NR-3VMA-T

Fuel metering function of the NR-3VMA-T

During start-up, acceleration, steady operation and throttle reduction, the NR-3VMA-T supplies the engine with the required fuel flow. It controls how much fuel reaches the combustion chamber through the fuel manifolds and working nozzles. The fuel schedule must be accurate: too little fuel may cause poor start or loss of acceleration, while excessive fuel can lead to temperature rise, unstable operation or incorrect power limitation. This is why condition, adjustment and bench-test results are especially important for this type of pump-regulator.

NR-3VMA-T in automatic engine regulation

The NR-3VMA-T works as part of the automatic regulation system of the TV3-117VMA engine. It supports regulation of turbocompressor speed and free turbine speed, participates in power synchronization on twin-engine helicopters, and provides fuel-flow limitation when required. In case of failure or absence of normal electronic correction, the hydromechanical part of the unit still provides reserve limiting functions within the design logic of the system. This makes the NR-3VMA-T important both in normal operation and in backup operating modes of the engine control system.

NR-3VMA-T with ERD-3VMA, IM-3A and IM-47

The pump-regulator operates together with electronic and electromechanical control units such as ERD-3VMA, ERD-3VM, IM-3A or IM-47, depending on the engine and aircraft configuration. The electronic regulator calculates correction commands, while the actuator physically changes the position of regulating elements inside the NR-3VMA-T. The RT-12-6 gas temperature regulator may also be involved in limiting fuel delivery when turbine gas temperature approaches the permitted limit. For troubleshooting, these units should be checked as one connected control chain rather than as isolated components.

Fuel system connections of the NR-3VMA-T

Fuel normally reaches the NR-3VMA-T after passing through the helicopter fuel supply system, booster pump and filtration elements. The DTsN-70A centrifugal booster pump supplies fuel to the inlet of the pump-regulator at the required preliminary pressure. After that, the NR-3VMA-T meters and distributes fuel to the first and second fuel circuits and then to the working nozzles. The unit is also connected with stop circuits, drainage elements and fuel return paths depending on the engine layout. Fuel cleanliness is critical because the aggregate contains precision hydraulic and metering components.

Compressor control through the NR-3VMA-T

The NR-3VMA-T also participates in compressor mechanization control. Fuel pressure is used as a hydraulic working medium for actuators that move inlet guide vanes and compressor guide vanes. The pump-regulator also works with compressor bleed valve control, which is needed during start-up and transient modes to prevent compressor instability. Because of this, a malfunction of the NR-3VMA-T can affect not only fuel supply but also acceleration quality, compressor stability, and the way the engine responds to pilot commands.

Installation of the NR-3VMA-T on Mi and Ka helicopters

The NR-3VMA-T is installed on TV3-117VMA engines used on Mi-8MT, Mi-17, Mi-8MTV, Mi-17-1V, Mi-172, Mi-24, Mi-35, Ka-32, Ka-50, Ka-52 and related helicopter types, depending on the engine version and aircraft configuration. The pump itself is located on the engine accessory gearbox in the powerplant compartment. It is mechanically driven from the engine and connected to fuel, hydraulic and electrical control lines. Exact applicability must be confirmed by the engine logbook, parts catalog and aircraft maintenance documentation.

NR-3VMA-T tropical execution and supply notes

The “T” index is generally associated with a tropical execution or a version intended for operation in more demanding climatic conditions, including high humidity and elevated ambient temperatures. For procurement, the designation NR-3VMA-T should be stated fully, because similar NR-3VM / NR-3VMA units may differ by execution, adjustment data, actuator connection or approved engine application. For aircraft use, the unit should be supplied with passport, test, repair or storage documentation. Bench-test confirmation is highly desirable, since external condition alone does not prove that the pump-regulator is correctly adjusted.