Citation I/SP (C501)
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor
Flight Crew Last 90 Days 85; Flight Crew Total 8270; Flight Crew Type 2400
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Other / Unknown
Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport; Flight Crew Overcame Equipment Problem; General Maintenance Action; General Flight Cancelled / Delayed
Aircraft
My copilot and I were doing a demonstration flight at 28;000 and then on heading back to ZZZ out of 28;000 to 18;000 I noticed smoke coming out of the right side of the flight deck we immediately put on our oxygen mask and ran the emergency checklist and [requested priority handling] and headed back to ZZZ Airport and landed without any incident.
Citation I Captain reported a fumes event during descent.