Aircrafts make extensive use of hoses to convey fluids. No matter if it is a plane or helicopter, a propeller or jet engine, civil or military: they all rely on hoses in critical applications. Aerospace hoses have standards and other official documents for specification and inspection, bearing many similarities with their land counterparts. It is beneficial to understand how hoses are used in an aircraft, their main characteristics, and what problems may affect their airworthiness.
By Davi Correia, Senior Mechanical Engineer
Hose Applications
Moving different fluids requires tubing and hoses, in what is commonly known as ‘aircraft plumbing’. As in a land application, hoses are used “to connect moving parts with stationary parts in locations subject to vibration or where a great amount of flexibility is needed. It can also serve as a connector in metal tubing systems.”[2]
Governing Bodies
In the United States, the Federal Aviation Administration (FAA) is the main government body regulating civil aviation. The rules and regulations set by the FAA are based on international standards published by the International Civil Aviation Organization (ICAO) and cover aviation safety, security, efficiency, regularity, and environmental protection. Documents issued by the FAA that affect hoses are often included in one of the categories below:
1) Advisory Circulars (AC): They are non-mandatory recommended practices that offer guidance on how to comply with airworthiness regulations within the 14 CFR Aeronautics and Space Title. One example is the AC 20-62, Eligibility, Quality, and Identification of Aeronautical Replacements Parts. This document provides information and guidance for determining the quality, eligibility, and traceability of aeronautical parts and materials.
2) Airworthiness Directive (AD): These mandatory documents are issued in relation to a specific group of aircraft models. It notifies owners and operators about some safety deficiency that must be corrected. For example, AD 95–26–13; Piper Aircraft, Inc. Airplanes. This AD covers an issue related to oil cooler hose assemblies that do not meet certain technical standard order (TSO) requirements. It requires inspections, replacement, and adjustment of said hose assemblies.
3) Technical Standard Order (TSO): TSOs specify minimum performance requirements for materials, parts, processes, and appliances used on civil aircraft. There used to be three TSOs related to hoses: C42, Propeller Feathering Hose Assemblies; C53c, Fuel and Engine Oil System Hose Assemblies; and C75, Hydraulic Hose Assemblies. They were combined in 2016 in a single TSO called TSOC140, Aerospace Fuel, Engine Oil, and Hydraulic Hose Assemblies. Some catalogues still make reference to the old TSO, as the FAA stated that articles approved under the cancelled TSOs could remain being manufactured under the provisions of their original approvals.
Hose Construction and Manufacturing Standards
A hose consists of an inner tube, a reinforcement, and a cover. The inner tube is designed to offer low resistance to flow and remain flexible, fluid-compatible and stable over the range of pressure and temperature of the application.
The typical materials of construction include Buna-N, neoprene, and PTFE. Buna-N is a synthetic rubber compound normally used with petroleum-based oils and solvents. Neoprene is also a synthetic rubber compound. It has better abrasion resistance than Buna-N but lower chemical compatibility with petroleum-based products. Finally, PTFE stands for tetrafluoroethylene and it has many advantages over the synthetic rubber. It has a broad operating temperature range (-65°F to +450°F or -19°C to 233°C) and it is compatible with nearly every substance or agent used. It offers little resistance to flow, and the fluid being conveyed usually does not adhere to the walls. It has less volumetric expansion than rubber, and the shelf and service life is practically limitless.[2]
The reinforcement layer is responsible for the pressure-retaining capacity of the hose. It accomplishes this by braiding or wrapping certain materials over the inner tube in various combinations and layers, depending upon the pressure requirements. Examples of reinforcement materials include Fabric (Cotton, Rayon, Dacron, Polyester) and wire (Carbon or Stainless Steel).
Combinations of reinforcement materials and layer design are used to create three pressure categories:[2]
- Low pressure—below 250 psi. Fabric braid reinforcement.
- Medium pressure—up to 3,000 psi. One wire braid reinforcement. Smaller sizes carry up to 3,000 psi. Larger sizes carry pressure up to 1,500 psi.
- High pressure—all sizes up to 3,000 psi operating pressures.
Figure 3. Aerospace hoses assembles with integral fire protection.[8]
The hose cover offers protection against environmental elements such as corrosion, abrasion, or UV damage. It does not add to the pressure retaining capacity of the hose. A particular type of cover worth mentioning is the fire-resistant cover, used in hoses carrying fuel, for example. Figure 3 presents an example of hoses with fire-resistant covers.
Hoses for aircraft applications also need to comply with manufacturing standards. Table 1 presents some common specifications for low, medium, and high pressure.
The materials used in hoses may be age-sensitive, that is, they may degrade even when stored in adequate conditions and without ever being put to use. In this case, they need to be subject to an age control that keeps track of time elapsed since manufacturing. Such control normally has three categories:
Acceptance life – The period of time from cure to the date of acceptance.
Shelf life – The period of time from the date of acceptance or delivery to the date of use.
Service life – The period of time from the date of installation to the date of removal. Installation date of the hose or hose assemblies are normally controlled by a tag.[6]
Standards such as the SAE AS5316 (Storage of Elastomer Seals and Seal Assemblies Which Include an Elastomer Element Prior to Hardware Assembly) and DOD 4140.27M (Shelf – Life Item Management Manual) prescribe procedures on managing shelf-life items. For example, the typical shelf-life for Buna-N and Neoprene is 15 years and PTFE components have no shelf-life limitations. It is common practice to store hoses in dark, cool, dry places with both ends sealed by caps. They should be protected from circulating air, sunlight, fuel, oil, water, dust, and ozone.
Service life for hose assemblies in aerospace applications is highly regulated and depends on many factors, such as type of aircraft, location within the aircraft, pressure (operational and surges), relative movement, temperature (fluid and ambient), installed bend radius, cleaning procedures, and exposition to degrading agents.
In many cases, the end of life is determined by inspection; the recommendation for hose assembly maintenance and/or replacement is conducted on an ‘on-condition’ basis. A common standard to guide in this situation is the SAE ARP1658 (Visual Inspection Guide for Installed Hose Assemblies). Another document that offers guidance is the Airworthiness Bulletin AWB 02-006 Issue 2 (Flexible Hose Assemblies – Maintenance Practices). Table 2 presents some examples of visual guidance found in this bulletin; almost all the cases provided would require immediate replacement.
Understanding the various forms of hose failure, and how to prevent its occurrence, is essential in aerospace applications. While it is important to mitigate the risk of hose damage in any application, there are high safety risks with a compromised hose during flight. It is therefore important to ensure that hoses are diligently selected to meet the specified requirements and that standards and maintenance practices are followed.
References
- Adapted from https://www.aircraftsystemstech.com/p/largeaircraft-hydraulic-systems.html)
- Federal Aviation Administration (FAA), Aviation Maintenance Technician Handbook – General: FAA-H-8083-30A, Aviation Supplies and Academics, Inc.; 2018th edition.
- EATON, How to Identify, Select and Assemble Aeroquip® Brand Aircraft Hose and Fittings, Available at https://www.eaton.com/content/dam/eaton/products/fc/flexible-hose-assemblies/eatonidentify-assemble-and-select-hoses-fittings-tf100-45b-en-us.pdf
- EATON, Rubber Hose, Fittings & Assemblies for Aerospace Applications, available for download at https://cdn.shopify.com/s/files/1/0317/1300/1612/files/2020_Eaton_Aeroquip_302a_303_306_Aircraft_Rubber_Hose_Fittings_Assemblies_for_Aerospace_Applications_Catalog_Assembly_Instructions_Certification_Specialty_Hose_FINAL_2020-Copy.pdf?v=1588535672
- Parker, Stratoflex Catalog, 171 PTFE High Pressure, Lightweight Hose Products for the Aerospace Industry, available at https://www.parker.com/literature/PTFE_Hose171.pdf
- https://navyaviation.tpub.com/14315/Figure-10-24-N-Factor-For-Flexing-Bends-358.html
- Civil Aviation Safety Authority, Airworthiness Bulletin AWB 02-006 Issue 2 (Flexible Hose Assemblies – Maintenance Practices), 8 May 2015, Australian Government.
- https://www.kitplanes.com/aircraft-plumbing-flexible-lines/
About the Author
Davi Correia is a Senior Mechanical Engineer who has worked at a major Brazil-based oil company for the last 15 years. Correia is part of multi-disciplinary team that provides technical support for topside piping and equipment of production platforms. During this period, he began to work with materials and corrosion, and later moved to piping and accessories technology, where he has become one of the lead technical advisors on valve issues. Correia was part of the task force that revised the IOGP S-562 standard, and wrote the S-611 standard. Correia has a master’s and a doctor’s degree in welding by the Universidade Federal de Uberlandia.