SA-227 AC Metro III
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Air Traffic Control Provided Assistance; Aircraft Aircraft Damaged; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Gate; General Flight Cancelled / Delayed; General Maintenance Action
Aircraft
After starting engines and running applicable checklists; ZZZ Ramp cleared me for a standard taxi east from the Company A Ramp via Taxiway XX. Upon leaving the parking space; I tested the brakes per the Taxi Checklist. Brakes checked good. Prior to the Taxiway XY hold-short bars; ZZZ Ground cleared me for an expedited taxi to [Runway] XXL via taxiways XZ; XA; and XB; crossing Runway XXR/XYL. Due to the expedited taxi; there was minimal braking. I only applied brakes when slowing for the tight turn from Taxiway XA onto Taxiway XB. There was no problem with the brakes during the turn.On Taxiway XB; ZZZ Tower cleared me for a full-length departure from Runway XXL ahead of a Company B aircraft holding short of Taxiway XB on Taxiway XC. I began slowing for the turn well in advance using ground idle power and brakes. Upon lining up on runway centerline; I noticed that the left brake had lost pressure and was worsening. Immediately following this realization; the Company B crew notified me on Tower frequency that my wheel was smoking. ZZZ Tower confirmed the condition. I notified ZZZ Tower that I would be exiting the runway at Taxiway XD.ZZZ Tower asked if I would like fire services and I replied in the affirmative. I was assigned a discrete frequency for coordination with the fire trucks. The fire crew hosed down my left main gear and I shut down engines. I asked the crew for an update on the wheel's condition and was told that the 'small flames' and smoke had been extinguished and that only steam was left. The crew notified me that they would be approaching the aircraft to meet me if I desired to exit the plane; which I did. The flames had burned through a portion of the brake lines and there was a steady stream of hydraulic fluid issuing from the hosing. The brakes' temperature was continually monitored. I put my hand near the right main gear and noticed no inordinate heat from its brake assembly. I called [the] Ramp Supervisor to coordinate for a tug and was towed back to the Company A Ramp by a Mechanic and escorted by ZZZ response crew.
Captain reported loss of the left main brake while positioning for takeoff and after being notified of smoke coming from a wheel; cancelled takeoff.