Date: 202010
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Takeoff / Launch
Aircraft component: Cockpit Window
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Distraction
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: Other Takeoff Roll
Result: Flight Crew Rejected Takeoff; Flight Crew Returned To Gate
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: We started our takeoff roll at ZZZ. It was the FO's (First Officer's) leg. Shortly after the 'set takeoff thrust call' my window on the Captain's side started to slide open. I'd estimate our speed at 40 kts. It was barely open so I said 'continue' and reached up to close it. However; the window would not close and actually felt like it was coming out of the track. I assumed the window would easily close and that we still had plenty of time to safely reject at low speed if it wouldn't. At that point I became distracted and was worried that the window was falling in. At that point the FO asked if I wanted to reject and I said 'yes'. He initiated the reject as he saw that I was handling the window. As I brought my scan away from the window the reject was started and I noticed the airspeed was about 100 kts. The aircraft quickly decelerated as it was very light. I expected the airspeed to be less than 80 kts based on the short amount of time we had been rolling. The FO continued to stop the aircraft. At that point I took controls. He called the Tower and I made a PA to the passengers to remain seated. We then taxied off the runway at Taxiway X and coordinated with Maintenance to have the brakes checked. They gave us a thumbs up and we taxied to the gate. At that point the window was stuck in a half open position. We shutdown at the gate and switched to another aircraft. Rejecting immediately when the window started to slide open would have prevented the distraction and the rejection would have started at a slower speed. I assumed two things: that the window would easily close and that we were still below 80 kts when the decision was made. It may have been; but at the light weight the aircraft accelerated quickly and I think by the time the actions began we were over 80 kts.


Second narrative: At approximately 40 kts during takeoff roll; Captain side window slid open. Captain called continue and attempted to close the window. A few seconds later I looked over and saw the Captain was struggling to get it closed. I stated that I recommend we reject and he agreed. His hands were still on the window to prevent it from slamming back so I initiated the reject procedure. I did not notice the top airspeed; but I would estimate visually about 100 kts; autobraking did engage. Aircraft came to a stop abeam Taxiway X. Notified the Tower. We were instructed to exit on Taxiway. We stopped on Taxiway and called Company. Maintenance met us on ramp and checked the brakes. We taxied in. Window was unable to be shut. When Maintenance came to the flight deck and were able to move the window forward the handle seemed loose and the squeeze grip seemed loose so we all conferred and removed the aircraft from service.I specifically remember observing the Captain check his window just prior to calling for the before takeoff checklist so I think it was secure. We should have rejected as soon as the window slid open to prevent from getting to the higher speed. The aircraft was light so acceleration was very rapid.


Synopsis: B737 Flight Crew reported cockpit window opening during takeoff roll.