BF-34 Batch A

BF-34 filter block in the AP-34B control system

The BF-34 is an aviation electronic filter block used in helicopter automatic control systems. In AP-34B autopilot installations, it works inside the signal-processing chain, where sensor signals must be filtered and stabilized before they are passed further into control circuits. The unit is not a cockpit control device and is not operated directly by the crew. Its role is technical: to improve the quality of signals used in automatic stabilization and to reduce unwanted disturbances in feedback circuits.

Signal filtering function of the BF-34 block

The BF-34 processes incoming signals from onboard sensors and related control circuits. In helicopter autopilot systems, signals from gyroscopes and other sources may contain noise, vibration-related components or unwanted oscillations. The filter block helps suppress these disturbances and prepares a more stable signal for downstream equipment. This is important because automatic control systems depend on clean feedback. If the signal contains excessive noise or oscillation, the autopilot may react too sharply, become unstable or fail functional checks.

BF-34 in feedback and stabilization circuits

In the AP-34B automatic flight control system, the BF-34 is connected with stabilization and feedback channels. It supports the correct transmission of control signals toward the amplifiers, logic units and executive elements of the autopilot. The block does not generate control commands by itself. It shapes and filters the electrical signals that are already present in the control loop. A faulty filter block can affect the behavior of the whole channel, especially if filtering parameters or time constants move outside the required range.

Application on Mi-8 / Mi-17 family helicopters

The BF-34 filter block is used on helicopters of the Mi-8, Mi-17, Mi-171 and Mi-172 family equipped with the AP-34B automatic flight control system. It is installed in onboard avionics compartments as part of the automatic stabilization equipment. Since Mi-8 family helicopters may differ by modification, wiring standard and autopilot configuration, the block should be checked against the aircraft parts catalog and AP-34B documentation before replacement. The drawing number 6A3.390.000 is useful for confirming the exact item.

Design and construction of BF-34

The BF-34 is built as an electronic module in a metal housing. The internal design includes electronic filtering circuits and matching elements used for signal conditioning in the control system. The metal case protects the unit and supports installation in the avionics area. During inspection, attention is usually given to housing condition, connectors, mounting points, markings, traces of corrosion, overheating, moisture ingress and previous repair. Since the block works with control signals, electrical testing is more important than appearance alone.

Power supply and operating checks

The BF-34 operates from onboard aircraft power supply circuits. Correct supply voltage, contact condition and grounding are important for stable filtering and signal transfer. During maintenance, technicians normally check the electrical parameters, time constants, continuity of circuits and the unit’s behavior under the required test conditions. A fault in wiring, connector contact or power supply can look like a failed filter block, so diagnostics should include the connected AP-34B circuits and not only the BF-34 itself.

Identification and supply of BF-34

The product may appear in documentation and supply requests as BF-34, БФ-34 or by drawing number 6A3.390.000. These designations usually refer to the same electronic filter block used in AP-34B-related control systems, but marking and documentation should still be confirmed before shipment. The unit may be supplied new, from storage, repaired, overhauled or serviceable depending on availability. For aircraft use, the manufacturer passport, technical condition and compatibility with the installed autopilot configuration should be checked.

Product Type: Electronic filter block
Designation: BF-34
Cyrillic Designation: БФ-34
Drawing Number: 6A3.390.000
Batch / Version: Batch A
Function: Signal filtering and stabilization in automatic control systems
Application / Where Used: Helicopter automatic control systems; AP-34B autopilot
System / Installation Area: Helicopter avionics compartment; AP-34B signal-processing and feedback circuits
Compatible Aircraft / Equipment: Mi-8, Mi-17, Mi-171 and Mi-172 family helicopters depending on AP-34B installation
System Role: Filtering of sensor and feedback signals before transmission to downstream autopilot control circuits and executive elements
Associated Equipment: AP-34B autopilot, gyroscopic sensors, control amplifiers and executive mechanisms
Construction: Metal housing with electronic filters and matching elements
Operating Voltage, AC: 36 V (+18 / −3.6 V)
AC Power Consumption: Not more than 0.1 A
Time Constant T1: Not more than 0.025 s
Time Constant T2: Not more than 0.03 s
Failure-Free Operation Time: Not less than 0.05 h, according to provided passport data
Mean Time Between Failures: Not more than 0.25 h, according to provided passport data
Weight: Not more than 1.2 kg
Condition: As applicable
Technical Documentation: BF-34 passport

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