Date: 202111
Local time of day: 1201-1800
Flight conditions: VMC
Light: Daylight
Aircraft: Citation X (C750)
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
Aircraft component: Air Conditioning and Pressurization Pack
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Troubleshooting
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Attendant
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Landed As Precaution; Flight Crew Diverted; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Overcame Equipment Problem; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: About halfway through our five hour flight from ZZZ to ZZZ1 up at cruise; the passenger came up to tell us; 'Hey guys; it smells like something is burning back here.'  I gave the controls to the Pilot Monitoring and left the flight deck to check the cabin.  There was a burning acrid odor like something electrical was smoldering but there was no visible smoke.  I advised the passenger to sit down and that I was going to lower the Masks so she could breathe fresh air and I assured her that I would be back to check on her after we formulated a plan of action. I returned to the flight deck and asked the PM to turn off the Interior Master switch to de energize the electrical systems in the cabin.  Next we pulled out and ran the checklist for Smoke and simultaneously advise ATC and diverted to ZZZ2.  We isolated the PACKs and determined that the odor went away after turning off the cabin pack.  We checked on our passenger several times and notified her of our intentions to divert to ZZZ2. Approximately fifteen minutes later; we landed and taxied in.  Upon arrival the fire trucks followed us to the FBO.  They checked for fire but couldn't find anything.  Follow up communication with Maintenance and [company] occurred and reports submitted.  We deplaned our passenger and informed her of the plan to have another jet pick her up shortly.  She was in excellent spirits given the situation.  We stayed onsite until she was safely onboard Aircraft Y.  We left the FBO when Aircraft Y taxied out.Once on the ground; after talking with Maintenance; it appears that the Cabin PACK had seized up and stopped functioning.  [Suggest] More frequent oversight and maintenance of the PACKs so they don't break down  in flight.


Second narrative: While in cruise flight; about 2.5 hours into the flight; the passenger approached the flight deck; and said; 'Something is burning back here!'. The Piot Flying (PF) transferred the flight to me; and left the flight deck to investigate. At that time; I smelled a smoldering metallic/electrical burning smell.  The PF calmed; and reassured the passenger; and returned to the flight deck.  I transferred the flight controls back to the PF; and formulated a course of action to take immediately. We diverted to ZZZ1; descended; and landed without incident. Follow up phone calls; and reports were submitted. Once on the ground; after conferring with Maintenance; it appears that the cabin pack seized; and stopped functioning. [Reporter suggested] more frequent oversight; and maintenance of the packs; so their reliability does not come into question.


Synopsis: Pilot reported that a passenger reported smoke in the cabin during cruise. The pilot elected to secure the cabin pack and the smoke dissipated. Pilot elected to divert and make a precautionary landing.