Flight Instructor and student reported brake malfunction during taxi resulted in taxiway excursion.
Synopsis
Flight Instructor and student reported brake malfunction during taxi resulted in taxiway excursion.
Narrative
I pre-flighted the aircraft; all of the wheels were in good condition. After discussing the flight plan with my instructor; we go into the aircraft and both performed our brake checks to which the brakes responded well. We then taxied from the West Ramp to runway XX via taxiway 1. During the taxi; we had a light tailwind so my instructor had me taxi full aft. Once we reached the end of the taxiway and were about to turn right to line up with the hold short lines; I noticed the plane was veering to the left; even though I was applying right rudder pressure. I applied full right rudder firmly to which the plane did not respond and continued veering left. My instructor attempted to help apply right rudder pressure; the plane still did not respond. My instructor told me to cut the engine; so I pulled the throttle fully out. My instructor then turned off electrical equipment and pulled mixture fully idle as we continued to veer off the runway. The plane stopped in a small ditch in the grass; we contacted UNICOM for assistance; then exited the airplane.
Second reporter narrative
My student and I did a quick review of our anticipated plan of flight to continue Part 61 private pilot training. The student preflighted the airplane before I arrived because we were going to try for a solo cross country flight. Ceiling at ZZZ was scattered at 1800; but the ceiling at destination was too low for solo XC; so we decided to conduct a dual instruction flight. I did a cursory glance of the airplane and noted no defects. Engine start went normally and we obtained weather as required. Winds were out of the south around 4 or 5 knots. We conducted a dual brake check before taxi. The student checked her brakes; and then handed controls to me; and I checked mine. The airplane was responsive to both brake checks. We continued our taxi from the south side of the West Ramp via the taxi intersection via 2 and continued left down 1 to runway XX. The airplane was taxiing at 1000rpms. I reminded the student during taxi to have wind correction and since we had a tailwind; we would have higher ground speed on taxi and to hold the yoke full aft to help lift the nose; correct for the wind; and slow the plane down before needing to apply brakes. I noted mentally that our speed was a few knots faster than normal. I double checked our power setting (which was at 1000rpms); and ground speed was around 15knts. I was going to suggest applying brakes to slow down; but we were by that time approaching the run-up area and the student was already applying brakes. However; the airplane made no noticeable response and was beginning to yaw to the left. I quickly reacted by reaching out for the right brake and rudder pedal only to notice that it was already full deflection to the right. I reached out even farther and suggested we pump the brakes in case to see if we had somehow locked them up or could get some pressure. By this time we were approaching the grass and a taxi light and continuing to veer to the left. I called out; 'Shut the engine down;' a couple of times. The plane veered into the grass and kept its left turn until it came to rest in the thick grass halfway down a ditch. As soon as the plane came to rest; I called again; 'Turn the engine off. Shut everything down.' The student turned the ignition switch to 'off' and I pulled the mixture to cutoff. The propellor cut some grass but did not strike terrain. After cutting the engine we heard a radio call from an aircraft taxiing behind us informing traffic we went off the taxiway. I made a follow-up call announcing our location and confirming that we were fine. Then I cut the master switch and avionics switch; and we egressed from the aircraft; performed a visual inspection; called maintenance and line personnel and returned to the airport.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.