B737 Captain and First Officer reported the aircraft was creeping forward unnoticed by the flight crew while waiting for pushback. The Marshaller signaled the flight crew and they stopped the aircraft from rolling further.
Synopsis
B737 Captain and First Officer reported the aircraft was creeping forward unnoticed by the flight crew while waiting for pushback. The Marshaller signaled the flight crew and they stopped the aircraft from rolling further.
Narrative
During the pushback and after-start sequence we had an uncommanded aircraft movement. During the pushback I had notified clearance delivery and confirmed we would be good for a release time (I believe 9 minutes from that time); and the Captain was on flight interphone with the ground crew. The number two engine was started during the push per normal procedures. As I recall; the Captain made all the normal calls to the ground crew as I was focusing on the engine start; from setting the brake through disconnecting the headset. I tend to point (just for myself) to the hydraulic pressure when the brake is set; and I do remember the indication being normal. This incident does have me questioning if I missed the brake light simply because of what happened. But I don't think either of us missed setting the brake in general and I know the aircraft did not move until after all communication with the ground crew.After the command to disconnect the headset; the After Start Checklist was called for and the flow initiated. Somewhere during the flow I noticed out front that the Marshaller was trying to get our attention and then both the Captain and I quickly realized the aircraft was creeping forward. The brakes were quickly applied and the ground crew cleared the aircraft without any further incident. The ground crew was already disconnected from the headset so there was no talking between parties. The Captain communicated with Ops before takeoff and also gave his phone number to discuss what happened and if everyone was OK. The Captain and I discussed the event thoroughly and also spoke with the tug driver over the phone upon parking in ZZZ1. I quite honestly don't know exactly what led to this event but it has made me sure to pay even more attention from here on out.
Second reporter narrative
Aircraft was pushed back and I set the brakes when the ramp crew asked for it. After the ramp was ready to disconnect; I gave them the Ok and also had the First Officer (FO) starting number 2 engine. At the same time clearance asked us if we would be ready in 6 min to take off or we would have to wait 45 minutes. We responded yes and at the same time the Marshaller was trying to get our attention frantically. I looked down and saw the brake light no longer on and applied the brakes. After the salute; I called Ops to talk to the Lead at which time he wasn't available. I gave them my number and asked to talk to him when I landed ZZZ1 which I did. He said we rolled about 5 feet before stopping and said everyone was clear so no one was injured. The FO and I talked about it and cannot figure out how the brakes were release prior to the salute and release. I'm wondering if I hit the rudder petals accidentally or I did not apply them firmly.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.