Date: 201311
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Dash 8-300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Ground ZZZ
Aircraft component: Hydraulic System Lines; Connectors; Fittings
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Captain; Pilot Flying
Anomaly: Aircraft Equipment Problem Critical
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: Taxi
Result: General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

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.


Second narrative: During taxi; we were number three or four for takeoff. While stopped with the parking brake set; the First Officer stated that we had a hydraulic ISO caution light. I looked down at the hydraulic panel and saw that the Number 2 quantity and pressure were below normal. I happened to look up to find that the aircraft was rolling forward and to the right (with the parking brake set). I applied the toe brakes to bring the aircraft to a stop. We contacted Ground Control and asked to pull off the taxiway into the non-movement ramp to our right. Other aircraft began saying on frequency that they observed smoke and fluid coming from our right wheel area. The Flight Attendant also called and said that passengers were seeing smoke coming from the wheels. I taxied into the ramp and brought the aircraft to a stop. At this point we saw that the Number 2 hydraulic quantity was basically zero; as was the parking brake pressure; and we had very little pressure. The First Officer then turned off the Number 2 standby pump. The Controller asked if we would like to have the fire trucks come to the aircraft. I said yes; and asked that they chock the wheels for us so that we could shut the engines down. The fire trucks arrived; but there was some confusion about what we needed from them. It took about five minutes for us to communicate that we needed to have the nosewheel chocked before we could shut down the engines. We completed a normal shutdown and kept the passengers seated until buses arrived to take them to the terminal.


Synopsis: 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.