If they lost their certificate, how do the still operate with the TAG-US call sign?
The U.S. operations of TAG and AMI have been purchased by Sentient. They have also purchased Atlantic Aviation Flight Services, Talon Air, Raytheon Aircraft Charter/Management, and Summit.
and JetDirect, and Regal, and Presidential, and Sunset, and a few more I’m missing. But my question was how/why do the use the Tag callsign if they are on the Sentient Cert.
and Air Group, and Southwest Jet, and Spirit, and Jetcorp.
Sentient doesn’t have it’s own certificate. Sentient is a charter broker/membership program that flies it’s clients on aircraft operated by its (Sentient’s) wholly owned subsidiary’s which are the actual certificate holders.
From Sentient’s literature:
All flights are operated by FAR Part 135 air carriers (“Operators”), who shall maintain full Operational Control of charter flights at all times. Operators providing service for Sentient Jet Membership must meet both FAA safety requirements and additional standards established by Sentient Jet, Inc. Some flights will be operated by Sentient Flight Group, LLC or by one of the wholly-owned subsidiaries of Sentient Jet Holdings, LLC, each as an individual FAR Part 135 air carrier with full Operational Control of their respective charter flights and each operating in full compliance with all FAA requirements and Sentient Jet, Inc. standards.
TAG is still TAG, under its own certificate…its just owned by Sentient Flight Group, or Holdings, or whatever way they have it structured.
Actually, Sentient DOES have it’s own 135 certificate, it just isn’t under the Sentient name…First Atlantic, then, uhhh, everyone else
BD Ventures a Philly based private equity firm purchased JetDirect, then Jetcorp, Summit, Regal. Then purchased Sentient who already owned Atlantic. The company just decided to move forward with the Sentient brand, and management team.
They (Sentient Flight Group/Holdings) own many certificates/operators now. But Sentient Jet, the original company, does not have a certificate with that name on it. The actual certificate holders are set up purposely under Sentient Flight Group/Holdings as separate entities at arms length from Sentient Jet for liability protection.
Yeah, yeah, yeah…I know. I had the pleasure of working for them. Don’t make me call you a name 
I can commiserate with you my friend… They are a PITA to conduct trips for… The incessant phone calls…and people think that I have control issues… 
Actually worked for Atlantic, left just as Sentient was taking over. We’d do charters for them and get the calls CONSTANTLY! And they were trying to exercise control of the crew and aircraft too. We’d pick up the phone, tell them to stop calling our cellphone/FBO/whatever, and hang up. The questions were great though: “What are you wearing?” Huh? Kinky bugger! Best ever was: " And make sure you keep the cabin at a comfortable temperature for our PAX…" Stuuuuupid people
We call them the Sentient robots. 
The TAG call signs after the certificate revocation were managed airplanes flying part 91.