Narrative:

Taxiing to the runway we stopped the aircraft short of the takeoff line. The parking brake was set with both engines running and out of feather. We received a number 2 rud iso valve caution light followed by a nose wheel steering caution light then noticed the aircraft moving forward. We stopped the aircraft using normal braking and noted the number 2 system quantity was at 0.5 quarts. We had lost all pressure from the main and standby pumps; and also the parking brake accumulator pressure. We alerted ATC and taxied onto the cargo ramp. The flight attendant called and said passengers noted fluid on the right main gear; so we decided not to taxi to the gate. Trailing aircraft also noted the fluid on the ramp and smoke from the brakes; but no indication of fire. The firefighters arrived and choked the aircraft since we had no parking brake; then we shut down the aircraft and the passengers were bused back to the terminal. Failure in the number 2 hydraulic system; most likely a hose or brake caliper failure on the right main. No abnormalities were noted on taxi out; or the previous flight. We discussed the amount of control remaining with the loss of the number 2 hydraulics and determined the aircraft was safe to move off the taxiway; but with the fluid on the wheels we decided it was not safe to return to the gate. We shut down in a safe location and the passengers were deplaned when the buses arrived. There were no indications on preflight or leading up to the event that suggested the failure was to occur. However it was my understanding; and that of every pilot I have talked to; that in the event of a hydraulic failure the parking brake accumulator operated outside the system and would hold pressure and retain the fluid regardless of system operation. It is worth noting that the accumulator; at least in this instance; will bleed down; and the aircraft must be secured prior to engine shutdown.

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Original NASA ASRS Text

Title: DHC8 flight crew experiences failure of the number two hydraulic system while holding short with the parking brake set. The aircraft begins to move and is stopped with with the normal brakes. The aircraft is taxied clear of the taxiway and shutdown and the passengers are bussed to the terminal.

Narrative: Taxiing to the runway we stopped the aircraft short of the takeoff line. The parking brake was set with both engines running and out of feather. We received a Number 2 RUD ISO Valve caution light followed by a Nose Wheel Steering caution light then noticed the aircraft moving forward. We stopped the aircraft using normal braking and noted the Number 2 system quantity was at 0.5 quarts. We had lost all pressure from the main and standby pumps; and also the parking brake accumulator pressure. We alerted ATC and taxied onto the cargo ramp. The Flight Attendant called and said passengers noted fluid on the right main gear; so we decided not to taxi to the gate. Trailing aircraft also noted the fluid on the ramp and smoke from the brakes; but no indication of fire. The firefighters arrived and choked the aircraft since we had no parking brake; then we shut down the aircraft and the passengers were bused back to the terminal. Failure in the Number 2 hydraulic system; most likely a hose or brake caliper failure on the right main. No abnormalities were noted on taxi out; or the previous flight. We discussed the amount of control remaining with the loss of the Number 2 hydraulics and determined the aircraft was safe to move off the taxiway; but with the fluid on the wheels we decided it was not safe to return to the gate. We shut down in a safe location and the passengers were deplaned when the buses arrived. There were no indications on preflight or leading up to the event that suggested the failure was to occur. However it was my understanding; and that of every pilot I have talked to; that in the event of a hydraulic failure the parking brake accumulator operated outside the system and would hold pressure and retain the fluid regardless of system operation. It is worth noting that the accumulator; at least in this instance; will bleed down; and the aircraft must be secured prior to engine shutdown.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.