Ord local controller and responsible pilots reported an unsafe situation due to a readback-hearback ...

Narrative:

Working #3 local control; rolling 28R. Aircraft X advised they were not ready for departure from the N5 intersection. I made the decision to taxi them down to the full length of runway 28R to follow out aircraft Y. As aircraft Y passed the gg intersection; I then crossed an aircraft and taxied him to his ramp. After asking aircraft X if they were ready (they were); I told them to 'luaw' on 28R and traffic was crossing downfield. Using anticipated aircraft taxi speeds; I knew that the taxiing aircraft would be no factor by the time that aircraft X even got into position on runway 28R. Aircraft X read back 'cleared for takeoff'. I missed the readback and did not correct them. I wasn't expecting to hear that kind of transmission. I then went to the next aircraft on taxiway papa and told them to hold short of runway 28R at gg. Aircraft X started takeoff roll. I questioned what they were doing and they said they thought they were cleared for takeoff. At that point; I was pretty sure wake turbulence with aircraft Y wasn't going to be an issue so I chose not to cancel their takeoff at their high rate of speed. They decided to abort and exit the runway. I was relieved from the position after another 15-20 minutes where I was informed of the readback/hearback error and the pilot deviation. The relieving controller was told to issue the brasher warning to aircraft X before they departed. In this situation; it was merely a missed readback on my end and a pilot deviation from aircraft X. There were no distractions.

Original NASA ASRS Text

Title: ORD Local Controller and responsible pilots reported an unsafe situation due to a readback-hearback error. Line up and wait traffic attempted to depart while wake vortices from the previous departure may have still been present.

Narrative: Working #3 Local Control; rolling 28R. Aircraft X advised they were not ready for departure from the N5 intersection. I made the decision to taxi them down to the full length of RWY 28R to follow out Aircraft Y. As Aircraft Y passed the GG intersection; I then crossed an aircraft and taxied him to his ramp. After asking Aircraft X if they were ready (they were); I told them to 'LUAW' on 28R and traffic was crossing downfield. Using anticipated aircraft taxi speeds; I knew that the taxiing aircraft would be no factor by the time that Aircraft X even got into position on RWY 28R. Aircraft X read back 'cleared for Takeoff'. I missed the readback and did not correct them. I wasn't expecting to hear that kind of transmission. I then went to the next aircraft on taxiway Papa and told them to hold short of RWY 28R at GG. Aircraft X started takeoff roll. I questioned what they were doing and they said they thought they were cleared for takeoff. At that point; I was pretty sure wake turbulence with Aircraft Y wasn't going to be an issue so I chose not to cancel their takeoff at their high rate of speed. They decided to abort and exit the runway. I was relieved from the position after another 15-20 minutes where I was informed of the readback/hearback error and the pilot deviation. The relieving controller was told to issue the Brasher warning to Aircraft X before they departed. In this situation; it was merely a missed readback on my end and a pilot deviation from Aircraft X. There were no distractions.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1411559
Time
Date	201612
Local Time Of Day	0601-1200
Place
Locale Reference	ORD.Tower
State Reference	IL
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	A321
Operating Under FAR Part	Part 121
Flight Phase	Takeoff
Route In Use	None
Flight Plan	IFR
Aircraft 2
Make Model Name	B777 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Climb
Flight Plan	IFR
Person 1
Function	Local
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 5
Person 2
Function	Pilot Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	ATC Issue All Types Deviation - Procedural Clearance Ground Incursion Runway

Ord local controller and responsible pilots reported an unsafe situation due to a readback-hearback error. Line up and wait traffic attempted to depart while wake vortices from the previous departure may have still been present.