Citation II S2/Bravo (C550)
Flight Crew Commercial; Flight Crew Flight Instructor; Flight Crew Multiengine; Flight Crew Instrument
Flight Crew Last 90 Days 125; Flight Crew Total 5300; Flight Crew Type 1000
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
General Flight Cancelled / Delayed; General Maintenance Action
Aircraft; MEL
We experienced a depressurization at FL400. I [requested priority handling with ATC] followed by a descent to below 10;000 feet. We then landed at ZZZ. Inspection of the airplane did not reveal any structural damage. This was discussed with the co-owner of the plane who is also a Mechanic and pilot with over 30;000 hours. He suggested that I fly the airplane back home unpressurized as allowed by the MEL. I agreed with him and filed a flight plan for 8;000 feet for the [next] morning. After uneventful preflight and startup we taxied to the runway. During takeoff at 90 knots IAS the airplane swerved to the left and vibrated due to a left main tire blowout. I was able to abort the takeoff and taxi the plane to the ramp. In my opinion these 2 events were not related. My concern after talking to the owner/mechanic again is that the MEL for unpressurized flight may not have been valid since the airplane was recently purchased. The MEL was resubmitted after purchase; but may still be in process with the FAA. For this reason it is possible that I unintentionally violated a regulation.
C550 pilot reported a loss of cabin pressure resulting in a diversion; subsequently departing back to home airport; possibly with an incorrect MEL.