Cessna 172 pilot and instructor reported a single brake malfunction resulting in a taxiway excursion. The aircraft was pushed back onto the taxiway and taxied to the parking spot.
Synopsis
Cessna 172 pilot and instructor reported a single brake malfunction resulting in a taxiway excursion. The aircraft was pushed back onto the taxiway and taxied to the parking spot.
Narrative
My instructor and I were taxiing out for an IPC (Instrument Proficiency Check) flight. After engine start and receiving a taxi clearance; we both checked our brakes as we departed the tie-down spot. I then a made a left turn on the apron to enter taxiway 1. I stopped on the apron to wait for a fuel truck to move and then stopped again to let a twin pass on taxiway 1. I made another left turn to enter taxiway 1; then a right turn onto taxiway 2. All of these turns were done with a combination of pedal and brakes. On taxiway 2; there was an approximate 10 kt. tailwind; so I throttled back to keep the speed low and was periodically lightly braking. As we approached taxiway 3 where I needed to make a left turn to get to the runup area for [Runway] XXR; I started to apply both brakes; but more on the left as I was starting my turn. The plane was not slowing down; nor was it turning left. When I felt the left pedal go all the way to the floor; I called out; 'brakes; brakes; brakes' to my instructor and pressed both brakes in. By this time; the plane had started to roll of taxiway 1 straight ahead and over a small; shallow embankment. With both of us on the right brake; it veered to the right in the dirt off of the taxiway. I shut off the engine and we called for a tug.The plane had not hit anything and there was no damage to any part that we could see and no injuries. When the tug arrived; we pushed the plane back on to the taxiway and tug pulled back to the apron. The tug driver; who was also the owner of the plane; conducted a taxi test with my instructor on board. He reported that both brakes and directional pedals worked correctly. My instructor taxied the plane back to its tie down spot.It is not clear to me with why both the left brake and left pedal failed at the same time. This seems like an odd coincidence as I understand they are separate systems. Neither my instructor nor I could think of something we could have done differently prior to the failure to avoid it as we both tested our brakes and I had made two left turns during the taxi.
Second reporter narrative
The Pilot/Student (left seat) and I (right seat) were taxiing out in Aircraft X from parking at ZZZ; route to Runway XXR 'taxi via 1; 2; and 3'. Upon clearing the tiedown; the pilot flying tested his breaks; and asked to to test mine. They felt a bit 'mushy' but effective; nothing I have not experienced in C-172s of this vintage. The left turn onto taxiway 1; the right turn onto taxiway 2 were uneventful.Upon trying to turn left onto taxiway 3; I noticed the pilot flying depress his left rudder and brake; and yell 'brakes' as I noticed that the aircraft was not turning. I immediately applied full left brake pressure on my side; with no effect. Recognizing that we were departing the taxiway; the pilot flying and I both depressed both brakes in an effort to stop the aircraft. it stopped just short of some shrubbery. The pilot flying shut down the engine with the mixture control and secured the aircraft (fuel/electrical off). No damage; no injuries.The pilot flying was taxiing at a safe airspeed; to which I attribute the safe outcome.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.