PA-34-200 Seneca I
Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 200; Flight Crew Total 4000; Flight Crew Type 100
Aircraft Equipment Problem Critical; Ground Event / Encounter Object; Ground Event / Encounter Loss Of Aircraft Control; Ground Excursion Taxiway
Aircraft Aircraft Damaged
Aircraft
As we completed our 3rd landing and taxied out onto taxiway; towards the beginning of runway; the airplane started to lose directional control and the student tried to regain control and slow the plane down using braking; when he reported to me as the instructor that there was a problem.I immediately exchanged and assumed control of the aircraft to find out that all efforts to maintain the plane from stopping and going off the runway were useless. The following correcting actions on my part as PIC where taken:1. Application of brakes with full right rudder deflection2. Made sure engines were at idle3. As we approach the left taxi lights and saw the taxi way airport sign and imminent collision with it and the left propeller; proceed to shut down the left engine by full idle mixture.4. And finally application of parking brake; but all were unsuccessful and left propeller struck with the sign knocking it down; but also creating damage to the propeller and the left leading edge wing/fuel tank.After the incident and no bodily injuries noticed; I proceed to inspect and access the damage to the call the FBO for assistance. Then a representative of the County arrived; temporarily closed that portion of the taxiway and awaited the instruction to tow the aircraft and reopen the taxiway. Minimum fuel was leaking and more was prevented by taping the wing with aviation speed tape in order to be towed and secured in the FBO parking area. The left tank was then drained/emptied by the FBO personnel as per request of its manager to avoid any further fuel leaks into the tarmac.Weather conditions at the time were VFR and winds less than 6 Knots approximately from 050 direction. My conclusion is a mechanical failure of some sort in the braking and steering hydraulic system; which caused the situation and not the students input or my assistance to fix the situation.
PA-34 Instructor Pilot reported his student experienced loss of directional control and even with the Instructor's efforts the aircraft left the taxiway and struck an airport sign. Instructor stated he felt the issue was with a mechanical failure rather than pilot technique.