723900-4AT

723900-4AT filter in the aircraft pneumatic system

The 723900-4AT is an aircraft air filter used in onboard pneumatic systems where compressed air must be cleaned before entering the working circuit. Its task is to retain mechanical impurities that may come with air supplied from a ground charging cylinder, compressor, or service equipment. In a helicopter pneumatic system, even small contamination can affect valves, reducers, control units, and downstream lines. The filter therefore works as a protective element at the system entry point, before compressed air reaches more sensitive pneumatic components.

723900-4AT on Mi-8 and Mi-17 helicopters

On Mi-8 and Mi-17 family helicopters, the 723900-4AT is associated with the air charging line of the pneumatic system. It is installed in the high-pressure pipeline used when the onboard air bottle is charged during ground servicing. In this position, the filter cleans air before it enters the helicopter pneumatic circuit. This is important because ground charging equipment is not always perfectly clean, and contamination introduced during servicing can later create problems inside the onboard pneumatic system.

Installation area of the 723900-4AT filter

The 723900-4AT is typically installed under the cockpit floor in the charging line, close to the onboard charging connection. Its position is not random: the filter must be placed early enough in the line to catch impurities before they travel further through the system. During maintenance, this area is checked together with the charging valve, high-pressure pipework, fittings, and line condition. If the filter is blocked, contaminated, damaged, or installed incorrectly, the pneumatic system may charge poorly or receive air that is not properly cleaned.

Cleaning function of 723900-4AT

The filter performs a mechanical cleaning function. It does not change the pressure regime by itself and does not control air distribution. Its role is to remove solid particles and protect the next elements in the pneumatic circuit. In service, this makes the filter a simple but important part: if it is ignored, contamination can move deeper into the system and reach components that are harder and more expensive to inspect or replace. For that reason, the condition of the filter should be considered during troubleshooting of charging or pneumatic system reliability issues.

723900-4AT and 723900-6AT distinction

The 723900-4AT should not be confused with the 723900-6AT. Both items are aircraft air filters used in pneumatic systems, but their installation positions and system roles are different. The 723900-4AT is installed at the system entry side, in the ground charging line under the cockpit floor. The 723900-6AT is used further in the pneumatic system, typically on a pneumatic panel or in an operating line connected with brake reducers and control units, depending on helicopter modification. This distinction matters when ordering, replacing, or troubleshooting the system.

Service and replacement of the 723900-4AT

When replacing the 723900-4AT, the full passport designation should be checked against the helicopter parts catalog and pneumatic system documentation. A visually similar filter may not match the required installation point or system configuration. Before installation, the filter should be inspected for housing condition, thread and fitting condition, cleanliness, storage damage, corrosion, and availability of documentation. In practical supply, it is useful to specify whether the request is for a new item, an item from storage, or a checked serviceable filter with passport or manufacturer documentation.

Product Type: Aircraft air filter
Designation: 723900-4AT
Passport Designation: 723900-4АТ
Function: Filtration of compressed air and removal of mechanical impurities
Application / Where Used: Aircraft onboard pneumatic and air systems
System / Installation Area: Pneumatic system charging line / high-pressure air line
Typical Installation Position: Under the cockpit floor, after the onboard charging connection / charging valve
Primary System Role: Cleaning of compressed air during charging of the onboard air bottle
Compatible Aircraft / Equipment: Mi-8 and Mi-17 family helicopters depending on pneumatic system configuration
Working Medium: Air
Operating / Ambient Medium: Air
Temperature Range: −60 to +100 °C
Filter Resistance: 400 mm Hg at flow rate 40 ml/min
Registration Number: 7-04156
Related Filter: 723900-6AT, used in another section of the pneumatic system depending on aircraft configuration
Condition: As applicable
Technical Documentation: Product passport / manufacturer documentation

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