Overview
The function of the emergency shutdown system is to stop the flow of fluids (fuel, hydraulic and air) to and from the engine.
Fire Handles
The emergency shutdown has three fire handles (one for each engine and one for the APU) which are on the fire extinguishing panel in the flight compartment.
Each fire handle has two possible movements:
- When pulled out, it arms the emergency system
- When turned counterclockwise or clockwise, it operates the applicable squib on the fire extinguishant containers
When you pull the fire handle out, the effects that follow occur:
- The applicable squib is grounded
- The applicable main generators de-energize
- The applicable fuel shutoff valve closes
- The applicable hydraulic shutoff valve closes
- The applicable high-pressure-bleed shutoff valve closes
06/01/16
Emergency Low Pressure Turbine Fuel Shutoff System
The system is designed to shut off fuel flow to the fuel manifolds in the event of a LP turbine shaft failure, in order to prevent possible overspeed of the LP turbines as a result.
A shaft breakage sensing mechanism is located aft of the Rear Bearing Chamber (RBC). A system operating cable passes through a turbine outlet guide vane at the engine BDC and is connected to a cable assembly via a quick release link adapter. The cable assembly is attached to an Overspeed and Splitter Unit (OSU).
An overspeed fuel shutoff valve and a splitter valve are parts of the OSU. The splitter valve is provided to "split" the metered fuel flow equally between the upper and lower fuel manifolds, which in turn, supply the fuel to the spray nozzles. The function of the overspeed SOV is to close the splitter valve, and thereby the fuel supply to the engine, in case a LP turbine shaft breakage is detected.
The emergency shutdown system automatically shuts off the fuel supply to the fuel spray nozzles in the combustion chambers through the overspeed (shutoff) valve, when it is activated by the spring-loaded pawl mechanism and cable. The pawl mechanism is a hollow cylindrical sleeve (pawl carrier), flanged and attached to the LP turbine stub shaft. Two flanges to the rear provide the attachment points for the two pawls.
A signal tube is mounted within the LP turbine shaft and is driven by the fan disc. It is supported within the pawl carrier sleeve at the rear.
During normal operation, shown as the maximum takeoff condition, the pawls are thrown out by centrifugal force until the heels of the pawls seat onto the signal tube. Failure of the fan or LP turbine shaft would result in an angular displacement between the LP turbine and signal tube (fan) increasing from a normal maximum takeoff condition (of approximately 15 degrees) to a failed shaft condition of 35 degrees.
This would align the pawl heels with slots in the signal tube and allow the pawls to move outwards and engage with the stirrup. As a result, the pawls will pull on the cable and allow the overspeed emergency shutoff valve in the OSU to move to the closed position.
