Date: 201511
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: Windshear; 10
Light: Night
Aircraft: Golden Eagle 421
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Training
Flight phase: Taxi
ATC / advisory: Ground ZZZ
Aircraft component: Normal Brake System
Component problem: Failed
Reporter organization: Corporate
Reporter function: Trainee; Pilot Flying
Reporter qualification: Flight Crew Commercial
Human factors: Situational Awareness
Second reporter function: Check Pilot; Instructor; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Critical; Ground Event / Encounter Loss Of Aircraft Control; Ground Excursion Taxiway
Detector: Person Flight Crew
When detected: Taxi
Result: Aircraft Aircraft Damaged
Contributing factors: Human Factors; Aircraft
Primary problem: Aircraft

Narrative: Left Seat Pilot: Physically Fit; Feeling Healthy; Rested; NourishedAircraft: Airworthy; my first flight at flight controls in this aircraft Mission: Aircraft Specific; C-421B; TrainingThe aircraft involved was recently purchased and new to all the pilots mentioned in this report. From the research I did it appeared that this plane was maintained in an above average quality manner. The day prior to this flight myself with another pilot at the controls flew the plane. After start up I had my feet on the right side brakes and they felt like I expected good brakes to feel. I assume I did the same thing upon each engine start/stop at each of our 3 fuel stops and landing upon our return but don't remember. The next day I had an flight training checkout in the same C421B. The plane is stored in a very clean hangar. A thorough preflight was done with no hydraulic fluid found on the floor under the plane in the hangar nor on the ramp where it was fueled prior to flight. Nor was any hydraulic on either landing gear assembly. After startup the brakes felt fine. After receiving taxi clearance my routine procedure is to increase throttle to start to move; decrease throttles and test the brakes. It's a habit and as such I can't specifically remember doing it and assume I did with positive results or else the flight would have ended at this point. Early in our lengthy taxi with multiple turns I noticed the steering to be a little mushy. The much more experienced instructor in the right seat said his felt normal and as the taxi continued my left seat steering actually improved and left me confident upon takeoff.After our training flight we did a normal landing exited the runway and taxied back for another takeoff without issue or evidence or any irregularity with the steering and braking. Our next trip around the pattern was simulating a short-field takeoff and landing. Also the prior takeoff used a short-field technique to maximize runway length and each takeoff had full brakes applied as the engines powered up. Takeoff went smoothly; we left the gear down. After landing we could not exit the assigned taxiway because we could not slowdown in time. After clearing the runway and speaking to ground we made a right turn and then a very sharp 135 degree turn to the left. When trying to get the nose wheel straight again from this left turn the right nose steering and brake inputs did little to assist getting the nose wheel back to the right.  Our momentum was taking us toward the grass to the left so I added a small input of left throttle recognizing the need to straighten wheel with as little power as necessary since the right brake wasn't working. This decision was necessary. The result would have been going off the taxiway at that time. Now we were on the taxiway centerline at an appropriate speed; for an airplane with braking capability; it was now apparent we didn't have any so I pulled the throttles back as far as they would go. Immediately shutting down both engines at this point would have had us roll into the barricade ahead of us as the only brake we had was the left brake (asymmetrical braking at this point was not an option as the plane had to remain straight).  Straight ahead of us our taxiway had a lighted low barricade across it and just prior to that was the 90 degree right turn we had to make. At this point Plan A was to make the right turn and shut down both engines and then brake to a stop in a big left turn as we would be on a big open ramp at that time. Plan B was roll into the barricade as I felt our speed was so slow that damage would be small if an he option I knew I did not want to do was have the plane go to the left as it was off the taxi way with lights and a small ditch in the near distance. Plan A with no damage was chosen and I tried to turn into the right turn early knowing the input was sluggish at best. I pushed as far as I could on the right rudder and brake. It went to the floor. I was pushing down on the aircraft floorboard in front of the seat with my left foot which raised me off the seat as far as my seat belt would allow so that I could reach as far as possible with my right leg. It was a helpless feeling and it was clear we were not going to the right with rudder/nosewheel and brake input. I suggested about going with some left throttle to help in the turn but during this process I realized how narrow the space to make the turn actually was and thought it too risky to apply left throttle as it could take us wide into another grassy area with lights if we skidded or into yet another bad scenario if the amount of throttle wasn't perfect and we turned too sharply to the right. Those two decisions happened in a small fraction of a second. Just at the moment I had resigned myself to going into the barrier the plane abruptly turned to the left. I closed the mixture controls immediately. I think the instructors hands were on top of mine.  I knew we didn't have enough speed to go too far in the grass and when we entered the grass it was soft and we stopped quickly. There was control input response up until the last right turn was needed. For this reason I kept trying to taxi and eventually got into a situation where we were moving and had no ground steering or braking. As far as contributing factors the lack of steering control may have been masked by use of asymmetric thrust leading to lack of awareness of loss of control abilities. Also I may have assumed the right turn onto the taxiway after being cleared by ground control was accomplished using control inputs from the stop/start. This would have given me a false sense of security in the ability of the planes ability to taxi. The plane may actually have stopped after clearing the runway with the nosewheel pointed to the right. So when the plane started to move again it was already in a right turn. After exiting the runway I thought/assumed the danger was behind us. Also we were so close to being parked and done; there may have been some 'get there-itis' to get back to the hangar. What we should have done - exited the runway and called for a tow back to the hangar.I fixated on traditional flight controls and in an emergency one may have to think out of the box; quicker. Even after we started to taxi back prior to the right turn to the ramp we could have shut off the right engine at the right turn to the ramp to slow us down and assist the turn. Even prior to that we could have shut off both engines and hit the low barrier. Ultimately we should have gotten a tow back to the hangar.As far as what caused us to turn to the left. I think it was the winds at the airport.  Airplanes experience wind shear turbulences with landings on 8R/26L when winds are from the south as a result of the ridge that the winds spill over and then onto the airport in the valley below. Prior to takeoff we saw a Pilatus rocking back and forth on short final exhibiting this windshear. Combine this with the wind gusts from ATIS and I think a left quartering tailwind hit our rudder pushing it to the left causing a yaw of the nose to the left causing our turn to the left. I haven't thought out all the physics of the situation but it's the only explanation I can think of at this point.


Second narrative: While taxiing in to ramp we noted that the right brake had failed. In attempting to stop the a/c veered to the left and went off the pavement and in so doing the left prop hit a taxi light and damaged the left prop.


Synopsis: C421 flight crew experienced a brake and steering failure during taxi in after a training flight. The engines were shut down; but the left prop struck a taxi light before it stopped.