A C208 ran off the taxiway and was mired in the mud when the brakes failed.
Synopsis
A C208 ran off the taxiway and was mired in the mud when the brakes failed.
Narrative
The PIC-in-training was sitting left seat and I was sitting right seat because the PIC-in-training was going through upgrade training. The PIC-in-training was taxiing the aircraft on the wet pavement. The Marshaller had the PIC-in-training go forward a few feet then turn sharply 90 degrees to the right. The PIC-in-training turned the plane to the right 90 degrees no problem while using a right brake assisted turn. The PIC-in-training followed my instructions to then stop the aircraft and contact ground for clearance and taxi. We received our clearance and taxi instructions from Ground. The PIC-in-training taxied the aircraft as instructed straight ahead to make a right turn to join the taxiway to Runway XX. I was watching the PIC-in-training as he taxied downhill and noticed he was not turning very sharply to the center line of Taxiway B. I immediately asked what was wrong and the PIC-in-training exclaimed that the plane would not turn to the right. I immediately got on the brakes with him and put the aircraft in full BETA. The right brake had very little pressure and would go all the way to the floor without much pressure being applied. We could not stop the aircraft from preceding forward into the grass and mud. Tower asked if we could power out of the spot and we tried to no avail. We advised that we would need a tow out of the mud. I decided to wait for company contract maintenance to arrive to do the tow since the aircraft would have to be towed from a non-standard spot due to the nosewheel being unavailable because of the deep mud in front of it. There was no damage to the aircraft or any airport property. Taxiway was closed until the aircraft was moved which did not create any major problems for the airport according to the Control Tower. The PIC-in-training wrote the aircraft up for the aircraft not having any right braking action. When maintenance arrived they deemed the problem was a badly leaking right brake main cylinder and said there was a lot of air in the right brake line. They repaired the problem and signed off the aircraft.
Second reporter narrative
We received clearance to taxi out of the ramp; which was a 90 degree turn from our present position onto the B. I proceeded to taxi as cleared. When I went to make the right turn the right brake had no braking action; which didn't allow any turn. I immediately stated the right brake wasn't working and put the plane into full beta and also applied what I could of the left brake using the right rudder to keep it stable. It had been raining all day so it was muddy in the grass; which caused us to get stuck.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.