Date: 202010
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: PA-31T Cheyenne I
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Other Post maint test flight
Flight phase: Cruise
Aircraft component: Air Conditioning and Pressurization Pack
Component problem: Malfunctioning
Reporter organization: Corporate
Reporter function: Pilot Flying; Single Pilot
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Landed As Precaution; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: This was a post maintenance test flight after ventilation system had been partially dismantled trying to troubleshoot a noise from vents.Departed ZZZ VFR and requested from Approach to depart to the southwest and climb to FL250 for post maintenance flight to check pressurization. Was given an IFR flight plan to ZZZ1 and eventually handed off to Center. Center cleared me to FL250 and was directed to maintain current heading...basically towards ZZZ1. Upon reaching FL250 I determined that the noise had not been resolved and I requested from ATC that I was done and would like to head back to ZZZ. Immediately there was a 'pop' and the aircraft began to rapidly depressurize followed by the cabin pressurization Master Warning light and buzzer. Everything happened before the controller could give me a new heading or altitude. I immediately pulled power back to idle; banked approximately 45 degrees to left and began a rapid descent. The controller stated that he 'noticed' I was in a left turn to which I replied that I was having some pressurization issues and needed to get lower. The controller asked if I wanted to declare an emergency to which I declined and stated that it's ok I just needed to get lower. I was then cleared to 17;000 feet and then handed off to Approach control. Remainder of flight back to ZZZ was uneventful.Although the controller did not call me out for altitude or heading deviation; I was concerned that I had initiated a turn and altitude change without clearance from the controlling ATC. My main concern was to fly the plane and handle the situation prior to advising ATC. The sky condition was unlimited visibility and ceiling and there was little to no traffic in the area. The time from the onset of the depressurization; me configuring the aircraft for a rapid descent and communicating with ATC was maybe 5-10 seconds.It was later determined that the shop had not securely reinstalled pressurized ducting in the nose of the aircraft and upon reaching FL250 that ducting separated from it's connection and vented pressurized air from the cabin to the outside.




Synopsis: PA-31T single pilot reported a pressurization system malfunction resulting in an uneventful mechanical air return to departure airport.