Widebody; Low Wing; 2 Turbojet Eng
Air Traffic Control Developmental
Communication Breakdown; Distraction
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Took Evasive Action; General Declared Emergency
Aircraft
Aircraft X was on route in level fight when I received an altitude out of conformance message generated in response to the aircraft's last ADS-C altitude report. The flight was cleared to and last reported at FL410; but the new altitude report showed the flight at FL400; and subsequent reports showed the flight continuing to descend. I sent a message for the pilot to confirm altitude via HF. When I saw the aircraft was continuing to descend I called Commercial Radio Operator for a phone patch with the aircraft; this occurred less than a minute after I sent the confirmation message via HF and before the pilot could respond to the confirmation.The pilot then declared an emergency via the CPDLC Emergency up-link and voiced; 'MAYDAY MAYDAY' on HF as heard by the Commercial Radio Operator. The HF frequency the pilot was using was unreadable and I had him switch to a different frequency; which was better. The pilot confirmed a number two engine shutdown due to a low oil pressure light; and the flight was descending to FL260 direct ZZZZ. I issued the appropriate clearance and there was no traffic. A colleague coordinated the emergency information with [a foreign] Center and [my Center] Supervisor; and I sent the new route and altitude to [the foreign Center] via AIDC; later completed by the Relieving Controller.No recommendations to stop the emergency; this type of emergency happens. I can only comment that it was an [Aircraft X type] and there have been previous issues with this equipment. I cannot comment on if this incident was related to the previous known issues. The HF frequency the pilot was using was unreadable and I could barely make out; after many attempts; the pilots intentions; and could not make out the nature of the emergency until the pilot changed to the new HF frequency. Even though the aircraft was CPDLC and ADS connected; I feel direct voice contact is the fastest and easiest way to communicate with the flight deck in emergency situations; and allows the flight crew to concentrate more on the aircraft and less on texting via CPDLC.The pilot having bad HF frequencies made this method unusable until new HF frequencies could be issued causing the pilot to add to his already high workload. HF frequencies for ADS/CPDLC connected aircraft could be better managed to ensure all aircraft have readable frequencies.
Oceanic Controller reports of an aircraft that is not at the correct altitude and continues its descent. Emergency is declared and aircraft is directed to airport.