Ramp Agent reported communications issues while training a tug driver resulted in broken tow bar shear bolts and a delay while Maintenance inspected the nose gear of the aircraft; finding no damage.

Narrative

Today on Day 1 I started my shift at XA:30. I checked the online app to confirm that I was still assigned to conduct push training with Person A. This was our 3rd day of OJT push training. I called. I looked at scheduled departures. On Day 0 it rained; and the ramp closed several times. The off schedule operation made it hard to pick flights to push. I wanted to choose gates that were straight pushes with a turn to get him comfortable with pushing. I intentionally used older Tug push back tractors because they require more effort to maneuver than the new push backs with power steering. This is a standard part of how I train candidates that have no push experience.We pushed 3 flights that night. 1 from [Gate] ZXX; 1 from [Gate] ZXY and 1 from [Gate] ZXZ. I noticed that the pushes from [Gate] ZXZ and [Gate] ZXY were wide and at odd angles until he pulled back. I knew after that it would be good to take him to the gates between [Gate] ZXA and [Gate] ZXB the next day. I wanted Person A to push from an area where he could have the opportunity to choose what angle he wanted to push at. This is the way I normally train to show how you can push through a bad angle. This is accomplished better on these gates because they are not parallel to an active taxi way and it is wide enough to make any adjustment necessary. When I met Person A approximately XB:00 on Day 1; I explained that our 1st push would be from [Gate] ZXB. We drove to XX concourse; and he practiced going forward and turning the push back tractor with only a tow bar on it. We drove back to the XY concourse approximately XD:00. I told him to take a break and meet me at ZXB XD:45 so he could complete his pre departure walk around. We arrived as scheduled. He spoke with the crew chief assigned to flight XXXX and got his permission to push. Person A performed his safety check and walk around. He checked tow bar [Number] and did not observe anything unusual. I then showed the diagram I draw for all my trainees explaining how I wanted him to find the angle that he is most comfortable with. When the CSA (Customer Service Agent) pulled the jet bridge; Person A informed the flight-deck that we were secure below; steering was disengaged with by-pass pen and that we were ready for push back. When given the clearance by flight-deck to push he signaled wing walkers by flashing the lights and honking horn. The wing walkers stepped in the roadway and gave Person A signal that he was clear to push. He started pushing in 1st gear as I taught him. When the wheels hit the roadway he began turning left. The headset cord got wrapped around the base of the steering wheel. Person A took his foot off the gas and the tractor continued to move left in the direction he was going until he hit the brake. I helped him untangle the cord and secure it around the flasher on top of tractor's dash. Once secure; I instructed Person A to continue pushing at angle tractor was at.After a few feet of pushing I told him to turn the wheel right in order to begin correction that would put plane on the north line. As he began to turn the wheel to the right I heard the shear pin(s) break. I told him to stop and he did. I told Person A to tell Captain to set brakes and he did. Once brakes were set; I looked at tow bar neck and saw that it was bent slightly confirming that shear pin(s) were damaged. I took the headset and spoke with the Captain to explain what had just happened. He said that everything was 'ok; the plane was fine'. He said when Maintenance comes out 'they will check it quickly and we'll be on our way'. I explained I needed to pull the tow bar away from the plane so mechanics could get to nose gear. He agreed; I disconnected the tow bar and pulled the tractor away from the plane. About this time the CSM (Customer Service Manager) Person B came to aircraft and I explained what was going on. I then drove off to get another tow bar. After I got back and waiting for more than 20 minutes the CSM Name1 spoke with the Pilot. Name1 told me The Captain now wanted to pulled to the gate so nose gear inspection could be done at gate. I got on the headset and told the Captain that I would need to remove the bypass pin to allow him to straighten nose gear before I could connect new bar and pull him back. I told to go ahead and I removed pin; replaced it when gear came back to center and connected new tow bar. I told The Captain that bypass is in installed and I was ready to tow them in. The Captain told me that brakes were released and I was clear to pull them in. I pulled the plane into (Gate) ZXB and parked it on the line designated stop line. I asked Captain to set brakes and he said; 'brakes set and thank you'. I then spoke with CSM Person B and we agreed that we would leave the gate and convene to conduct investigation. Before we began investigation. I spoke with Person C (Duty Manager) to explain what happened. During our conversation I explained that when the cord got stuck and the push moved to a strange angle; I saw it as a teachable moment to demonstrate how adjustments can be made to get a push back on track when over steering in one direction occurs. I also said that Person A was only following my directions which is essential when training on any process. He said he under and for that reason I would be the subject of the investigation and that they would take a witness statement from Person A. Once Person A began pushing the aircraft after a full stop to secure the headset I told him to turn the steering wheel to the right. When he did and the push back began moving I heard what I thought based on my 23 years of experience was the sound of shear pins braking as they are designed to do. Once at a stop and after having Captain set brakes and looked at the tow bar head and saw that there had been damage to shear pins.The shear pins did what they were designed to do but I question whether the pins were in good condition. Having to resecure the headset cord caused the agent to have to stop and allowed the push back to drift out of position. Only being able to teach on live flights help contribute to incident today. Perhaps more frequent replacement of shear pins. There are times like today when pins are stressed. I don't think that angle of the push out or tow bar should have caused the pins to fail. Having a place to secure headset cords added to driver's side; maybe near or on the seats would be great. Cordless headsets would eliminate the problem with the cord catching like it did today. Cords can get in the way for even experienced agents. If we could eliminate that concern; it would be one less area that a trainee will have to worry about while trying to master pushing. It would make it safer for everyone. Having a push simulator would allow trainees to practice coming back from oversteering without fear of damaging nose gear.