Date: 201309
Local time of day: 1801-2400
Location: ZLA.ARTCC, CA
Flight conditions: VMC
Light: Daylight
Aircraft: Embraer Jet Undifferentiated or Other Model
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZLA
ATC / advisory: Center ZLA
Second aircraft: Embraer Jet Undifferentiated or Other Model
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Workload; Situational Awareness; Confusion; Communication Breakdown
Second reporter function: Captain; Pilot Flying
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Other / Unknown
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance
Contributing factors: Human Factors; Airspace Structure
Primary problem: Human Factors

Narrative: I was working a low altitude sector; feeding a high altitude sector. High altitude has three departure traffic flows all going over one NAVAID; as well as one departure traffic flow that goes head on into the three previously mentioned flows. In addition; high altitude has high altitude crossing traffic from both east and west; some still climbing to requested altitude. My low altitude sector has departures heading east to west; as well as arrivals heading west to east and one more flow of arrivals from northwest to southeast. In addition there is traffic from both east and west traveling level enroute or climbing to requested altitude. [Company] X19 and [Company] X90 were both departures heading from the west to the east. [Company] X19 was ahead and requesting FL370 as a final. [Company] X90 had LAS as its destination and was requesting FL210 as a final. Both aircraft were on frequency; with [Company] X19 being step-climbed to FL230 for converging traffic level at FL220. This traffic was a Kingair. [Company] X90 was climbed to the requested final of FL210 on check-on. [A B737] was an arrival from high altitude; descending to FL230. All four aircraft were converging at PMD VORTAC. As [Company] X19 cleared [the Kingair] (level at FL220); I climbed [Company] X19 to FL230. It took two calls to get a readback from [Company] X19. [Company] X90 was climbing to requested final of FL210 with [the Kingair] converging at FL220 and [the B737] descending to FL230 - again all converging over the same NAVAID. I issued [Company] X19 a frequency change to the high altitude sector; climbing to FL230. [Company] X90 read the frequency change back; and I failed to recognize the bad readback. I was thinking about the traffic call I needed to issue to [the B737] and [the Kingair]. [Company] X90 checks on to the high altitude frequency; clearly stating her call sign and her assigned altitude of FL210; two thousand feet below high altitudes stratum. However; high altitude had radar on 14 aircraft and only one was a [Company]. My guess is the high altitude Controller heard the pilot state '[Company]' and perhaps the 'nine' in the numbers and issued a climb. The high altitude Controller clearly stated '[Company] X-nineteen; LA Center; climb and maintain FL250.' To which the response from the pilot was '[Company] X-ninety; climb and maintain FL250.'I noticed that [Company] X19 was level at 230 but had 250 assigned as his climb clearance; and I call out '[Company] X19?' [Company] X19 answers and I issue the frequency change again. I did not think this was unusual; it happens quite often that pilots will forget to flip the switch or to check on. And [Company] X19 had already failed to respond to a climb clearance the first time; I assumed they were just distracted. In addition; this was the second time I knew high altitude was not talking to an aircraft but the data block reflected that they had issued a climb. When [Company] X19 checks in with the high altitude Controller; the Controller immediately senses something is wrong and asks; 'Wait a minute; [Company] X19 didn't you just check in and I climbed you to FL250?' At which point both aircraft attempt to answer at once. Meanwhile; [Company] X90 is climbing into [the Kingair] (FL220) and the B737 (descending to FL230). I key up and say '[Company] X90 descend and maintain FL210; turn 30 degrees right immediately;' but of course the aircraft is not on my frequency to respond. The high altitude Controller has quickly located [Company] X90 and checked my sector seeing the traffic and between ourselves we verbally coordinate high altitude by turning [Company] X90 right and descending him back to FL210. At the same time I have issued a traffic alert to [the Kingair]; letting him know the traffic will pass behind - and have stopped the B737 at FL240 to prevent him from catching the [Company] aircraft.This was a hearback/readback error for all four of us; both Controllers and both [Company] pilots. Had I been listening closely to the frequency change readback and stopped [Company] X90 from taking [Company] X19's frequency change; this would have all been prevented. If the high altitude Controller had heard the call sign or assigned altitude; this could have been prevented. If the pilots had heard the correct call sign; [Company] X19; with the frequency change or the altitude assignment - this would have been prevented. It seems like a case of the 'Swiss-cheese' theory that can happen in ATC. Had one of us been listening better; this never would have happened.


Second narrative: Climbing out...on...departure we were talking with SOCAL and then L.A. Center. When we checked on as [Company] X90 there was another '[Company]' on frequency ([Company] X19). Both aircraft were climbing out of the LA area heading eastbound at the same time. We were told by LA Center to climb and maintain FL250. We left FL210 and began our climb. Through FL220 we were asked by ATC why were climbing and where we were going. We told them that we were climbing to FL250 as assigned and read back for [Company] X90. The issue was that altitude was meant to be given for [Company] X19 even though it was given to us as [Company] X90. We immediately descended back down to FL210. [Company] X19 was then given instructions to climb and maintain FL250 while we remained at FL210 to KLAS.


Synopsis: Three Controllers and two pilots reported on an altitude deviation resulting from similar call signs which involved four aircraft converging on the same fix. Controllers recognized the problem and separated traffic.