SD-29A

SD-29A pressure switch for helicopter fuel systems

SD-29A is an aircraft pressure switch used for monitoring pressure in helicopter fuel system circuits. The unit operates as a pressure-controlled electrical signalling device: when the pressure of the measured medium reaches the preset threshold, the internal contacts change state and send a signal to the aircraft indication circuit. In Mi-8 / Mi-17 applications, it is associated with service tank pipelines, manifolds and related fuel system monitoring points.

Pressure signalling through SD-29A

The switch monitors gauge pressure of liquid or gaseous media and provides a discrete output signal. This means the unit does not display pressure continuously like a gauge. Instead, it reacts when pressure falls below or rises above the configured threshold, depending on the circuit design. The signal can be used to activate cockpit warning indication or system status lamps. Correct operation depends on the pressure setting, clean pressure port, sealed connection and serviceable electrical contacts.

SD-29A in service tank pressure control

In helicopter fuel systems, the pressure switch may be used for monitoring the service tank, associated manifolds or pipelines. It can provide timely warning when pressure in the controlled section reaches a critical level. The monitored medium may include aviation fuel and fuel vapors, depending on installation. Because fuel system pressure monitoring is safety-related, the unit should be checked together with the pipeline connection, wiring, warning lamp circuit and aircraft maintenance documentation.

Construction of the SD-29A pressure switch

The unit typically includes a pressure connection, sensitive diaphragm, adjustable setpoint spring and sealed microswitch assembly. Pressure from the controlled line acts on the sensing element, and the mechanical movement changes the state of the contacts at the set pressure point. This construction allows the unit to work as a compact pressure relay in aircraft systems exposed to vibration, temperature changes and fuel or oil environment. The housing execution is selected for aviation operating conditions.

Identification and ordering of SD-29A

Before ordering, the full marking should be checked against the aircraft parts catalog, system documentation or product passport. SD-29A is associated with part registration number S-05847 and technical specification TU-SD-SD-A-SNN-70. For correct selection, provide the aircraft type, installation system, pressure setting if known, existing unit marking and a photo of the data plate. The product is also known under alternative designations SD-29A and СД-29А.

Product Type: Aircraft pressure switch / pressure signalling device
Function: Monitoring of gauge pressure and generation of discrete electrical signal at preset threshold
Application / Where Used: Helicopter fuel and oil system monitoring circuits
System / Installation Area: Service tank manifolds, pipelines and related pressure control points
Compatible Aircraft / Equipment: Mi-8 / Mi-17 family helicopters, depending on configuration
Part Registration Number: S-05847
Technical Specification: TU-SD-SD-A-SNN-70
Measured Media: Aviation fuel T-1, TS-1, RT; fuel vapors; compatible liquids or gases depending on installation
Nominal Controlled Pressure: Up to approx. 0.15–0.2 MPa, depending on system mode
Typical Switching Threshold: Approx. 0.02–0.05 MPa, depending on aircraft system setting
Supply Voltage: 27 V DC aircraft network
Output Signal Type: Discrete dry contact, make or break depending on circuit design
Operating Temperature Range: −60 to +60 °C
Construction: Pressure port, sensitive diaphragm, adjustable setpoint spring, sealed microswitch block
Environmental Resistance: Vibration-resistant, shock-resistant, protected for aircraft fuel / oil environments
Equipment Category: Aircraft control and measuring equipment
Alternative Designations: SD-29A; СД-29А
Technical Documentation: Product passport, aircraft system documentation and maintenance documentation

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