B737 flight crew reported during pushback there was a loud bang followed by uncommanded roll of the aircraft due to sheared tow bar pin. Captain set brakes to stop aircraft.
Synopsis
B737 flight crew reported during pushback there was a loud bang followed by uncommanded roll of the aircraft due to sheared tow bar pin. Captain set brakes to stop aircraft.
Narrative
During Pushback; after sitting approx 10 min waiting for aircraft to clear behind us; Parking Brake Off; we begin push but before moving more than 2 feet aircraft came to a sudden stop with a loud bang. I initiated comm with ground crew asking them if we had sheered the pin. They said yes; it had". For a second there I thought I had forgotten to release the brake for pushback because it was bit violent and so sudden; but; I did verify the parking brake was OFF. The ground crew said they were contacting Mx (Maintenance). At this time; the parking brake is OFF and Hydraulics ON. We had made no changes to our configurations nor had we been asked to. I then contacted Mx; and began writing an ACARS ELB (Electronic Log Book). I do remember wondering about the Parking Brake being OFF and even queried the ground guys but got no response. After a few minutes though (could have been longer); the aircraft jerked a second time and maybe rolled a half a foot before again coming to a stop. This was a bit more concerning as we were given no heads up about something being done to the aircraft. Again; Parking brake is OFF and Hydraulics ON. I finally do get in contact with the ground guys and query them about setting the Parking brake. They say "Yes; Set Brake" and I do. Apparently Mx is on the scene now and notice the tow bar has made contact with the left wheel. After some time; two Mx personnel enter the flight deck asking me what had happened. I told them what I knew; we sheered the pin; and just sat there. That we were given "No instructions" what so ever about anything. I think they did ask me about whether or not I was asked to set the brake after the occurrence. I told them we did not receive; nor did we read back any instructions concerning the Parking Brake or Hydraulics until "I" queried them after the second jerk of the aircraft."
Second reporter narrative
Immediately after pushing back; there was a loud bang. Captain asked the ground crew (via headset) if it was a broken sheer pin. They said yes; it was. The ground crew said they were going to contact maintenance and the Capt began entering an ELB (Electronic Log Book) via ACARS. He asked ground whether the brakes should be set; but we got no response. We stayed on headset in case they tried to communicate. A few minutes later; the aircraft rolled forward a few inches. We motioned for them to communicate with us via headset; at which point the Capt asked if we should set the brakes. They said yes. We stayed in the flight deck; but generally had no communication. Several minutes later; 2 guys came up (maintenance? ground manager?) and asked what had happened. We relayed everything. It was finally decided to change the tire and then carry on our way.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.