PMK-21

PMK-21 program mechanism for rotor anti-icing

PMK-21 is an electromechanical program mechanism used in the rotor anti-icing system. Its purpose is to generate a fixed time sequence and issue commands that switch the heating elements of the main- and tail-rotor blades. The mechanism distributes the electrical load between separate heater sections instead of energising every section at the same time. This controlled sequence keeps the demand on the generating system within the intended operating pattern while maintaining periodic heating of the blade surfaces.

The unit belongs to the program mechanisms and control units used in onboard electrical systems. It is not a cockpit control panel and does not measure ice directly. PMK-21 performs the switching programme after the anti-icing system has been activated through the helicopter control circuits.

Timed switching sequence produced by PMK-21

Technical documentation for the Mi-8 anti-icing installation describes six parallel output commands. Four commands operate the main-rotor heater groups, while two commands operate the tail-rotor heater groups. Each command lasts 38.5 ± 2 seconds. The complete heating cycle is approximately 150 seconds for all four main-rotor sections and 77 seconds for the two tail-rotor sections.

In this arrangement, four power contactors connect corresponding heater sections on all five main-rotor blades. Two additional contactors switch the corresponding sections on the three tail-rotor blades. The programmed sequence is therefore part of the load-management logic of the anti-icing system. Correct operation also depends on the contactors, wiring, generator, blade heating elements and the controls fitted to the individual helicopter.

PMK-21 in the Mi-8 blade-heating circuit

In the documented Mi-8 installation, the blade heaters are connected in sequence to a two-phase 208 V, 400 Hz supply from the SGO-30U generator. This value describes the switched heating circuit; it should not be confused with the control supply of the program mechanism itself. Technical data published for the PMK-21 installation on the Mi-2 specifies a 27 ± 3 V DC unit supply and current consumption of no more than 1 A.

Because electrical diagrams and installed versions may differ, the helicopter model, PMK-21 series, connector arrangement and wiring diagram should be checked before replacement. A PMK-21TV or a series-indexed unit must not be treated as automatically interchangeable with an unqualified PMK-21 designation.

Electromechanical construction and associated equipment

The PMK-21 is installed in the radio-equipment compartment in the documented Mi-8 configuration. Its internal timing element is the PM5-154 program mechanism, built as a motor-driven time relay. The assembly includes a D2-RT electric motor with a centrifugal speed governor, a cam-operated switching device, a switch block, power contactors and an interference-suppression filter.

Rotation of the motor drives the programmed cams, which change the state of the switching contacts at defined intervals. Those contacts control the six output channels connected to the rotor-heating contactors. This mechanical timing principle explains why inspection should cover not only electrical continuity, but also timing accuracy, contact condition and stable operation of the drive mechanism.

Applicability, identification and ordering

The designation appears as PMK-21 and ПМК-21; common search forms also include PMK 21 and ПМК 21. Documentation confirms use of the mechanism in Mi-8 and Mi-2 anti-icing systems, but the precise aircraft modification and installed series must be verified against the illustrated parts catalogue, wiring diagram or maintenance manual for the individual helicopter.

Before ordering, confirm the marking on the identification plate, series or execution, connector configuration, required condition and documentation set. Reliable public data for the exact overall dimensions and connector pin count of this version were not found, so these values should be checked from the product passport or the unit offered for supply.

Product Type: Electromechanical program mechanism / cyclic timer

Designation: PMK-21

Alternative Designations: ПМК-21; PMK 21; ПМК 21

Function: Generation of a fixed time programme and sequential commands for rotor-blade heating circuits

Application / Where Used: Main- and tail-rotor anti-icing systems

Confirmed Helicopter Applications: Mi-8; Mi-2, subject to aircraft modification and installed series

Installation Area: Radio-equipment compartment in the documented Mi-8 installation

Control Supply: 27 ± 3 V DC according to Mi-2 technical documentation

Current Consumption: Not more than 1 A according to Mi-2 technical documentation

Switched Heating Circuit: Two-phase 208 V AC, 400 Hz in the documented Mi-8 installation

Heating Power Source: SGO-30U generator in the documented Mi-8 installation

Output Commands: 6 parallel commands

Command Duration: 38.5 ± 2 s

Main-Rotor Heating Cycle: Approximately 150 s for four heater sections

Tail-Rotor Heating Cycle: Approximately 77 s for two heater sections

Main-Rotor Switching: 4 power contactors controlling corresponding heater sections on five blades

Tail-Rotor Switching: 2 power contactors controlling corresponding heater sections on three blades

Internal Program Mechanism: PM5-154 motor-driven time relay

Drive Motor: D2-RT with centrifugal speed governor

Construction: Cam switching device, switch block, power contactors and interference-suppression filter

Operating Mode: Not more than 2 h according to Mi-2 technical documentation

Mass: Not more than 2.5 kg according to Mi-2 technical documentation

Dimensions and Connector Pin Count: Reliable public data were not found

Interchangeability Note: PMK-21TV and series-indexed versions require separate documentation confirmation

Technical Documentation: Product passport, anti-icing system description, aircraft wiring diagram and maintenance documentation when available

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