Date: 201503
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
ATC / advisory: Tower ZZZ
Aircraft component: Cockpit Window
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Physiological - Other; Distraction; Time Pressure; Workload; Communication Breakdown
Second reporter function: First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport; Flight Crew Took Evasive Action; General Maintenance Action
Contributing factors: Human Factors; Procedure; Aircraft
Primary problem: Human Factors

Narrative: I was the Pilot Flying (PF). On takeoff roll just past 80 knots; my side window slid open. I continued the takeoff. At approximately 500 feet I transferred control to the First Officer (FO). I attempted to close the window; but was not successful; so I requested a level off and return to the departure through ATC. I was able to move the window forward to reduce the amount of wind and noise in the cockpit but it would not latch. I requested an extended downwind so we would have time to run the QRH checklist and get performance numbers. When I attempted again (per the checklist) to close the window at maneuvering speed for flaps 10; it broke away from the frame and started to pull inward toward me. It was essential for me to continue to hold the window in place for the remainder of the flight. We notified the Passengers and Flight Attendants of the return and asked ATC to advise Company. I ran the performance numbers which confirmed an overweight landing with ample runway for an autobrake 2 landing on Runway 30L. ATC cleared us to land and the First Officer made a very smooth landing with normal braking. We taxied to the gate where we requested Maintenance; wrote up the event and overweight landing in the logbook; and talked to Dispatch. Maintenance examined the window and was able to close and latch it and ensure that it operated normally. They performed the overweight landing inspection and signed off the logbook. I discussed the event with the Flight Attendants who indicated that they were okay to continue. After getting new paperwork and more fuel; we continued uneventfully to our destination.I believed the window was secure at pushback; but Maintenance showed it to function normally upon their inspection after the event. Therefore; I can't rule out that it may not have been completely latched. I will double check my window in the future. Maintenance told me that the checklist may as well require a return to departure airport as the window is virtually impossible to close in flight. This certainly was my experience. When the window fell inward as I attempted to close it airborne; I assumed this probably meant that there was an issue with the window; but the Maintainer indicated that this was more likely due to the design of the window. Based on my experience during the event and conversation with Maintenance; I believe Pilots need to be prepared to transfer control to the other Pilot and return to the departure airport should this event occur. Communications were significantly hindered due to wind noise; something that should also be anticipated.


Second narrative: At about 1000 feet Captain turned the controls over to me to try to close the window. The window would not close; so I continued to be the Pilot Flying (PF) and the Captain became the Pilot Monitoring/communicating with ATC. A return to the departure airport was made and an uneventful landing was made. (Overweight at 139;000 pounds. Very smooth with minimal braking.)


Synopsis: A B737NG Captain's sliding window which was closed but not locked; opened during the takeoff roll. Unable to close and lock it; the flight returned to the departure airport.