Date: 201905
Local time of day: 1801-2400
Location: MDW.Airport, IL
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
ATC / advisory: Tower MDW
Second aircraft: B737 Undifferentiated or Other Model
Reporter organization: Government
Reporter function: Ground
Reporter qualification: Air Traffic Control Developmental
Human factors: Situational Awareness; Communication Breakdown
Second reporter function: Pilot Not Flying; First Officer
Anomaly: ATC Issue All Types; Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Detector: Person Air Traffic Control
When detected: In-flight; Taxi
Result: Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Procedure
Primary problem: Procedure

Narrative: The Local Controller cleared Aircraft X for takeoff from Rwy 22L; however Aircraft Y tried to Depart Rwy 31C the intersecting runway. The Local Controller took action to stop Aircraft Y to avoid a further incident and Aircraft X aborted Takeoff clearance.Pior to both Aircraft reaching the runway I was working Ground Control; I pushed and Taxied Aircraft X to RY 22L. Then about 3-4 minutes Later Aircraft Y strip was passed to me and I pushed Aircraft Y off his gate. About 90 seconds later LC asked me to switch Aircraft X so he could depart; I reached out to Aircraft X but I never got a response; so to the best of my knowledge Aircraft X was no longer on my frequency and I was not talking to both aircraft at the same time. Then it was a full 7-8 Minutes before I taxied Aircraft Y to Rwy 31C; approx. 3 minutes after Aircraft Y pushed back. I was unaware that Aircraft X had not departed at this point; so I didn't give the 'similar sounding call sign' phraseology as I only had Aircraft Y on my frequency at the time; then 2 minutes later the incident happened when Aircraft Y had reached 31I would recommend that if there is a chance that the first aircraft could get delayed at the runway then within at least 10 minutes of having a similar sounding call sign aircraft; then just issue the phraseology so that at least 1 aircraft is aware they may encounter a similar sounding call sign when switching from GC to LC; or sector to sector. Furthermore you could advise the next section to point it out that they may have both aircraft at the same time.


Second narrative: We were running about an hour and fifteen minutes behind schedule because of an aircraft swap. During the taxi out; we were assigned Runway 31C. Taxiing towards the southeast on Golf; and approaching Taxiway Echo 'E' and the runway 31C hold position; we were finishing up the 'Before Takeoff Checklist'. At the same time [Other Carrier] Aircraft X was given instructions to line up and wait on Runway 22L. We were unaware of this because we were still on Ground Control frequency when Tower gave [Other Carrier] Aircraft X the lineup and wait instructions.I switched to Tower frequency; several seconds later Tower issued a takeoff clearance to [Other Carrier] Aircraft X. The [Other Carrier] Aircraft X call sign was a little muffled and hard to clearly understand. Being number one at 31C; and just switching to Tower frequency and hearing 'XXX' cleared for takeoff; we fell into the expectation bias trap and heard what we expected to hear and not the actual clearance. I read back cleared for takeoff 31C left heading 180 degrees [Company XXX]. Tower immediately corrected us and said '[Company XXX] that was not for you' but in same transmission gave us line up and wait 31C.' We were approaching the hold position line and it would have been an abrupt stop to get the aircraft to stop before the hold position line. However; being given the lineup and wait in the same transmission we rolled on to 31C and held our position.[Other Carrier] hearing what was happening rejected their takeoff. We were given a possible Pilot Deviation phone number to call while in position on 31C. After departing we continued the flight without further incident. Several factors led to this event. Not being told by either Ground or Tower that [Other Carrier] was on frequency and taxiing out for departure at the same time as [Company]. We had to do a bleeds off Takeoff for performance which is not unusual; but not done every leg so I was a little distracted by thinking of that procedure. I also think in the few moments before takeoff; Pilots are doing a personal mental checklist of items and slightly distracted; and then hearing XXX cleared for takeoff; the expectation bias kicked in and we fell into the trap of hearing what we expected to hear not the clearance.I think if the call sign numbers are identical [Company XXX] versus [Other Carrier XXX]; there needs to be a positive affirmation by both aircraft that each other are on frequency; especially for critical operations such as takeoff and landing. Switching to Tower frequency a bit earlier to be able to listen and create mental picture of what ATC is doing may have helped; since we possibly would have heard the [Other Carrier XXX] line up and wait instructions when we were still a long ways from 31C. Clarification of any garbled radio transmissions. We heard the XXX clearly; but not the [Other Carrier] and because we were [Company XXX] we assumed it was our clearance.


Synopsis: MDW Controller and an air carrier flight crew reported an unsafe situation due to similar sounding call signs.