Date: 201606
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Taxi
Aircraft component: Parking Brake
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Vehicle
Detector: Person Maintenance; Person Flight Crew
When detected: Taxi
Result: Aircraft Aircraft Damaged; Flight Crew Returned To Gate; General Maintenance Action; General Flight Cancelled / Delayed
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Human Factors

Narrative: We were operating a flight delayed approximately 3 1/2 hrs. We showed up to the aircraft and everything was normal. We ran all of the appropriate checklists and everything was sop. We called center on the ground and got our IFR clearance with a hold for release time as the tower was closed. Agent asked if it was ok to close and we said yes. I established communication with ground crew and they requested permission for pushback. We ran the 'before push' checklist and I called them back and reported 'brakes released; cleared to push'. During pushback ground crew called and said 'cleared to start both engines'. I acknowledged the ground crew and asked the First Officer (FO) to start both engines. While this was happening we heard a flight on the radio inbound and I was thinking about how this was going to work out as they were doing a straight in approach and we were going to taxi to depart the opposite runway. All of this time I had my eyes out of the cockpit and was monitoring the pushback and watching the ground crew. Next I heard the ground crew state 'set brakes'. I reached down and set the parking brake and thought I saw the red parking brake light illuminate. I stated 'brakes set' while keeping my eyes on the ground crew. Next thing I knew was I was feeling a weird vibration under the aircraft and I was looking outside trying to figure out what was going on and if the ground crew was having trouble disconnecting the tow bar.  Then the FO stated 'the parking brakes not set'. I then immediately reached down and reset the parking brake. During this time we had NO communication from the ground crew that the aircraft was moving or that there was anything wrong. I called the ground crew and asked what was going on and they said the parking brake wasn't set. I said we realized that and reset it. We had maintenance come out and do a quick inspection and they said they wanted us to taxi to the gate so they could do a more thorough inspection. Before I moved the aircraft I called the lead Flight Attendant and told them what had happened and that we would be returning to the gate and then I made a PA to the passengers explaining to them we would be returning to the gate. We went through an after landing flow and when the ground crew was back at the gate we taxied the aircraft back to the gate. After returning to the gate we did the parking checklist and once I was satisfied everything was under control I called the [Duty Manager] and informed him of who I was and what had just happened. We were getting no information from the local station so I exited the aircraft to go talk to maintenance and visually check for myself what had happened. That is when maintenance told me they were cancelling the flight as a more in depth inspection of the nose gear would be required.  I returned to the cockpit and told the crew what was going to happen. The station deplaned the passengers and we as a crew remained on the aircraft trying to communicate with scheduling and see what their plan was. After a while we accomplished the termination check list and deplaned the aircraft. The First Officer and I remained at the airport until all of the passengers had left in taxis for hotels and we knew the flight attendants had rooms and I knew what was going on with their schedule. Both the flight attendants and the cockpit crew left the airport at the same time. I have always thought I was very vigilant of the parking brake. I even state to my First Officer's when I set the parking brake during lengthy taxis. All SOP's and checklists were followed and adhered to. In retrospect I know I will be much more vigilant of verifying that the parking brake is actually set when I call 'parking brake set'. The only thing I can think that happened is that I had a false set of the brake or somehow I may have kicked the brakes off without realizing it. I think a good procedure to implement in the future would be for the First Officer's to visualize and verbalize on the push back that the parking brake is indeed set with a positive indication. Another good procedure I feel should be implemented is that when the ground crew sees or feels anything out of the ordinary when they are in headset communication with the flight crew is that they should verbalize that concern immediately and request 'brakes set'. During this whole incident we had NO communication with the headset operator that anything was wrong until I called down to check what was going on after I reset the brake. Again; we had NO call that anything was out of the ordinary as I had no sense of aircraft movement. I just heard strange noises and thought they were having difficulty disconnecting the tow bar.


Second narrative: While operating a delayed flight we were conducting routine pushback and engine start from hard stand 1. Captain (CA) was coordinating pushback procedure; I First Officer (FO) was in the process of engines start; after being cleared by ground crew and receiving CA command for both engine start.After completion of both engines start; I stood by for CA command for 'after engine start' to conduct my flow; due to warm climate and increased outside temperature; I set the pneumatic panel to ventilate main cabin and provide air conditioned air to passenger; for comfort; I waited for the rest of the flow for CA command. As I recall; just as I was finishing my pneumatic setup for PACK 2 I heard ground crew request for parking brake; and notice CA setting break and flash of red light; it was in my peripheral vision; but I'm positive that CA has made the motion of setting the parking brake.Shortly after; we heard and felt the typical commotion of tow bar disconnect; flowed by additional noises and unfamiliar 'grinding' like noise. We looked at each other and I asked 'what's going on down there?'; I noticed then a very subtle; relative motion at the background; I looked down and noticed no red light for the parking break; and told immediately CA; 'parking brake; Brake is not set!' CA immediately set parking break; and I could tell he was very surprised that the break was not set. He contacted ground crew right away to coordinate; this is when they have informed him for the first time that the aircraft was in motion during the tow bar disconnect process.The entire ordeal lasted perhaps less than 5 seconds; aircraft motion was physically undetectable from the flight deck; besides initial tow bar disconnect noises i.e. knocking; clicking; and then followed by unfamiliar sounds of very gentle grinding; knocking.After setting parking brake; CA coordinated with ground crew feasibility of concluding push back procedure; however; at this time tug operator (I notice a driver with headsets on) informed the CA; that there is a possible damage to the aircraft as a result of motion; and they need a maintenance personnel to have a look and inspect the nose gear.Maintenance personnel arrived and inspected nose gear and determined that they will not allow flight to continue without an extensive inspection to the entire nose gear as they noticed some damage to nose gear assembly.After consultation with maintenance personnel; we ran after landing flow; taxied aircraft to the hard stand; and ran parking flow and checklist. After few minutes at the hard stand; we noticed a lot of commotion including maintenance personnel and ops; but they did not open the door for at least 5 more minutes. After contacting ops over local freq. ops finally opened the main door and CA was able to go outside and consult further option with maintenance personnel.Maintenance decided not to allow further ops for the flight without an extensive nose gear inspection; we coordinated with ground and station personnel deplaning procedure; and ran terminating checklist and concluded the flight.At no point; during the whole ordeal; we have heard from the ground crew an regency or any calls over the intercom to coordinate an 'emergency stop; or break set verification'.The entire time; tug operator; had his headset on and was connected to the intercom; and to the best of my knowledge we never lost communication with ground crew.In conclusion; this event; which I would classify as 'false parking brake engagement'; was not caused by flight deck crew procedure omission; negligence; lack of discipline; fatigue; or carelessness; in my humble opinion; I believe this was a misfortune chain of events.We both followed our procedures to the best of our knowledge and ability. Flight deck tone was set by CA as highly professional and procedural.In addition; the reason I did not stepped on breaks the very moment I realized the aircraft motion was to prevent double engagement and disengagement of parking break. This would have potentially created more confusion and possibly even more damage. I allowed the CA to react and he did exactly that; he immediately pressed breaks and set the parking break in less than a second.


Synopsis: B737 flight crew reported setting the parking brake after pushback then starting engines. At some point it becomes apparent that the brake is not set and the nose gear makes contact with the tow bar as the aircraft moved forward unnoticed by the crew. Maintenance determines that an inspection is necessary and the flight is canceled.