EN 4556:2003

Overview

EN 4556:2003 is a European standard that specifies the characteristics of cap assemblies for aerospace pipe couplings. These pipe couplings feature a 37° angle, are manufactured from heat-resisting steel, and come in inch series dimensions. The standard is developed by CEN (European Committee for Standardization) and focuses on critical requirements ensuring compatibility and performance in aerospace fluid systems. EN 4556 cap assemblies are designed to mate seamlessly with pipe couplings compliant with related standards such as EN 4552, EN 4553, and EN 4554.

The standard applies to fittings operating at nominal pressure Class D according to ISO 6771, making it highly relevant for robust aerospace applications requiring reliability under high temperature and pressure conditions.

Key Topics

• Scope and Applicability
  EN 4556 covers inch series cap assemblies for aerospace pipe couplings with a 37° cone angle constructed from heat-resisting steel. It ensures interchangeability with other AECMA aerospace coupling standards.

• Materials and Surface Treatment
  Caps must be made from steel grades conforming to EN 3468 or EN 3487 with minimum hardness HB > 140.
  Surface treatments for nuts include options such as molybdenum disulphide coatings and electrolytic silver plating to enhance wear and corrosion resistance.

• Configuration and Dimensions
  Detailed dimensions, tolerances, and mass specifications are provided with precise figures for nominal diameters ranging typically from 4.763 mm up to 25.4 mm (inch-based sizes).
  The standard defines locking wire hole options and nut threading according to unified thread standards (UNJF and UNJ).

• Cap Assembly and Marking
  Specific guidance on assembly staking, ensuring the nut spins freely and the cap is securely fitted without damage, ensures safety and functional integrity.
  Marking prescribed under EN 2424 guarantees traceability and standard identification, crucial for aerospace maintenance and inspection.

• Designation System
  A systematic coding scheme is used for identifying cap assemblies by standard number, dimensional code, nut surface treatment, and locking wire hole presence. This uniform system simplifies part identification and inventory management.

Applications

EN 4556:2003 cap assemblies are critical components in aerospace fluid systems, particularly where high-temperature resistance and pressure sealing integrity are essential. Applications include:

• Aircraft hydraulic, fuel, and pneumatic piping systems requiring secure and durable couplings.
• Fluid transfer lines in aerospace engine systems exposed to extreme thermal and mechanical stress.
• Maintenance and repair operations in aerospace industry ensuring standardization and interchangeability of pipe fittings.
• Integration into high-performance aerospace equipment where component reliability under operational stress is mandatory.

By adhering to EN 4556, manufacturers and maintenance organizations guarantee safety, compatibility, and compliance with international aerospace engineering standards.

Related Standards

To ensure full system compatibility and performance, EN 4556 references and is mateable with the following related standards:

• EN 4552: Aerospace pipe couplings 37°, spherical, straight nipples in heat-resisting steel (inch series).
• EN 4553: Aerospace pipe couplings 37°, spherical, 90° elbow nipples in heat-resisting steel (inch series).
• EN 4554: Aerospace pipe couplings 37°, spherical, straight unions (threaded) in heat-resisting steel (inch series).
• ISO 6771: Aerospace - Fluid systems and components - Pressure and temperature classifications.
• EN 2424: Aerospace products marking requirements.
• EN 2491: Molybdenum disulphide dry lubricants - coating methods.
• EN 2786: Electrolytic silver plating of fasteners.
• EN 4560: Technical specifications for aerospace pipe couplings operating up to 21,000 kPa.

Together, these standards form an integrated framework for aerospace fluid system components, facilitating consistent product performance, safety, and reliability.

Keywords: EN 4556, aerospace pipe couplings, heat resisting steel, cap assemblies, inch series, 37 degrees coupling, aerospace fluid systems, pipe coupling standards, ISO 6771 nominal pressure Class D, aerospace maintenance, CEN standards, aerospace fasteners, surface treatment aerospace, molybdenum disulphide coating, silver plating aerospace, aerospace component marking.