Date: 202104
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Gulfstream G650
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Second aircraft: PA-44 Seminole/Turbo Seminole
Reporter organization: Corporate
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Human factors: Workload; Distraction; Communication Breakdown
Second reporter function: Pilot Not Flying; Captain
Anomaly: Conflict NMAC
Detector: Automation Aircraft RA
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Took Evasive Action; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Aircraft; Airspace Structure; Human Factors
Primary problem: Airspace Structure

Narrative: On the Approach ATC advised us of uncontrolled VFR traffic at 2;500 feet  who was ducking under the Class Bravo.  We called visual and saw the aircraft on TCAS; which showed the aircraft 400 feet above us. We were at 2;000 feet on the approach. When the VFR aircraft was about a mile and a half; TCAS showed a descent of the offending traffic; and an RA to monitor vertical speed. I believed the descent mode of the TCAS to be inhibited; and since the aircraft was now showing 300 feet above us; I initiated a descent; bottoming out at 1;850 feet.Upon landing; we called the TRACON on the telephone; and were told by the person who answered that he believed the target had gone to a satellite airport. I spoke to the supervisor at that tower; and the Supervisor was able to tell me that the aircraft had actually gone to a different satellite airport; but the radar playback showed the targets merging with us at 1;900 feet; and the offending aircraft 300 feet above us. He allegedly then threaded the needle between the top of Class D; and the Class B; and went to its destination. It's been very problematic in the last few years of VFR aircraft flying under the Class Bravo; and us having to take evasive action beyond the TCAS RA to avoid collisions; especially on the approaches. This airspace really needs to extend the 30 mile ring to the surface; in order to protect all of the approaches into the very busy satellite airports in the area.


Second narrative: While on the RNAV Approach ATC pointed out traffic at our 11:00 o'clock position 500 feet above us; heading our way. We acquired the traffic visually; but shortly thereafter we got an RA to descend as the other aircraft; a light twin; started a descent. He passed right over head us. After landing; I called the TRACON to find out about that aircraft; but the Supervisor had no information as the shift had just changed. So we called a Tower. The controller there opened an investigation; and got back to us later. The controller indicated that the radar showed the two aircraft merged; and the vertical separation was only 300 feet. The controller also indicated that the light twin did not violate anybody's airspace. It threaded the needle between the top of Class D; and the bottom of the overlying Class B; on its way to its destination; and at the intersection on the approach it was in class E. Therefore this guy was completely legal the whole time. I believe the time has come to lower the floor of the Class B in this airspace to protect the approaches into some of the busiest General Aviation airports in America. I do not believe that the other pilot ever saw us; and it is time the FAA put a stop to guys threading the needle in these busy airspace locations.


Synopsis: Flight crew reported receiving an RA caused by a VFR light twin; while they were on final approach .