Date: 202012
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
Aircraft component: Parking Brake
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Other / Unknown
Detector: Person Ground Personnel
When detected: Taxi
Result: Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Before push checklist and Ramp tower clearance to push were completed / obtained per procedures. Following standard pushback verbiage from the ground crew; I released the brakes. Pushback and No 2 engine start proceeded without incident. Ground crew instructed to 'Set Brakes' - at this time I read back 'Parking brake set; pressure normal' while the FO (First Officer) and I visually checked the red parking brake indicator for illumination and Brake Press gauge for 2800 PSI. Ground crew then announced 'Aircraft disconnected by-pass pin removed' to which I replied 'Disconnect headset.'Shortly after this statement the aircraft appeared to move forward unexpectedly. The ramp agent came back on headset with 'Set brakes set brakes' to which I applied brakes.Apparently the parking brake was not set; as we had intended and thought; and an uncommanded aircraft movement had occurred. The ramp agent came on the interphone and indicated that all was fine on the ground; and standard signals were given for ramp departure. Upon crew debrief; one of the threats that we identified was the sun shining into the cockpit; making the red parking brake light less apparent to the crew. Another threat was the general distraction of normalcy and not blocking out the sunlight's glare to verify aircraft condition.We are glad no one on the ground was injured and will learn from this experience.


Second narrative: Before push checklist was completed. Received our push back clearance and the Captain communicated with the ground crew standard tow bar less pushback clearance and released the brakes appropriately. Pushback was completed uneventfully and during so we were cleared for engine start after which the Captain had me start the No. 2 engine. The ground crew instructed to 'Set Brakes.' I saw good brake pressure - 2800 psi; and what I believed to be the red Parking Brake light illuminated so I continued my duties of setting the packs appropriately.The ground crew announced 'aircraft disconnected - bypass pin removed' and I heard the Captain respond with 'Disconnect Headset.' Shortly after this the aircraft began an uncommanded movement forward; ground crew came on headset Immediately saying 'Set Brakes Set Brakes' to which brakes were applied and the aircraft stopped the slow roll and the Parking Brake was appropriately verified engaged at this time. The ramp crew came back on the inter phone and indicated all was fine on the ground; and standard signals were given for departure. Apparently our parking brake was not set after all when we initially thought; and upon debrief we both agreed the threats encountered were the glare of the sun on the parking brake light illuminating it more red than usual; as well as the general distraction associated with starting engines and push back tasks in general. We did not properly block the sun's glare and we should have been more vigilant to ensure it was set properly. We are both thankful nobody on the ground was injured and we will learn from this experience in order to prevent it from happening again.


Synopsis: B737 flight crew reported uncommanded aircraft movement occurred following pushback without brakes set.