EMB ERJ 145 ER/LR
Air Traffic Control Fully Certified
Communication Breakdown
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance
Company Policy; Human Factors; Procedure
Air Carrier XXXX and a Company Air Carrier XXXY on the frequency at same time; not advised of similar sounding call signs; but both had been on same previous frequencies since SDF and CID; their departure locations. After Air Carrier XXXX checked on and was answered; the next transmission was for Air Carrier XXXY to descend now to FL340 then cross SAYGE at FL190 at 250 KTS. Air Carrier XXXY clearly read clearance back without any audible overlapping transmissions and the data block reflected that Air Carrier XXXY began descent. I noticed Air Carrier XXXX also descending when Mode C indicated FL357 and transmitted for Air Carrier XXXX to maintain FL360. The answer was stepped on by Air Carrier XXXY questioning the clearance. A second clearance to Air Carrier XXXX was given to turn 40 degrees right and maintain FL360. Response was also stepped on; but I heard Air Carrier XXXX say descending through FL355. After third transmission to Air Carrier XXXX asking him to verify level FL360; Air Carrier XXXX responded and verified level at FL360. The descent of Air Carrier XXXX put him in close proximity to an Airbus on another frequency (R9); responded to an RA after being turned by that controller to the right as well. The similar call signs directly contributed to this extremely close situation as well as the fact that the aircraft responded at exactly the same time. Assign aircraft distinctly different call signs and allow controllers to add letters to call signs if needed for clarity. After doing this before; I was corrected as it interrupted aircraft tracking for company trackers and passenger services.
[Narrative #2 contained no additional information.]
ZDV Controller described a loss of separation event listing similar call signs as the direct cause.