Sea local controller reported an A321 climbed at a slower than normal and expected airspeed resultin...

Narrative:

It was an overcast day with ceilings too low to do pilot to pilot visual separation; but I could still see aircraft some ways out to achieve tower visual separation. Departed aircraft X (A321) and then next in line departed aircraft Y with what would normally be plenty of space to achieve 3 miles of separation. Aircraft X's speed on departure was not uncommonly slow immediately off the departure end; but then did not accelerate at all which is not typical. They stayed constant at about 170 knots.when aircraft Y tagged up I saw that there was not going to be 3 miles of separation immediately; but still had some room to achieve tower visual separation. So I was still thinking aircraft X should be speeding up at any moment and that 3 miles of separation would develop before I lost visual. I held on to aircraft Y and was going to wait for required separation to develop. I then pretty quickly realized that aircraft X was showing no signs of speeding up and decided to cap aircraft Y's altitude. At the time aircraft Y was showing either 2400 feet or 2500 feet and I instructed them to maintain 3000 feet. In response aircraft Y said that he would descend to 3000 feet as he was actually at 3300 feet at the time. I observed its mode C was indicating him at 2500 feet. At that point my local assist asked if I was going to leave them at 3000 feet and ship them to departure. I said yes and he called over to coordinate it. During the attempt to coordinate the departure controller was combative; unprofessional; and completely unhelpful and made no attempt to maybe help resolve the situation. After coordination had been made I shipped aircraft Y to departure.

Original NASA ASRS Text

Title: SEA Local Controller reported an A321 climbed at a slower than normal and expected airspeed resulting in the subsequent departure overtaking the A321.

Narrative: It was an overcast day with ceilings too low to do pilot to pilot Visual Separation; but I could still see aircraft some ways out to achieve Tower Visual Separation. Departed Aircraft X (A321) and then next in line departed Aircraft Y with what would normally be plenty of space to achieve 3 miles of separation. Aircraft X's speed on departure was not uncommonly slow immediately off the departure end; but then did not accelerate at all which is not typical. They stayed constant at about 170 knots.When Aircraft Y tagged up I saw that there was not going to be 3 miles of separation immediately; but still had some room to achieve Tower Visual Separation. So I was still thinking Aircraft X should be speeding up at any moment and that 3 miles of separation would develop before I lost Visual. I held on to Aircraft Y and was going to wait for required separation to develop. I then pretty quickly realized that Aircraft X was showing no signs of speeding up and decided to cap Aircraft Y's altitude. At the time Aircraft Y was showing either 2400 feet or 2500 feet and I instructed them to maintain 3000 feet. In response Aircraft Y said that he would descend to 3000 feet as he was actually at 3300 feet at the time. I observed its Mode C was indicating him at 2500 feet. At that point my Local Assist asked if I was going to leave them at 3000 feet and ship them to departure. I said yes and he called over to coordinate it. During the attempt to coordinate the departure controller was combative; unprofessional; and completely unhelpful and made no attempt to maybe help resolve the situation. After coordination had been made I shipped Aircraft Y to departure.

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	1443849
Time
Date	201704
Local Time Of Day	0601-1200
Place
Locale Reference	SEA.Tower
State Reference	WA
Environment
Flight Conditions	Marginal
Light	Daylight
Aircraft 1
Make Model Name	A321
Operating Under FAR Part	Part 121
Flight Phase	Initial Climb
Flight Plan	IFR
Aircraft 2
Make Model Name	Medium Transport Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 121
Flight Phase	Initial Climb
Flight Plan	IFR
Person 1
Function	Local
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 1
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict Deviation - Procedural Published Material / Policy Deviation - Speed All Types

Sea local controller reported an A321 climbed at a slower than normal and expected airspeed resulting in the subsequent departure overtaking the A321.