SPN-4-3T

SPN-4-3T surface-discharge ignition plug

The SPN-4-3T is an aviation surface-discharge ignition plug used in gas turbine engine starting systems. It is installed in the ignition section of the combustion chamber or starting unit, depending on the engine layout. The plug produces the spark needed to ignite the fuel-air mixture during engine start. In service documents it may be described as an electroerosion surface-discharge plug, because the spark path is formed along the ceramic insulator surface rather than across a conventional open electrode gap.

Ignition function of the SPN-4-3T plug

The working area of the plug is the surface of the ceramic insulator between the central and side electrodes. During operation, the ignition unit sends a high-voltage pulse to the plug. The discharge passes over the ceramic surface and ignites atomized fuel in the start zone. This type of construction is used where the ignition system must work under fuel spray, pressure changes, vibration and repeated start cycles. The plug itself does not generate voltage. It receives the pulse from the ignition unit and creates the discharge at the firing end.

SPN-4-3T in AI-20 and TG-16M ignition systems

The SPN-4-3T is used with AI-20 aircraft gas turbine engines and TG-16M turbine generator units. In these applications it works as part of the starting ignition circuit, not as a standalone component. The typical system includes the aircraft DC supply, the starting ignition coil or ignition unit, shielded high-voltage wiring, the plug, and the engine combustion or start chamber. The plug is also documented in ignition arrangements that use the 1KNI-11B-T low-voltage starting ignition coil. For replacement work, the plug should be matched not only by designation, but also by the engine manual and the actual ignition harness fitted on the aircraft.

Construction notes for SPN-4-3T

The plug has a non-disassemblable screened body with ceramic insulation. The screened design reduces interference from the ignition pulse and allows the plug to work with shielded aircraft ignition leads. The threaded body is used for installation into the engine or starting unit, while the screened elbow connection links the plug to the ignition cable. The surface-discharge design gives the plug a different firing pattern from common automotive-type plugs. In practical maintenance, this matters because checking the part by appearance alone is not enough. The thread, firing surface, shielding connection and part marking all need to match the technical documentation.

Ordering and identification of SPN-4-3T

Before ordering, the marking on the plug should be compared with the engine or aircraft parts catalog. A plug removed from storage should also be checked for condition of the ceramic area, thread, shielding interface and sealing surface. For AI-20 and TG-16M applications, it is useful to provide the engine type, the ignition unit type and the part number from the logbook or maintenance list. If the aircraft uses the 1KNI-11B-T ignition coil, this should be stated in the inquiry. The product is also known under alternative designations SPN-4-3T, SPN-4-3-T and СПН-4-3Т.

Product Type: Electroerosion surface-discharge ignition plug
Function: Ignition of the fuel-air mixture during gas turbine engine start
Application / Where Used: Aircraft gas turbine engine and turbine generator starting ignition systems
System / Installation Area: Engine ignition system / combustion or start chamber area
Part of System / Included in Set: Works with starting ignition equipment, including 1KNI-11B-T ignition coil
Compatible Aircraft / Equipment: AI-20 aircraft engines, TG-16M turbine generator units
Thread of Installed Section: M18×1, class 2a
Thread Length: 11 mm
Thread for Shielding Elbow: M18×1, class 2a
Spark Gap / Annular Belt on Ceramic: 0.8–1.0 mm
Precious Metal Content: Silver, 1.3339 g
Weight: Not more than 0.085 kg
Technical Documentation: Engine maintenance documentation, aircraft parts catalog, manufacturer technical documentation

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