The indicating system supplies the aural and visual APU data necessary for the crew. The data concentration unit (DCU), electronic control unit (ECU) and the hourmeter comprise the input for indicating. The ECU relies on input from the APU sensors. Input from the APU sensors goes to the ECU, which supplies this data to the DCU. Communication between the ECU and DCU is via ARINC 429 data bus.

The DCU supplies input for flight compartment indication of the APU parameters and messages, which are shown by the engine indicating and crew alerting system (EICAS). Display of the EICAS is on the left multifunction display (MFD), unless the left MFD becomes defective or has been reverted. If the left MFD becomes defective, the EICAS data is automatically shown on the right MFD. As well, if both MFDs become defective, the crew has the option of showing EICAS data on the primary flight display.

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The hourmeter records the number of APU operating hours. It is a solid-state device that operates with a 28 VDC input from the ECU. An instrument can be connected to the hourmeter to get a digital indication of the hours of operation. The hourmeter attaches to a bracket above the fuel solenoid valve.

When the APU is at 95% of its operating speed, the ECU energizes the hourmeter. The hourmeter stays energized until the APU is shut down. After shutdown, the ECU reads and stores the data given by the hourmeter. During the pre-start BIT of the subsequent start-up cycle, the ECU compares the stored value with that shown by the hourmeter.

If the value from the hourmeter does not agree with the stored ECU value, the ECU aligns with the value given by the hourmeter.

The EGT sensor supplies the ECU with temperature data from the exhaust gases. If this sensor fails, the EICAS shows an EGT warning message. The speed sensor supplies indicated speed for on-speed control and overspeed protection of the APU. If an overspeed condition occurs, the EICAS shows a built-in test (BIT) fault message.

During usual APU operation, APU RPM and EGT are shown on the EICAS status page. The APU RPM is displayed as a percentage of maximum allowable RPM (100% at 58,737rpm). When APU speed reaches 106% (overspeed), this value is shown in red. The APU EGT is displayed in degrees Celsius. This number changes color if the normal operating temperature parameters are exceeded. When RUN is selected on the APU master switch, the ECU sends a signal to the DCU, which activates the display. If the data bus fails, the values are shown as dashes.

The EICAS messages that follow are related to the indicating system:

EICAS MESSAGE(S)	LEVEL (COLOR)
APU FIRE	WARNING (red)
APU OVERTEMP	WARNING (red)
APU BLEED ALT LIMIT	CAUTION (amber)
APU OIL PRESS LOW	CAUTION (amber)
APU OIL TEMP HIGH	CAUTION (amber)
APU OVERSPEED	CAUTION (amber)
APU STARTER FAIL ON	CAUTION (amber)
APU FAULT	ADVISORY (cyan)
APU SHUTDOWN	ADVISORY (cyan)

When the APU FIRE PBA is pressed, the FIRE signal to the ECU is activated. The ECU shuts down the APU and sends a signal to display the APU SHUTDOWN advisory message on the EICAS.

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The indicating system has interfaces with the systems/components that follow:

• The Engine Indicating and crew alerting system (EICAS)
• The data concentration unit (DCU)
• The electronic control unit (ECU)
• The APU Control Panel
• The APU Fire-Detection System

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