The master warning/master caution control system is used to supply the flight crew with different level of alerts. The alerts are given to the flight crew with a priority level, through light indications, crew alerting system (CAS) messages, and/or aural warnings.

The components of the master warning/master caution control system operate the annunciation switch/lights in the flight compartment. The data concentrator unit (DCU) controls the logic and the inputs for the aural warnings and the master warning/caution switch/lights. The radio interface unit (RIU) uses the logic from the DCU to supply aural warnings.

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The lamp driver unit (LDU) is installed in the flight compartment on the floor below the main instrument panel, at approximately FS258.00 about the center. The LDU is a part of the control circuit for the annunciation lamps. Flight compartment lamps are used to identify switch/lights that can be used by the flight crew to correct some aircraft conditions. They also supply switch/lights position and system status. The LDU supplies the lamp test function.

The LDU has two isolated channels that are the same. They supply control of the annunciation signals in the flight compartment. The channel A of the LDU receives 28 VDC electrical power from the L ESS BUS and the channel B receives 28 VDC electrical power from the R MAIN BUS. The internal power supply of the LDU changes that voltage into the necessary voltage levels for internal use. The LDU microprocessor uses the 8-bit bi-directional data-bus to control the internal data movement.

Each channel supplies the function to operate 96 outputs. The DCU controls the data on the output bus of the lamp driver.

Each of the two LDU channels can receive data from the two DCU channels. If a fault condition prevents the correct operation of one channel, the other channel does the necessary task. Usually, the LDU channel A uses data from the DCU channel A, and the B channel uses data from DCU channel B. If one of the DCU channels is defective, the two LDU channels will use the serviceable DCU channel.

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The master WARNING/CAUTION switch/lights are found on the WARNING/CAUTION panels which are part of the glareshield in the flight compartment. The top portion of this switch/light is a WARNING indication and the bottom one is a CAUTION indication. The two switch/lights are electrically the same. The master WARNING/CAUTION switch/lights advise the flight crew of the warning and caution messages on the engine indication and crew alerting system (EICAS). The activation of the lamps stops when the switches are pushed or when the conditions that set the WARNING/CAUTION are removed.

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The aural warning switch/lights are on the AURAL WARNING control panel which is on the aft bulkhead behind the copilot's seat in the flight compartment. One is the DCU A OFF switch/light and one is the DCU B OFF switch/light. The two switch/lights are electrically the same. The OFF switch/light comes on white when it is pushed. These switch/lights are used to stop the sound of the aural warnings from the applicable DCU channel.

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The WARNING/CAUTION panels are installed on the glareshield in the flight compartment on the left and right sides. They are located to be in a clear view to the pilot and copilot. The left and right WARNING/CAUTION panels have one master WARNING/CAUTION switch/light each.

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The AURAL WARNING control panel is in the flight compartment on the aft bulkhead behind the copilot's seat. It has two switch/lights (DCU A OFF and DCU B OFF).

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There are two lamp dimmer power supplies. They are installed in the right side console in the flight compartment. They supply bright or dim control to the flight compartment switch/lights. The flight crew sets the bright/dim function with the ANNUN - BRT/DIM switch on the COCKPIT LIGHTS control panel in the flight compartment. The lamp dimmer power supply No. 1 receives 28 VDC power from the L ESS BUS and the R MAIN BUS. The lamp dimmer power supply No. 2 receives 28 VDC power from the R ESS BUS and the L MAIN BUS.

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The SYSTEMS TEST control panel is installed in the center pedestal. The switch on this panel starts a pilot-initiated test (PIT) of different systems. The switch rotates through 10 positions to select a specific system test. The switch is then momentarily pushed to start the applicable test. Usually, the switch gives a ground signal when it is pushed and it starts a test sequence that is different from one system to the other.

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Switch/Lights Operation

Master WARNING/CAUTION Switch/Lights

When a new warning/caution message shows on the EICAS, the two master warning/caution switch/lights operate with an aural warning. The switch/lights flash at a 1 Hz rate.

The WARNING part of the master warning/caution switch/lights flashes red when a new CAS warning message is shown. The CAUTION part of these switch/lights flashes amber when a new CAS caution message is shown.

When the switch/lights are pushed, or the condition that set the warning/caution is removed, a signal is sent to the two channels of the DCU. This effect causes the DCU to stop the applicable switch/light to flash and to stop the aural warning.

The two master warning/caution switch/lights are electrically connected to each of the DCU channels. One DCU channel will operate the master warning/caution switch/lights at a time, although each of the two channels can do this function.

Annunciation Switch/lights

Each of the DCU channels sends input data to the LDUs through an aeronautical radio incorporated (ARINC) data bus. The LDU uses this data to control the grounds to the annunciation switch/lights in the flight compartment. At power-up, the LDU lamp drivers are set to the off state and ARINC data inputs must agree with a set label before the data can be used.

The flight crew sets the light intensity level for the annunciation switch/lights with a switch (ANNUN - BRT/DIM) on the COCKPIT LIGHTS control-panel. The intensity level is set by the voltage supplied to the switch/lights by the lamp-dimmer power supplies.

This voltage level is related to the position of the ANNUN - BRT/DIM switch (intensity is low if selection is DIM, intensity is high if selection is BRT).

Aural Warnings Operation

Aural Warnings Control

The flight crew can stop the transmission of aural warning commands from one of the two or the two DCU channels (A or B). This is done with the selection of the DCU A OFF or DCU B OFF switch/light on the AURAL WARNING control-panel.

When these switch/lights are pushed, the applicable OFF switch/light comes on and the applicable AURAL WARN A (B) MUTED status (white) message will be shown on the EICAS. When these switch/lights are pushed, the applicable DCU channel stops the transmission of aural warning commands to the RIU. When the two switch/lights are pushed, no aural warnings can be heard. When one switch/light is pushed, the channel not set off will give the aural warning function.

The aural warning function supplies triple or single chimes, aural warning tones and voice alarms.

Aural Warnings Priority Sequence

The DCU assigns the priority sequence to the aural warnings. The DCU channels give the command to the RIU channels to operate the aural warning. The RIU supplies aural warnings of different tones and voices. The table that follows shows the different aural warnings in a priority sequence (higher priority first): 

Aural Warnings

ALARM	DESCRIPTION	CHIME
STALL	Voice	No
TAWS aurals	Voice	No
TCAS aurals	Voice	No
NORMAL BRAKES FAIL	Voice	Triple
LEFT ENGINE FIRE	Voice	Triple
RIGHT ENGINE FIRE	Voice	Triple
APU FIRE	Voice	Triple
CONFIGURATION	Voice	Triple
OVERSPEED	Voice	No
Trim clacker	Aural	No
Autopilot disconnect	Aural (Cavalry Charge)	No
CABIN ALTITUDE	Voice	No
GEAR	Voice	Triple
Warning chime	Chime only	Triple
Preselect altitude alert	Aural (C Chord)	No
Caution chime	Chime only	Single
Vertical track alert	Aural	No
SELCAL	Voice	No

The RIU can produce no more than two aural warnings at the same time. It can give the two most important aural warning tones, or the most important voice alarm and the most important aural warning tone. It cannot produce two voice alarms at the same time.

Each aural warning has a related minimum time specified in the RIU. The aural warning tones are heard for this minimum time before another tone starts. This occurs even if the DCU sends a command of a new warning. Each voice alarm is heard fully even if the DCU sends a command of a new warning.

The TCAS (terrain alert and collision avoidance system) and TAWS (terrain avoidance warning system) warnings cancel a voice alarm that is in progress and the STALL warning cancels all voice alarms.

Also, there is a 1.0 ±0.2 second interval separation between the voice alarms. There is an interval even if the DCU sends a command of a new warning in less than 1 second from a previous voice alarm. The voice alarms may or may not be preceded by a chime. The DCU tells the RIU if a chime is necessary. The triple and single chimes are considered voice alarms.

Systems Test Operation

The master warning/master caution control system includes the SYSTEMS TEST panel that lets personnel start a test for some specific systems. In usual condition, the PUSH TO TEST switch, on the SYSTEMS TEST panel, is in the OFF position. In this condition, all the system tests related to the SYSTEMS TEST panel are off. To start a test, the switch is first set to one of the following positions:

• ANNUN A
• ANNUN B
• PROBES
• STALL/RUD LIM
• ICE DET
• FIRE DET
• TAWS

Then, the PUSHT TO TEST switch must be pushed for more than a given time (the duration is related to the specific test) to start the test.

The ANNUN A and ANNUN B selections are for the master warning/master caution control system. With these two selections, a system test can be started for the aural warning function and for the annunciation switch/lights circuits (lamp test).

The other test selections on the SYSTEMS TEST control panel are for the following systems:

• Fire detection and extinguishing (FIREX) system and the smoke detection system for the FIRE DET selection
• Stall protection system for the STALL RUD/LIM selection
• Air data probes and sensors for the PROBES selection
• Ice detection system for the ICE DET selection
• Terrain-avoidance warning system (TAWS) for the TAWS selection

The EICAS messages that follow are related to the master warning/master caution control system: 

EICAS MESSAGE(S)	LEVEL (COLOR)
AURAL WARN A MUTED	STATUS (white)
AURAL WARN B MUTED	STATUS (white)

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The master warning/master caution control system has interfaces with the components that follow:

• Data Concentrator Unit (DCU)
• COCKPIT LIGHTS Control Panel

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On the SYSTEMS TEST control panel, the flight crew can test the aural warning function and lamp circuits.

Aural Warning Test

To do the aural warning test, the PUSH TO TEST rotary switch on the SYSTEMS TEST control panel is first set to ANNUN A or ANNUN B position. Then, the switch is pushed and held for more than 15 seconds. In this condition, the test of each DCU and related RIU channel starts and the aural warnings, other than the TCAS and TAWS (which do not go to the DCU), are heard in a sequence.

When the aural warning test has started, the test switch can be released. If the switch is pushed again, it will cancel the test.

Lamp Test

The test of the LDU that causes the lamps from the two channels to come on is done on the SYSTEMS TEST control panel as follows:

• For channel A, the rotary switch is set to the ANNUN A position. The rotary switch is then pushed to start the test. In this condition, the LDU grounds all channel A outputs at the same time and all the flight compartment lamps come on


• For channel B, the rotary switch is set to the ANNUN B position. The rotary switch is then pushed to start the test. In this condition, the LDU grounds all channel B outputs at the same time and all the flight compartment lamps come on

The LDU can do five lamp tests, one after the other. It does not do a test again for one minute.

The lamp test also does a test of the master WARNING/CAUTION switch/lights. The light of the master WARNING/CAUTION switch/lights stays on until the lamp test is done, or the master WARNING/CAUTION switch/lights are pushed. Also, a new warning or caution message causes the light of the master WARNING/CAUTION switch/lights to go off momentarily.

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The LDU supplies a continuous internal monitor function of the following circuits:

• RAM (random access memory)
• PROM (programmable read only memory)
• Processor
• Power supply

Fault conditions could cause an incorrect operation of the lamp drivers in a channel. If this fault condition occurs, all lamp driver outputs from that channel are disabled.

Each of the lamp driver outputs is monitored to make sure that it agrees with the input command. If the output does not agree with the input command, the applicable LDU lamp driver sends a test word to the DCU. The LDU diagnostic label sends the results from the input bus test of defective buses to the DCU.

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Component Location Index
IDENT	DESCRIPTION	LOCATION	IPC REF
A52	LAMP DRIVER UNIT	ZONE(S) 212	31-52-01
SDS55	MASTER WARNING/CAUTION SWITCH/LIGHT (LH)	ZONE(S) 211	31-52-05
SDS54	MASTER WARNING/CAUTION SWITCH/LIGHT (RH)	ZONE(S) 212	31-52-05
SDS64	AURAL WARNING SWITCH/LIGHT	ZONE(S) 212	31-52-09
SDS65	AURAL WARNING SWITCH/LIGHT	ZONE(S) 212	31-52-09
PL5	WARNING/CAUTION PANEL (LH)	ZONE(S) 211	31-52-13
PL6	WARNING/CAUTION PANEL (RH)	ZONE(S) 212	31-52-13
PL20	AURAL WARNING CONTROL-PANEL	ZONE(S) 212	31-52-17
A163	LAMP-DIMMER POWER SUPPLY (NO. 1)	ZONE(S) 212	31-52-21
A72	LAMP-DIMMER POWER SUPPLY (NO. 2)	ZONE(S) 212	31-52-21
PL22	SYSTEM TEST CONTROL-PANEL	ZONE(S) 211	31-52-25

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