Date: 202104
Local time of day: 0601-1200
Location: DAL.Airport, TX
Flight conditions: Mixed
Weather elements / visibility: 7
Light: Daylight
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Ground DAL; Tower DAL
Second aircraft: Commercial Fixed Wing
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Communication Breakdown; Distraction
Second reporter function: Captain; Pilot Flying
Anomaly: ATC Issue All Types; Conflict Ground Conflict; Critical; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Detector: Person Flight Crew
When detected: Taxi
Result: Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented
Contributing factors: Procedure; Human Factors
Primary problem: Ambiguous

Narrative: During single runway operations immediately after a runway change from 31R to 13L due to winds; all Admin; Ground Operations; and checklists through taxi out were uneventful until Taxiway B; holding short of Taxiway M5. We estimated being #10 for takeoff. The Ground Controller gave us a slot time of (time) Zulu time; cleared us to cross 13L at M5 and told us to continue taxiing A5 then Alpha to Runway 13L. On the same radio the Ground Controller advised us the RJ (Regional Jet) ahead was cleared for takeoff and another aircraft was on three-mile final. That left one RJ on Alpha at the end of runway to taxi past while contacting Tower and running the Before Takeoff Checklist. With Lots of urgency in voice with words and tone from both Ground and Tower Controllers to all aircraft; the Tower Controller cleared us to line up and wait. We completed the Before Takeoff checklist and noticed an aircraft begin to taxi expeditiously; on taxiway Bravo on the other side of the runway. The Captain noticed the aircraft taxi and landing lights; stated his observation and asked F/O (First Officer); 'Why are his lights on?' F/O noticed the rate of Taxi Speed and lights then queried Tower Controller; 'Tower Confirm Aircraft X is cleared to line up and wait?' Tower responded but the radio call was quick and interrupted by another person transmitting. Captain and F/O were 100% confident Aircraft X was originally cleared to line up and wait. Due to a visual conflict with the other aircraft possibly receiving the same clearance; our Captain stopped our aircraft during the query and response. The other aircraft appeared to slow down on the other side of the runway. The next radio call was; 'Aircraft X; CLEARED FOR TAKEOFF 13L (text represents a QUICK radio call strung together). This was followed by 'Aircraft X you are past the hold short line 13L; continue to cross without delay join Bravo and Contact Ground. F/O repeated the call including 'Sorry for the mixup'. The rest of taxi; ground operations; and return to 13L for takeoff were uneventful with the following exception: all communications from Tower; although a different voice assumed to be a supervisor; continued to contain LOTS of urgency in voice; words; and tone from both Ground and Tower Controllers to all aircraft. When conducting single runway operations; especially during deteriorating weather conditions; Ground and Tower Controllers should make a conscious effort to avoid speaking too fast and share information when similar sounding flight numbers are on frequency. When recognized Controller saturation and urgency; we can increase our guard and awareness against expectation bias. Additionally; we could possibly slow the cadence when appropriate while reading back clearances.


Second narrative: Scenario in DAL wasn't optimum. Runway change had just occurred due to winds out of limits. 13R closed. Winds shifting and Rain/low visibility to the south of field. During taxi; four to five Aircraft are shutdown on Taxiway B due to weather at east gates. Six Aircraft shutdown on M for the same reason. Taxi instructions for Aircraft X; 'Taxi to 13L via B hold short of M5;' given initially. EDCT (Estimated Departure Clearance Time) time now given (not known prior to this) by Ground of (time) void by (time). Ground then gave us hold short 13L at M5. Cross 13L; A5; A; 13L; contact Tower. Corporate jet in hold area just north of Taxiway A; approaching 13L holding short. Ground Controller and Tower Controller sounded rushed / voice inflection during transmissions from taxi out to this point with all aircraft. Tower Controller stated; 'Aircraft X; lineup and wait Runway 13L.' F/O read back instructions to Controller verbatim; to lineup and wait 13L; with no response from ATC which seemed normal. 'Before Takeoff Checklists complete; final clear; 13L verified;' and I looked right; to double check final clear; rounding corner on A I looked left and saw a Company aircraft rounding corner on M6 toward 13L with LUAW (Lineup and Wait) Lights configured. I stated this to F/O; while simultaneously stopping aircraft at 45 degrees facing 290 degree estimate; just past the hold short line. Tower Controller then cleared other Company aircraft for takeoff 13L. He then stated to us 'Aircraft X; it appears you're past hold short line; taxi without delay across 13L to B; contact Ground and we will get you right back out. Never heard COMM jam or readback from other Aircraft X. When we were cleared LUAW and we read it back (all sounded normal in headset and transmission) and therefore I don't know how I could have prevented mistake. First time we knew of other Company aircraft call sign was when Tower cleared the aircraft for takeoff. The rest of DAL Ground Ops through departure phase was uneventful. Runway change / single runway operations / weather changing / departure gate closed; definitely played into non-standard operations both for ATC and Aircrew. Similar call signs should have been noted or announced by ATC to help prevent any confusion on ATC to aircraft instructions. Cadence and voice inflection are critical components in managing risk during less than optimal conditions for ATC. Although the task loading and pace for our phase of flight and internal communications did not feel rushed; the additive conditions; especially late notice EDCT could have contributed.  When I recognized Controller saturation potential and their urgency; we could have slowed down our transmissions and pace or have been more directive to them to slow down. Not only our craft; but all the aircraft that were on the taxi; could have helped slow things a bit.


Synopsis: Flight crew reported after receiving clearance to line up and wait; they observed another aircraft taxi onto the runway.