Small Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 6.0
ATC Issue All Types; Airspace Violation All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Air Traffic Control Separated Traffic; Flight Crew Exited Penetrated Airspace; Flight Crew Returned To Departure Airport
Manuals; Human Factors; Chart Or Publication; Airspace Structure; Procedure
Aircraft X checking in descending to 24000 feet. I continued to descend him to 14000 feet. The high altitude sector previously had MOA active 260B280 feet. [A military flight of 2] exited the airspace. However the controllers did not realize that [the military flight] was actually a flight of 2 and ONLY Aircraft Y departed. Aircraft Z remained in the MOA with their transponder off. The high sector declared MOA INACTIVE and had no clue Aircraft Z was still there. When the high sector controller realized there was still an aircraft in the MOA; they thought when they spoke to Aircraft Z he was at 25000 feet. So they immediately told me to stop Aircraft X at 26000 feet who was already descending out of 28800 feet. Unfortunately Aircraft Z was actually at 27000 feet when Aircraft Z actually began to squawk his beacon code. I called traffic immediately to Aircraft X.He told me he got a TCAS alert but no RA.  Prior to this incident while we thought the MOA was inactive; another aircraft departed going southeast bound and via coordination with the High altitude sector they allowed me to climb him to 27000 feet through the what we thought was inactive MOA. We don't think the aircraft ever got close but we are very uncertain of the situation.Controllers to verify number of aircraft in a flight when entering a MOA. Controllers should update Flight Plan to flight of 2 if they knew he was a formation flight and they decided to drop the second track!
MOA was activated and deactivated with Aircraft Y. A departure; traversed the deactivated MOA; climbing to 27000 feet; southeast bound. I noticed a fight plan in URET (User Request Evaluation Tool) on Aircraft Z and the route display indicated a position within the MOA. I asked the sector to confirm with Aircraft Y; their flight formation status and if they worked with Aircraft Z earlier in the MOA. Aircraft Y confirmed that they were no longer a formation flight and that Aircraft Z was still operating in the MOA. Seconds later; Aircraft Z checks in on our frequency at 25000 feet in MOA. I immediately called a sector to amend Aircraft X altitude and they stated they were no longer talking to Aircraft X. I instructed a sector to amend Aircraft X altitude to 26000 feet for traffic. Aircraft Z was assigned a discrete beacon code and the target and data block auto acquired 3 miles south of Aircraft X at 27000 feet. Aircraft Z was east bound and Aircraft X was northwest bound; descending through 27000 feet. Aircraft Z was issued traffic and a clearance to 25000 feet and recovery to destination.During a request for individual recovery; the flight leader will state the callsign and number of aircraft remaining in the MOA. Each sector will initiate a handoff on all aircraft in a formation flight with individual flight plans.
Center controllers report of a civilian aircraft receiving a TCAS alert from military aircraft operating within a mistakenly deactivated MOA. Confusion resulted from the misunderstanding that the military aircraft was not a single aircraft but rather a flight of two that had ceased joint operations and were subsequently operating independently.