Emergency shutdown of the engines can occur automatically because of an overspeed condition, or manually in the event of a fire.

There are two types of overspeed protection:

• N1 overspeed protection prevents low-pressure (LP) turbine overspeed and failure if the LP turbine shaft breaks.
• N2 overspeed protection prevents high-pressure (HP) spool overspeed due to an ECU or HMU fault condition.

The fire extinguishing system lets the flight crew shut down the engines in the event of a fire in the engine or nacelle.

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The emergency shutoff system is activated by low pressure turbine axial movement in the event of an N1 shaft failure.

The N1 overspeed shutoff detector (OSSD) is a plunger with a frangible conductive link connected to two electrical leads. The detector is attached to the No. 5 bearing housing with the plunger near to the LPT turbine shaft. The leads go through the number 6 vane of the exit guide vane (EGV) and connect to the W5P10 and W5P11 connectors on the W5 (yellow) wiring harness. Each ECU has two independent overspeed circuits which receive input signals through the two electrical connectors.

A severed LPT shaft causes the shaft to move aft which is sensed by the overspeed detector. The plunger moves aft and breaks the frangible link that causes the continuity to open in the link circuit.

The overspeed circuits in the two ECUs sense an aft movement of the LPT and send a signal to the overspeed shutoff valve (OSSV) torque motor in the HMU. The OSSV has a solenoid, two coils and a fuel bypass valve. The fuel bypass valve has two positions: one for normal operation and the other position is for overspeed fuel shutoff. When the ECUs transmit a signal to activate the solenoid, the valve moves to the bypass position and shuts off fuel flow to the fuel nozzles.

A fault in the ECU or the HMU can cause an overspeed condition of the engine(s). When this occurs, the N2 speed sensor sends an overspeed signal to the ECU. When the ECU senses this signal, it sends a signal to the DCU to display the L(R) ENGINE EXCEEDANCE warning message on the EICAS and sends a control signal to close the overspeed and cockpit shutoff EHSV.

A record of the event is stored in ECU memory. The software does not allow another start until maintenance action is taken. The latching SOV remains in the shutoff position.

Complete loss of one ECU provides LP turbine overspeed protection through the other functional ECU.

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The N2 speed sensor is a two-coil, non-contact, variable reluctance pickup (monopole) that measures rotational speed of the HP spool, one coil dedicated to each ECU.

The N2 sensor has a stainless steel housing and is attached to the AGB and extends to the generator and hydraulic pump drive gear. A single captive bolt provides attachment and must only be slightly loosened to allow a 90 degree rotation to remove the sensor from the AGB. No measurements or shims are required.

The N2 sensor converts the AGB hydraulic pump drive gear tooth passage into an AC signal, with its frequency proportional to engine speed.

The N2 sensor is the N2 secondary signal. With data sharing via the cross-channel data link (CCDL), each ECU has access to both secondary N2 signals.

N2 Limiter

The ECU is designed to limit N2 speed by a reduction in fuel flow. This limit (99.5%) is greater than and distinct from the maximum scheduled N2 applied by the N1 power management fuel schedules.

N2 Overspeed Protection

The overspeed and cockpit shutoff EHSV is tested on all engine starts and shutdowns. It is controlled by one ECU and supplies pilot-commanded (manual) fuel shutoff and N1 or N2 overspeed (automatic) protection fuel shutoff.

The N2 overspeed system provides protection against overspeed driven by control system failures. These failures may include software errors, and electrical malfunctions of the engine FADEC controller, or HMU mechanical malfunctions that result in uncommanded acceleration or overfueling of the engine. The N2 overspeed protection is implemented through dedicated circuits within the engine FADEC controller. These circuits operate independently of the CPU and software.

Each ECU contains two N 2 overspeed detection circuits. One circuit monitors the PMA N2 speed signal, while the other monitors the gearbox-mounted N2 monopole signal. When both circuits detect that an overspeed condition exists (N2 > 103%), the shutoff solenoid is energized and the engine is shut down.

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In the event of a fire, a L(R) ENGINE FIRE is displayed on the EICAS, prompting an engine shutdown. When the flight crew pushes the L(R) ENG FIRE push button annunciator (PBA) on the engine control panel, a signal is sent to the ECU, which commands the fuel feed shutoff valve (SOV) and overspeed and cockpit shutoff EHSV to close. This stops the fuel flow and shuts down the related engine.

When the left (right) engine is shut down, a L(R) ENGINE SHUTDOWN status message is displayed on the EICAS.

The EICAS messages that follow are related to the emergency shutdown system:

EICAS MESSAGE(S)	LEVEL (COLOR)
L ENGINE EXCEEDANCE	WARNING (red)
R ENGINE EXCEEDANCE	WARNING (red)
L ENGINE FIRE	WARNING (red)
R ENGINE FIRE	WARNING (red)
L ENGINE SHUTDOWN	STATUS (white)
R ENGINE SHUTDOWN	STATUS (white)

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The emergency shutdown system interfaces with the following systems/components:

• Engine Fire Detection System
• Engine Feed Shutoff Valve
• Turbines
• Hydromechanical Unit (HMU)
• Electronic Control Unit (ECU)
• N2 Sensor

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