2416-17.5

2416-17.5 thermal switch in aircraft heating systems

The 2416-17.5 is an aircraft thermal switch used in onboard heating control circuits of Mi-8, Mi-17, Mi-171, and related helicopter platforms. The unit is installed in the air path of the KO-50 kerosene-fired heater and monitors the temperature of heated air at the heater outlet. Its main function is to protect the heating system from operation beyond the permitted thermal limit. When the controlled air temperature reaches the specified threshold, the thermal switch changes the state of its contacts and sends a command to the aircraft control circuit.

KO-50 heater protection with 2416-17.5

In the KO-50 heating and ventilation installation, the 2416-17.5 is used as part of the automatic protection and fuel shut-off logic. The unit is connected to the electrical circuit that controls fuel supply to the heater. If the air temperature at the outlet of the heater rises above the permitted value, the thermal switch opens the circuit and supports emergency interruption of fuel delivery. This prevents further heating and helps keep the heater, air ducts, wiring, and downstream equipment within the required operating conditions.

2416-17.5 fuel shut-off control function

The 2416-17.5 may operate in the control circuit of an electromagnetic fuel valve used with the KO-50 heater. In this role, it interrupts the power supply circuit of the valve when the temperature limit is exceeded. Depending on the aircraft configuration, the thermal switch may be associated with valves such as 772D or 610200A, as well as relay and contactor circuits used in the heater control system. The exact connection must always be checked against the wiring diagram and technical documentation of the specific helicopter modification.

Contact operation of the 2416-17.5 switch

The 2416-17.5 is normally closed during standard heater operation and opens its contacts when the sensitive element reaches the actuation temperature. This makes the unit suitable for protection circuits where loss of continuity must result in fuel shut-off or heater shutdown logic. The device does not provide a proportional temperature reading. It works as a discrete safety and control element, giving the system a clear electrical signal when the air temperature has reached the specified limit. This simple switching principle is well suited to aircraft systems where reliability and predictable response are required.

2416-17.5 sensing element and construction

The thermal action of the 2416-17.5 is based on expansion of internal elements made from heat-resistant materials. The design uses a temperature-sensitive assembly that reacts to heating in the airflow and mechanically acts on the contact group. The body and sensing components are intended for installation in a hot-air environment at the outlet of the KO-50 heater. The switch must withstand vibration, local heating, and the mechanical loads typical of helicopter operation. During inspection, attention is normally paid to the housing, mounting condition, wiring approach, and condition of the lead termination.

System compatibility of 2416-17.5 with related switches

In KO-50 heater systems, the 2416-17.5 may work together with other temperature-control devices, including the 2416-4 thermal switch and the 1374A-5 thermal switch. Each of these units has its own operating point and function within the heater control logic. The 2416-17.5 is associated with overtemperature protection, while lower-temperature switches may be used for fan control, heater readiness, or other automatic functions. For correct replacement, the designation must be checked carefully, because similar-looking thermal switches can have different actuation temperatures and different roles in the circuit.

Identification, supply, and maintenance of 2416-17.5

The 2416-17.5 belongs to aircraft technical equipment and is supplied with passport, label, or quality documentation depending on condition and source. It may be available as new production or as a first-category storage item, subject to documentation and acceptance status. Before installation, the part should be matched with the aircraft parts catalog, KO-50 heater documentation, and maintenance procedures for the relevant helicopter version. During scheduled maintenance, the unit is inspected for secure installation, visible damage, cracks, wiring condition, and correct routing of the connected harnesses.

Product Type: Aircraft thermal switch
Designation: 2416-17.5
Function: Automatic opening of the electrical control circuit when the air temperature limit is reached
Application / Where Used: Aircraft heating and ventilation systems
System / Installation Area: KO-50 kerosene-fired heater air outlet / heated-air duct
Primary System Function: Overtemperature protection of the heater and automatic interruption of fuel supply
Compatible Aircraft / Equipment: Mi-8T, Mi-8MTV, Mi-17, Mi-171, and selected Mi-24 modifications depending on heating system configuration
Associated Heater: KO-50 kerosene-fired aircraft heater
Associated Components: 2416-4 thermal switch, 1374A-5 thermal switch, electromagnetic fuel valves, relay and contactor circuits depending on configuration
Typical Controlled Components: Fuel shut-off valve, heater control circuit, electromagnetic valve control line
Nominal Actuation Temperature: +175 °C
Permissible Actuation Range: Approximately +165 to +200 °C depending on production series, adjustment, and service condition
Marking Interpretation: “17.5” corresponds to the actuation temperature expressed in tens of degrees Celsius
Contact Type: Normally closed contacts, opening on heating
Operating Voltage: 27 V DC aircraft electrical system
Switching Function: Opening / closing of low-current aircraft control circuits
Typical Associated Valves: 772D or 610200A electromagnetic fuel valve depending on system configuration
Sensing Element Material: Heat-resistant alloy EI946, according to technical description data
Body Material: EI946 alloy, according to technical description data
Operating Principle: Thermal expansion of the sensing assembly with mechanical action on the contact group
Installation Position: In the heated airflow at the outlet of the KO-50 heater calorifier
Inspection During Maintenance: Mounting security, housing condition, absence of cracks, wire harness routing, and terminal condition
Supply Category: Aircraft technical item; new production or first-category storage item depending on availability
Technical Documentation: Product passport, quality label, aircraft maintenance documentation, and KO-50 heater documentation as applicable

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