Air Traffic Control Fully Certified
Confusion; Situational Awareness; Training / Qualification
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Separated Traffic; Air Traffic Control Issued New Clearance; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Took Evasive Action
Human Factors; Procedure; Aircraft; Airspace Structure
The following event occurred while I was working and all aircraft involved were in the PHL Bravo airspace at the time. Air Carrier Z was level at 4;000 and southbound on the downwind for Runway 35. A VFR Helicopter X; was northbound at 3;500; opposite direction to Air Carrier Z. A VFR Aircraft Y at 4;500; also northbound; following roughly the same track as Helicopter X; but about 2.5 miles behind it. All aircraft were on my frequency; and both VFR aircraft had been cleared into the Bravo by me and had been assigned their respective altitudes. In fact; a short time prior to this event; I gave Helicopter X the current altimeter; restated his altitude assignment of 3;500 and informed him were would be multiple targets opposite direction to him at 4;000.When the aircraft were southeast I issued traffic to Air Carrier Z at 11 to 12 o'clock and 3 miles; opposite direction; VFR at 3;500. He acknowledged my transmission. I can't remember if I again called traffic to Helicopter X. A very short time later Air Carrier Z advised me they had climbed due to a TCAS Resolution Advisory (RA). I looked at their target and observed them at 4;400; now nose-to-nose with Aircraft Y who was at 4;500; maybe only about 3 miles apart. I told Air Carrier Z I needed him 'back down' due to opposite direction traffic at 4;500; and he said roger. Seeing Air Carrier Z had now passed Helicopter X I descended them further to help expedite his descent away from the traffic; then turned Aircraft Y to the northeast to ensure that I had the airplanes pried apart.All three aircraft were level at valid; assigned altitudes. All were separated as per requirements; and the situation was safe and under control. Due to Air Carrier Z's response to a TCAS RA; a safe situation became very dangerous and chaotic for no reason.I would recommend that the 'tolerances' or 'sensitivity' of the TCAS on this aircraft be checked. I don't see any reason a TCAS RA should occur in congested Bravo airspace due to two vertically separated aircraft passing each other. A safe situation was turned into a near disaster by the TCAS RA and the crew's response to it. Additionally; perhaps flight crews should be given discretion as to whether or not they should respond to a TCAS RA in congested Bravo airspace; when the TCAS traffic in question has been issued to them by ATC.
A PHL area Approach Controller reported the controlled fixed altitude relationship of one IFR and two VFR aircraft with required 500 FT separation turned chaotic when the Air Carrier in the middle of the altitude sandwich responded to a TCAS RA. His suggested resolutions included giving flight crew discretion as to whether to respond to RA's in congested airspace.