Two Center Controllers reported separated incidents on the same day of the same aircraft descending below their terrain minimums and diverting due to a rough running engine.
Synopsis
Two Center Controllers reported separated incidents on the same day of the same aircraft descending below their terrain minimums and diverting due to a rough running engine.
Narrative
I was working XXX with more than moderate traffic when Aircraft X [requested priority handling] with a rough running engine. The aircraft was enroute to ZZZ1 from ZZZ and wanted to divert to ZZZ2. I cleared the airplane direct and asked if they wanted a heading; and they did; so I assigned a 060 heading to ZZZ2. I asked if the pilot was familiar with ZZZ2 and they were. The airplane was slowly descending and I asked if they wanted an approach. They did want an approach so I cleared them direct ZZZZZ with a heading to get them started. Soon after the airplane descended below the MIA. Since this was a [priority situation] and I knew the engine was rough running and it would be dangerous to have the pilot climb I advised the pilot that the approach had a minimum altitude of 8200 ft. At ZZZZZ and after that it went to 6900 ft.Since the aircraft had just descended below the MIA I also advised the pilot of what the MIA was in that area; 8900 ft.; and that the ground would be at least 2000 ft. under that. The aircraft was below the MIA and I didn't want to force the pilot into a more dangerous situation; and he was about 8 miles or less from ZZZZZ and I cleared him for the approach. Since I had already given him all the information and he was below the MIA I did not give him a crossing restriction as that would possibly force him to try and climb and his speed had already slowed to around 80 kts. I kept the airplane until I was worried about losing him on radios and advised him to change to advisories and that I would send a plane over CTAF to make sure he landed safely. Later I observed the target land at the airport I sent and aircraft over and got a cancellation from the airplane. This was an event that had to do with the airplane and could not be avoided by me
Second reporter narrative
Aircraft X departed ZZZ2 after a prior [problem] earlier in the shift that one of my co-workers had been on position for. He originally diverted because of a rough running engine in the aircraft; so we knew we had to keep a close eye on them. Aircraft X departed the ZZZ2 airport in a slow climb on the ODP and then climbed in hold over the ZZZ3 VOR. After reaching the MEA on VXX he proceeded north on the airway. After some time passed I noticed the aircraft had descended below the MEA and MIA and began issuing safety alerts to the aircraft and treated it as an [priority] situation. The aircraft was unable to maintain a safe altitude and decided to divert to the ZZZ4 airport. I did my best to help the pilot try to navigate to the airport; at which time the aircraft entered VMC conditions and cancelled his IFR clearance and continued to his original destination (ZZZ5). This dangerous situation only developed because the pilot took an unnecessary risk in trying to fly his plane back home knowing that it wasn't running correctly; and in the dark in IMC conditions. Better training and maybe even some common sense could have avoided this situation entirely.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.