An air carrier pilot reported loss of nose wheel steering due to hydraulic leak while taxiing. After being towed in to the gate ground personnel opened the cabin door while the engine was still running.

Narrative

Flight ABCD; operating ZZZ-LCH; had an on-time departure with an FAA jumpseater on board. As the aircraft began pushback; the ground crew reported a towbar shear pin failure during the disconnect procedure. In response; maintenance was contacted by the Captain while the ground crew was instructed to stand by. Maintenance control dispatched local maintenance to the aircraft's position; and they arrived shortly thereafter to inspect the nose gear assembly and found no issues; allowing the crew to proceed with taxi via the route to Runway XXL.Approximately 10 minutes after block out; while taxiing on 1 between 2 and 3; the crew received a Spoiler Fail" caution message; and instantaneously messages for 'HYD SYS 1 Fail' and 'LO QTY"; along with the associated systems (AIL; RUD; etc). The First Officer immediately turned to the HYD MFD page; and both crew members observed hydraulic fluid from System 1 depleting at a rapid rate; reaching the lowest indicated quantity within seconds. Simultaneously; the Captain began to lose nosewheel steering and triggered the steering disengagement button; realigning the nosewheel with the taxi line via rudders; and brought the aircraft to a stop. After assessing the situation; the Captain determined there was no immediate safety threat and instructed the First Officer to inform the tower that they had lost steering and needed to run checklists. The Captain proceeded to call for the HYD SYS 1 Failure QRH. After completing the checklist and turning off HYD Pump 1; the Captain contacted dispatch to report the situation and request assistance at their location. The CA also requested to be transferred to Maintenance control. While waiting for Maintenance control; the CA advised the FO to secure engine 1.Maintenance control was notified; confirming that the QRH was completed and that Engine 1 had been secured. Maintenance then advised the crew to write up the discrepancy upon returning to the gate. Following this; the Captain asked to be reconnected with dispatch; who advised that coordination was still ongoing for a tow back to Gate X. Meanwhile; the First Officer handled all communications with ATC; and the crew reached out to operations via comm 2 to get an estimated time of arrival for the tow vehicle. The Captain informed both the Flight Attendant and the passengers of the issue; ensuring them that the aircraft was safe but would need to be towed back to the gate due to a failure in the steering system on the ground.At approximately XA:17 local time; with no tow vehicle having arrived; ATC informed the crew that fire trucks would be dispatched due to the aircraft's position on the field and the nature of the aircraft being disabled. The Captain clarified that there was no [priority handling] onboard and that they were merely waiting for a tow. ATC requested FOB and SOB. Instead of the anticipated fire trucks; the Fire Chief arrived in a single; small white pickup truck with flashing lights. Communicating with both the Fire Chief and the tower on a separate frequency; the crew requested that the Chief inspect the aircraft for any visible abnormalities. The Chief reported seeing a significant amount of red fluid leaking from the "right bogie." The crew acknowledged the observation and continued waiting for assistance; keeping the passengers and Flight Attendant updated; particularly to ease concerns stemming from the flashing lights of the [priority handling] vehicle circumnavigating the aircraft. During this time; the crew ran Engine 2 and the APU; completing an after-landing check while awaiting tow; but did not retract flaps out of concern of disturbing the aircraft more given the sudden hydraulic loss. The tow team arrived with a ground operations vehicle and a super tug. The crew coordinated with the super tug for a safe capture via the super tug frequency. The ground ops vehicle led the convoy and managed all movement across the runways with ATC/ramp; and safely guided the aircraft backto the gate.Upon arrival at Gate X; another safety issue arose. The move team failed to communicate that the parking brake needed to be set or that chocks had been applied; or that the aircraft was released from the super tug. While the aircraft was still running Engine 2; the Move team's super tug driver opened the main cabin door externally without first ensuring the aircraft was fully secured. The Captain immediately set the parking brake; shut down Engine 2; and requested the completion of the shutdown checklist. This oversight by the move team presented a serious safety hazard; as the aircraft was still live with no brakes applied when the cabin door was opened.Once the shutdown checklist was completed; the passengers were safely deplaned. Following this; the crew conducted an extensive debrief and discussed the hydraulic failure; the lengthy wait for a tow; and the serious communication breakdown during the parking process. The FA appreciated the communications between flight deck and cabin; but was startled by the door being opened unbeknownst to her. The crew debriefed the excellent CRM practiced with splitting duties and communicating amongst one another; along with our FAA Jumpseater. A briefing was held with the move team; where the Captain emphasized the extreme risk posed by not confirming the application of chocks or ensuring the aircraft was secure before opening the cabin door. Dispatch was notified about this issue as well in a post-incident call. The discrepancy was also written up. The flight was given a new aircraft and was completed without further incident; albeit delayed. It's worth noting that the crew's training and communication paid off in this instance; along with dispatch over the phone. Additional review regarding communication amongst all involved parties during a tow operation could benefit the system in the event this occurs again. Additionally; an investigation into the cause of the sudden hydraulic system failure should be conducted; as this could've been a much more intense [priority handling] had it been in-air. Lastly; the move team should undergo further training to prevent such incidents in the future. The crew could have mitigated this part of the incident by briefing the move team before pulling into the gate; but at the time; the crew assumed the move team was trained in such scenarios and were told that there was nothing they needed to do after capture. The repetitive nature of relocating aircraft could have led to a lapse in judgement or complacency; not remembering that there were passengers on-board or an engine running; It felt as if the move team was on 'autopilot' and that this was another 'normal' movement of an aircraft."