BU-1

BU-1 in the SAS-4-9 annunciation system

The BU-1 is an aircraft advisory signal unit used as part of the SAS-4-9 onboard alerting and annunciation system. Its function is to process advisory, service and status signals and transmit them to cockpit annunciators and indicator lamps. Unlike emergency or caution units, the BU-1 is not intended to generate warning alerts. It supports the crew by showing non-emergency information related to system status changes, equipment activation, confirmation signals and other operational states during flight and ground operation.

Advisory signal processing by BU-1

The BU-1 receives discrete signals from aircraft systems through relay contacts, sensor channels and monitored control circuits. These inputs are processed by the unit and then converted into command outputs for advisory indication in the cockpit. The unit can support up to 30 discrete input signals, which allows it to cover multiple aircraft systems within the SAS-4-9 logic. In practical use, the BU-1 helps separate normal system-state messages from more serious caution and emergency alerts, keeping the crew interface more organized.

BU-1 and cockpit advisory indication

The BU-1 sends processed outputs to advisory annunciators and cockpit indicator lamps. These messages may confirm that a subsystem has switched on, changed state, reached a defined condition or completed a commanded action. This type of indication is important because the crew needs confirmation that onboard systems are operating as expected. If the BU-1 fails or loses channel functionality, advisory information may not appear in the cockpit even when the connected aircraft system itself is working normally.

BU-1 with BAP-1 and BK-7 units

The BU-1 works together with the BAP-1 emergency and caution warning unit and the BK-7 switching unit. Together, these blocks form the SAS-4-9 alerting and annunciation suite. The BAP-1 processes emergency and caution signals, the BU-1 handles advisory and non-emergency indications, while the BK-7 performs switching, routing and distribution of signals inside the system. This separation of functions helps the crew distinguish between critical failures, caution states and normal operational messages.

Power supply and reduced-voltage operation of BU-1

The BU-1 operates from the aircraft 24–29.4 V DC power supply and remains functional under reduced voltage conditions down to 18 V. This is important because the annunciation system must remain available even during degraded electrical states. The unit is intended for continuous service as part of onboard avionics. Stable power supply, correct connector condition, reliable grounding and clean wiring interfaces are important for proper processing of advisory channels and stable output to cockpit indicators.

Installation of BU-1 in the avionics rack

The BU-1 is installed in an avionics mounting frame or equipment rack, depending on the aircraft configuration. The unit may be supplied with or without a mounting frame, so this point should be clarified before procurement. During replacement, technicians should check the exact unit designation, frame availability, connector arrangement, installation position and compatibility with the existing SAS-4-9 set. Mechanical fit and electrical compatibility should be verified together because the unit must integrate correctly with the complete annunciation system.

Application of BU-1 on Yak-42 and Il-86 aircraft

The BU-1 is used on aircraft equipped with the SAS-4-9 alert and annunciation system, including Yak-42, Il-86 and compatible aircraft types. It is part of the system architecture that provides cockpit status information to the crew. Before installation, the unit should be checked against the aircraft maintenance documentation, SAS-4-9 consolidated documentation and wiring diagrams. Similar-looking avionics blocks may differ by channel configuration, connector layout or approved installation version, so substitution should not be made by appearance alone.

Supply and documentation for BU-1

For procurement, the full designation BU-1 should be specified together with required condition, documentation and mounting frame requirement. The unit may be supplied from storage, serviceable, repaired or in another agreed condition. For aircraft installation, passport 632.068.008 PS and SAS-4-9 consolidated documentation are important for confirming designation, configuration, acceptance and traceability. Metal content data may also be required for warehouse accounting, customs records, repair documentation or disposal control.

Product Type: Advisory signal processing unit
Designation: BU-1
Alternative Designation: БУ-1
Function: Processing and output of advisory / non-emergency cockpit signals
Application / Where Used: Aircraft onboard alert and annunciation systems
System / Installation Area: SAS-4-9 alert and annunciation system
Compatible Aircraft / Equipment: Yak-42, Il-86 and compatible aircraft equipped with SAS-4-9
Associated Units: BAP-1 emergency / warning unit; BK-7 signal switching unit
Input Signal Type: Discrete signals from relay contacts, sensor channels and aircraft system circuits
Number of Inputs: Up to 30 discrete input signals
Output Function: Commands to advisory annunciators and cockpit indicator lamps
Alert Category: Advisory / service / status indications; non-emergency signals only
Power Supply, Normal: 24–29.4 V DC
Power Supply, Emergency: Down to 18 V
Power Consumption: Not more than 7 W per channel at 27 V
Operating Mode: Continuous service
Installation: Avionics mounting frame / rack installation
Weight: Not more than 2.0 kg
Package Contents: Mounting frame when applicable, depending on configuration
Condition: As applicable
Material Content: Gold 0.289 g; silver 61.83 g; palladium 0.117 g; aluminum 0.6 kg
Technical Documentation: Passport 632.068.008 PS; SAS-4-9 consolidated documentation

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Warranty
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