AT-8-3

AT-8-3 autotransformer for cockpit windshield heating

The AT-8-3 is a single-phase aircraft autotransformer used in the AC electrical system of Mi-8 family helicopters. Its main purpose is to supply the cockpit windshield heating elements with the voltage required for a specific glass installation. The unit receives AC power from the aircraft network and provides several output taps for the heating circuit. It is not a voltage regulator in the usual sense and does not control glass temperature by itself. It only provides the required voltage level for the heating elements connected to it.

Windshield heating power supply through AT-8-3

In service, the AT-8-3 works in the windshield heating circuit. The correct output tap is selected according to the voltage specified for the installed cockpit glass. This is an important point during maintenance, because the transformer may be serviceable while the heating circuit works incorrectly due to a wrong tap connection, damaged wiring, faulty heating element, or related temperature control equipment. On Mi-8 systems, the windshield heating circuit may also work together with temperature regulators and protection devices installed elsewhere in the aircraft electrical system.

Electrical design of the AT-8-3 unit

The AT-8-3 is an open-type autotransformer with natural air cooling. Its magnetic core is made from laminated electrical steel plates, and the winding is made from copper wire. Since it is an autotransformer, the primary and secondary circuits have a common electrical connection. This should be considered when checking insulation, wiring, and circuit layout. The unit has several voltage taps, allowing the heating supply to be matched to the glass passport data and aircraft wiring scheme.

Mi-8 helicopter installation of AT-8-3

On Mi-8 type helicopters, the AT-8-3 is normally installed in the radio compartment on the left side, between fuselage frames 19 and 20. It is used in the AC power supply system for cockpit glass heating on Mi-8T, Mi-8MT, Mi-8AMT and related configurations where this equipment is specified by the aircraft documentation. Exact applicability should still be checked by the parts catalog, because cockpit glass type, wiring, heating control circuit, and aircraft modification may differ after repair or modernization.

Identification and ordering checks for AT-8-3

Before ordering, the marking on the installed autotransformer should be compared with the aircraft parts catalog, wiring diagram, or maintenance documentation. AT-8-3 should not be confused with low-power transformers used in radio equipment, even if the designation looks similar. For a correct request, it is useful to provide the aircraft type, installation location, required condition, and a photo of the marking if available. The product is also known under alternative designation АТ-8-3.

Product Type: Single-phase aircraft autotransformer
Function: Voltage transformation for cockpit windshield heating elements
Application / Where Used: Aircraft AC power supply systems / cockpit glass heating
System / Installation Area: Mi-8 radio compartment, left side between frames 19 and 20, subject to aircraft configuration
Compatible Aircraft / Equipment: Mi-8T, Mi-8MT, Mi-8AMT and applicable Mi-8 family configurations
Input Voltage: 208 V AC
Output Voltage Taps: 190 V, 200 V, 230 V, 250 V
Cooling: Natural air cooling
Design Type: Open-type armored single-phase autotransformer
Core Material: Laminated electrical steel, Ш-shaped plates, 0.35 mm
Winding Material: Copper wire, ПЭВ-2 or ПЭВП type
Circuit Feature: Common galvanic connection between primary and secondary circuits
Technical Documentation: Mi-8 electrical system documentation / aircraft parts catalog

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