Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Distraction
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Other / Unknown
Aircraft Aircraft Damaged; General Flight Cancelled / Delayed
Environment - Non Weather Related; Human Factors
Pushback from the gate commenced while the parking brake was set. The tow bar's shear pin broke.I; the CA; failed to release the parking brake during my pushback flow. Typically; this oversight in a flow would be caused by rushing or by being interrupted. That was not the case here. Even though a mechanical had caused us to change planes and be delayed; I did not feel rushed. Especially since we had to wait 30 plus minutes to be fueled. Most flows have a cadence between the CA and the FO. The pushback flow also involves the gate agent; the lead FA; the Ramp Controller; and the pushback crew. The cadence of this pushback was different because everything had been completed except fueling the plane. The cargo door had been closed and the agents had been on the tug for 30 minutes. As soon as the fueling was finished; everyone was ready to go. The FA left the cockpit and we started our flows. This is where the normal cadence changed. Everyone was ahead of me. The first part of my flow ends with turning on the rotating beacon. Then I ask the FO for pushback clearance and I tell the pushback crew 'brakes released; hold the push. I'll call you back.' This time the FO called for the push before I finished the first part of my flow. The push back crew also called earlier than usual. Just as I turned the beacon on; my FO said: 'cleared to push.' Normally for me; the call to the pushback crew is three steps. The FO states 'cleared to push;' I release the brakes (a physical motion); and I state 'brakes released; cleared to push.' This time I received the 'cleared to push;' I turned on the beacon (a physical motion); and I gave the pushback command. That is the only reason I can figure as to why I skipped releasing the brakes. The FO did nothing wrong. Some call for pushback clearance as soon as the yellow hydraulic pump comes on indicating the cargo door is being shut. This time everyone was ready at the same time; which is not normally the case. I flew two legs the day before after having been off for two months. I had been reviewing my flows; etc. in preparation for returning to the line. I felt completely comfortable and satisfied with those two flights; which; by-the-way were completely 'normal.'Our flows are interrupted regularly. Most of the time it is easy to just start over. I did not realize that I had deviated from my flow. I had been off for a while. My plan was to go to the training center and practice my flows in a procedure trainer before I returned to work; but was unable to due to our training center being closed due to the COVID-19 virus. My practice at home went well. I had flown two legs the day before and all went well. I suppose I was overconfident. I probably should have used our flow charts for a few flights.
Air carrier Captain reported pushing back before brakes were release causing the tow bar shear pin to break.