723900-6AT

723900-6AT filter in the helicopter pneumatic system

The 723900-6AT is an aircraft straight-through air filter used in the high-pressure pneumatic system of Mi-8 and Mi-17 family helicopters. Its purpose is to clean compressed air before it reaches the control units of the wheel brake system and other pneumatic components located downstream. The filter retains mechanical impurities, dust particles and small debris that may appear in the pneumatic line during operation or servicing. In this position, the filter is not a control unit by itself, but it protects the units that actually regulate and distribute compressed air.

Protection of brake control units with 723900-6AT

In the Mi-8 / Mi-17 pneumatic system, the 723900-6AT is installed before the brake control units. It protects elements such as the PU-7 or UP25/2 pressure-reducing valve and the UPO-3/2 or UPO-3/2M reducing accelerator, depending on the helicopter configuration. These units are sensitive to contamination because small particles can affect valve seating, movement and sealing. A clean air supply reduces the risk of sticking, unstable pressure reduction, leakage or uneven brake response. For this reason, the filter is important for the reliability of the braking pneumatic circuit.

Installation area of the 723900-6AT filter

The 723900-6AT is installed in the cockpit area under the floor, in the high-pressure line leading to the wheel brake control equipment. This location places the filter close to the pneumatic control block and allows it to clean compressed air just before it enters the brake-control components. During maintenance, this area is usually checked together with the pressure-reducing valve, reducing accelerator, fittings, pipework and mounting points. If the filter is dirty, damaged or incorrectly installed, the downstream units may receive contaminated air even when the main pneumatic pressure appears normal.

Straight-through design of the 723900-6AT

The 723900-6AT is described as a straight-through high-pressure air filter. Its construction allows compressed air to pass through the filter body while the filtering element catches mechanical impurities. The filter does not replace the function of a pressure reducer, valve or accelerator. It only prepares the working medium for the next units in the system. This is why its condition is often checked when there are complaints about brake pneumatic operation, unstable pressure behavior or repeated contamination of downstream components.

Difference between 723900-6AT and 723900-4AT

The 723900-6AT is often confused with the 723900-4AT because both are aircraft air filters used in the pneumatic system. The difference is in their position and role. The 723900-6AT is installed under the cockpit floor in the line before brake-control units such as PU-7 and UPO-3/2. The 723900-4AT is used in another section of the system, in the ground charging line, typically on the pneumatic panel in the cargo cabin area on the right side between frames No. 5 and No. 6, depending on helicopter modification. For ordering, the full designation must be checked carefully.

Service and supply of 723900-6AT

When replacing the 723900-6AT, the part should be matched with the helicopter parts catalog and pneumatic system documentation. Similar filters may have close appearance but different installation positions or system use. Before installation, the housing, fittings, threads, cleanliness, corrosion traces and storage condition should be inspected. For aviation supply, it is better to specify whether the required item is new, from storage, inspected or supplied with passport documentation. The filter should not be selected only by dimensions or visual similarity, especially when the request concerns the brake-control pneumatic line.

Product Type: Aircraft straight-through air filter
Designation: 723900-6AT
Function: Filtration of compressed air and removal of mechanical impurities
Application / Where Used: Aircraft pneumatic systems
System / Installation Area: High-pressure pneumatic line before wheel brake control units
Typical Installation Position: Cockpit area under the floor, near the pneumatic brake control block
Primary System Role: Final cleaning of compressed air before pressure-reducing and brake-control units
Compatible Aircraft / Equipment: Mi-8, Mi-8MTV-1, Mi-17 and related helicopter versions depending on pneumatic system configuration
Protected Components: PU-7 or UP25/2 pressure-reducing valve; UPO-3/2 or UPO-3/2M reducing accelerator depending on configuration
Working Medium: Air
Operating / Ambient Medium: Air
Temperature Range: −60 to +100 °C
Filter Resistance: Not more than 400 mm Hg at nominal air flow rate of 40 ml/min
Related Filter: 723900-4AT, installed in another section of the pneumatic system depending on aircraft configuration
Condition: As applicable
Technical Documentation: Product passport / manufacturer documentation

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