BMP 6S2.399.021

BMP mechanical transition block in aircraft radio navigation

The BMP mechanical transition block, drawing number 6S2.399.021, is an aircraft radio-electronic unit used in automatic direction finder and radio-navigation circuits. It is installed where the signal from an antenna unit or angular transmitter has to be adapted before it can be correctly displayed on cockpit indicators. In these systems, the block works inside the signal transmission chain rather than as a standalone navigation instrument. Its role is to make different electromechanical elements of the system work together without distorting the transmitted bearing or course-angle information.

BMP block for matching transformers and selsyns

The main technical purpose of the BMP block is matching sine-cosine transformers with selsyn receiving circuits. This is needed when the angle source and the receiving indicator use different electrical principles. The transmitter may form angular data through a sine-cosine transformer, while the receiving side may use a selsyn circuit connected to an indicator. The BMP provides the mechanical and electrical transition between these elements, allowing the angular position to be passed further through the system in a usable form.

Course-angle transmission through the BMP unit

In automatic radio compass systems, the BMP block helps transmit the course angle or bearing data from the radio station antenna unit to the cockpit indicator. This is especially relevant in systems where the crew works with “Compass” and “Antenna” modes and expects the indicator to show the correct direction without delay or unstable movement. The block does not determine the bearing itself. It supports the transfer of already formed angular information from the radio-navigation equipment to the receiving indicator circuit.

BMP 6S2.399.021 in ARK equipment

The BMP block with drawing number 6S2.399.021 is associated with automatic direction finder equipment such as ARK-UD and ARK-15, depending on the aircraft configuration. It may be used in civil and military aircraft radio-navigation systems where antenna position data, bearing information, and indicator drive signals have to be coordinated. In such systems, even a small mismatch in the transmission chain can lead to incorrect indicator behavior. That is why the BMP is treated as a functional part of the radio compass set, not just as a passive mechanical component.

Aircraft application of the BMP transition block

The BMP mechanical transition block can be found on aircraft such as An-26, Il-76TD and other civil or military aircraft fitted with compatible radio compass equipment. Its actual use depends less on the aircraft name alone and more on the installed radio-navigation set. Two aircraft of the same family may have different avionics configurations, so the block should always be checked against the equipment list, wiring diagram, and parts catalog for the exact aircraft modification. For procurement, the drawing number is often the safest identifier.

Operating conditions for the BMP block

The BMP block is designed for normal aircraft service conditions, including vibration, temperature variation and pressure changes. It is normally installed in technical or radio-equipment areas as part of the navigation system wiring and signal path. During maintenance, attention is usually paid to secure mounting, connector condition, housing integrity, wiring condition and stable operation of the connected indicator. A fault in this block may not always look like a complete system failure. It may appear as wrong indication, unstable pointer movement, or incorrect response in one of the radio compass modes.

Identification and supply of BMP 6S2.399.021

When ordering or replacing the unit, the designation “BMP” should be confirmed together with the drawing number 6S2.399.021. This is important because similar transition units may differ by series, internal circuit, connector version or aircraft applicability. The block may also be referenced together with related designations used in documentation for radio compass equipment. Before supply, it is useful to confirm aircraft type, installed ARK system, required condition, documentation status and whether the unit is needed for installation, bench replacement or stock replenishment.

Product Type: Aircraft mechanical transition block / radio-navigation signal matching unit
Designation: BMP
Drawing Number: 6S2.399.021
Function: Matching of antenna unit signal circuits, sine-cosine transformers and selsyn receiving circuits
Application / Where Used: Aircraft radio-navigation systems and automatic direction finder equipment
System / Installation Area: Automatic radio compass signal transmission and indication circuits
Primary System Function: Transfer and adaptation of course-angle / bearing data to cockpit indicators
Typical Operating Modes Supported: “Compass” and “Antenna” indication circuits, depending on ARK system configuration
Associated Equipment: ARK-UD, ARK-15 and related automatic direction finder systems depending on aircraft configuration
Compatible Aircraft / Equipment: An-26, Il-76TD and other civil or military aircraft fitted with compatible ARK equipment
Signal Matching: Sine-cosine transformer to selsyn-based receiving circuit
DC Supply Voltage: 27 ±2.7 V
DC Current Consumption: Not more than 0.12 A
AC Supply Voltage: 32.4–37.8 V
AC Supply Frequency: 400 ±8 Hz
AC Current Consumption: Not more than 0.6 A
Dimensions: 270 × 93 × 84 mm
Operating Conditions: Aircraft vibration, temperature and pressure conditions according to applicable technical documentation
Technical Documentation: Product passport, acceptance documents and aircraft radio-navigation documentation as applicable

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