilot deviation. We verified the aircraft was again stable and then vectored Aircraft X to the west to get the aircraft back in our airspace.I did not receive coordination with the previous sector about Aircraft X's situation; if the pilot had made it known. It would have been easier to assist the pilot if Approach was able to answer the line quicker; and when they did; used good phraseology to let me know the aircraft was point out approved and my control before hanging up. I think the pilot did what they had to do to survive; but I would have liked to know how dire the situation was. It seemed like the pilot thought they were running low on oxygen; but were really out. Had we known the aircraft was going to descend that rapidly and in close proximity to Aircraft Y; we could have used a more aggressive vector.




Synopsis: ARTCC Controller reports of an aircraft that needs to descend due to lack of oxygen. Aircraft descends through another Controller's airspace. Aircraft Mode C shows XXX [not known altitude; usually when climbing or descending rapidly] and its path is in conflict with an arrival aircraft. Loss of separation occurs because aircraft in rapid descent Mode C works again and indicates 1;000 FT lower than conflicting traffic.