AS5174D0303 - Textron Aviation OEM Flared Tube to O-Ring Boss Union Fitting

AS5174D0303 – Textron Aviation Flared Tube to O-Ring Boss Union Fitting (AN816-3D)

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The AS5174D0303 is a high-quality union fitting manufactured to SAE Aerospace Standard AS5174 and specified for use by Textron Aviation and other leading OEMs. This is a crucial adapter fitting, designed to connect a standard 37° flared tube line to a component with a straight-thread O-ring boss port (per AS5202/MS33649). It effectively combines two of the most common sealing technologies in aviation into a single, reliable component. This fitting is the modern, aerospace-standard replacement for the legacy AN816-3D fitting.

Manufactured from high-strength 2024 aluminum alloy and finished with a blue anodized coating for identification and corrosion resistance, this fitting is intended for low-to-medium pressure (up to 1500 PSI) fluid systems. It is commonly used to connect rigid hydraulic, fuel, or oil lines to component housings such as pumps, valves, actuators, and filter assemblies.

Key Features

• SAE Aerospace Standard (AS) Certified: Manufactured to the exact material and dimensional tolerances of the AS5174 specification, ensuring reliable performance and interchangeability.

• Connects Flared Tube to O-Ring Boss: A versatile adapter that joins a 37° flared tube end (AN-style) to a straight-thread O-ring boss port, a common requirement when connecting lines to components.

• High-Strength 2024 Aluminum Alloy: Offers an excellent strength-to-weight ratio, ideal for airframe plumbing applications.

• Anodized Blue Finish: Provides a hard, corrosion-resistant surface (per MIL-A-8625) and positively identifies the fitting as an aluminum alloy component.

• Supersedes AN816-3D: This is the current aerospace standard fitting, directly replacing the older Army-Navy specification part.

Technical Specifications

• Part Number: AS5174D0303

• Component Type: Union, Straight, 37° Flared Tube to O-Ring Boss

• Governing Specification: AS5174 (supersedes AN816).

• OEM Applicability: Textron Aviation, and all other manufacturers using AS/AN standard fittings.

• Material: 2024 Aluminum Alloy.

• Finish: Anodized Blue per MIL-A-8625, Type II.

• End 1 (Flared Tube): For 3/16 inch OD tubing (AN size -3).

• End 2 (O-Ring Boss): For -3 size O-Ring Boss (AS5202), with 3/8-24 UNF-3A straight thread.

• Required O-Ring (Sold Separately): Requires a -903 size O-ring (e.g., M83248/1-903 for fuel/oil or MS28775-903 for Skydrol), depending on the fluid type.

• Nominal Working Pressure: 1500 PSI.

Aircraft Compatibility

The AS5174D0303 is a standard hardware component used in the hydraulic, fuel, and oil systems of countless aircraft, including many Textron (Cessna and Beechcraft) models. It is not specific to any one airframe but is a universal adapter fitting.

Crucial Note: The correct fitting part number for a specific application must be verified against the aircraft's Illustrated Parts Catalog (IPC) or maintenance manual.

Applications

• Adapting a rigid tube line to a component housing that has a machined O-ring boss port.

• Commonly used on: hydraulic power packs, pumps, valves, actuators, and filter housings to connect them to the system's rigid plumbing.

• Used in hydraulic systems utilizing MIL-PRF-5606 or MIL-PRF-83282 fluids, as well as fuel, oil, and pneumatic systems.

Why Choose AS5174D0303?

Choosing a genuine, certified AS5174D0303 fitting is a critical decision for ensuring a leak-free fluid system. This fitting incorporates two precision-machined sealing surfaces. A non-certified part risks incorrect tolerances on either the 37° flare seat or the O-ring boss sealing surface, which will result in a guaranteed leak. The AS standard is the only assurance that the fitting is made from the correct, certified 2024 aluminum alloy and that all sealing surfaces and threads are machined to the exacting standards required for flight safety.

The AS5174D0303 is a study in two distinct sealing philosophies combined in a single component. The flared tube end creates a metal-to-metal interference seal. Here, the threads of the B-nut provide only the axial clamping force, which is concentrated on the narrow, circular line of contact between the tube's 37° flare and the fitting's 37° cone. The immense compressive stress at this interface causes localized deformation, creating a perfect seal. The O-ring boss end, by contrast, creates an elastomeric compression seal. The straight threads provide mechanical retention and resist hydraulic pressure loads, but they perform no sealing function. The seal is accomplished entirely by a separate O-ring, which is compressed into a specially machined gland between the shoulder of the fitting and the face of the boss port. The system's fluid pressure then further energizes this O-ring, increasing the sealing force. This fitting expertly bridges these two robust, proven sealing technologies.

Note: This is a flight-critical fluid system component. A new, correct specification O-ring must be used on the boss end for every installation. The sealing surfaces on both ends must be protected from nicks, scratches, or debris. The mating tube flare must be inspected for cracks or imperfections. Both the B-nut on the flare end and the fitting itself into the boss must be tightened to the correct torque values specified in standard maintenance practices (e.g., AC 43.13-1B).

Documentation

The definitive technical requirements for this component are detailed in SAE Aerospace Standard AS5174. Its legacy equivalent is defined in Army-Navy specification AN816.