Aviation Flexible Hose Assemblies , despite their critical function in the aircraft systems, have not received adequate focus as a component requiring maintenance. Many aircraft maintenance manuals contain minimal hose related data and information from the hose manufacturers is generally fragmented. Widespread use of hoses manufactured in the course of maintenance can make identifying the necessary maintenance tasks difficult.
The service life of a hose assembly depends on the application and the environment in which it is used. Most hose assembly failures result from kinking, chafing, impact, flexing and temperature cycling. They also age and become brittle due to molecular changes with time and reactions with both the internal and external fluids with which they come into contact. One of the catastrophic modes of failure is when the hose comes out of its end fittings. This is primarily because of the permanent set that the hose material undergoes which reduces the retention forces on the end fittings and is the underlying reason for specifying a calendar time life for the hoses. Federal Aviation Administration (FAA) AC No.20-7N, General Aviation Inspection Aids Summary, recommends, “all flexible flammable fluid carrying hoses in the engine compartment be replaced at engine overhaul or every five years which ever occurs first”. Even when a calendar time life is specified for the hoses, it is necessary to have regular inspections for damage due to external forces to ensure continued airworthiness. Specification of a service life for hoses is not a substitute for regular inspection and maintenance of hoses.
Hose Certification Standards
It is important to note that original hose assemblies are made to different standards depending on the application and the environment in which they operate. A broad understanding of the requirements of these standards is helpful in developing hose maintenance practices including manufacture of replacement hoses.
Fuel and engine oil system hose assemblies are certified to FAA Technical Standards Order (TSO) TSO-C53a and classified into Types A to D depending on the operating environment and fire resistance characteristics. The term “fire resistant” means that the hose will perform its intended function while withstanding 2000 (± 150) ºF flame for 5 minutes under standard test conditions while “fire proof” means that it will withstand that temperature for 15 minutes.
- Type A. Non-fire resistant “normal” temperature hose intended for use outside the fire zone where fluid and ambient temperatures do not exceed 250 ºF.
- Type B. Non-fire resistant “high” temperature hose intended for use outside the fire zone where fluid and ambient temperatures do not exceed 450 ºF.
- Type C. Fire resistant “normal” temperature hose intended to be used in the fire zone where fluid and ambient temperatures do not exceed 250 ºF.
- Type D. Fire resistant “high” temperature hose intended to be used in the fire zone where fluid and ambient temperatures do not exceed 450 ºF.
Hydraulic hose assemblies are certified to FAA TSO-C75 or another equivalent standard and include testing to meet the application requirements such as cold temperature flexing, vacuum and bending, thermal shock and hydraulic impulse.
Propeller feathering hose assemblies, because of their safety critical function and location inside the fire zone, are certified to FAA TSO-C42 or another equivalent standard.
In July 2002 the FAA introduced a new standard for all aircraft hoses, TSO-C140, which incorporates new testing and data requirements. All new approvals by FAA for hose manufacture will have to meet this new standard and will require the hose manufacturer to provide the inspection and evaluation necessary for continued airworthiness of the hose assembly.
Hose Maintenance
Correct maintenance procedures including inspection, removal, installation, handling and storage are critical to ensure reliability and durability of hose assemblies and to ensure continued airworthiness. It is important that all Occupational Health and Safety (OH&S) precautions are taken during maintenance and testing of hoses. Where manufacturers maintenance data is not available, SAE Aerospace Information Report 1569 (AIR 1569), “Handling and Installation Practice for Aerospace Hose Assemblies”; and SAE AIR Report ARP 1658, “Hose Assemblies, Installed, Visual Inspection Guide for” are recommended, and when used it should be referenced in the aircraft logbook or in the maintenance documents as appropriate. Some of the critical elements from the SAE reports are reproduced below.
Visual Inspection
- Inspection of hose assemblies may be carried out in-situ at the intervals specified in the aircraft maintenance schedule. Considering the potential consequences of a major hose failure a maintenance schedule that does not include hose inspection should be viewed as deficient and should be amended. During each inspection the date of manufacture and date of pressure test should be checked. The inspected hoses are to be identified by part number and location and the details of the inspection findings and corrective actions recorded, where feasible.
- There are no simple rules to determine the serviceability or otherwise of a hose in-situ. The safety limitations, hose design and the installation environment that could cause a defect to develop determines the inspection requirements. Following is a list of defects with guidelines for minor and major defects categorization. In the context of hoses, a major defect is considered to be that which could cause a control system failure, or could cause fire in an aircraft. Defects that are not major are minor defects. Minor defects are to be recorded for future observation, and two or more minor defects indicate general deterioration and should be treated as major defects requiring hose replacement.
General condition
Deterioration of a hose is characterized by discolorations, flaking, hardening and crazing. Installation should be checked to ensure that any supports or clips are correctly fitted with no corrosion or chafing underneath them, and the hose is not twisted or stressed.
a. Kinks and Twists This defect is usually caused by incorrect installation or handling, is permanent damage restricting flow and the hose should be replaced.
b. Broken Braids Isolated random breakage of the braid wires is a minor defect. If breakage of several wires are concentrated in one area, or two or more wires in a braid is broken, it is a major defect.
c. Chafing and cuts Light scuffing, cuts and abrasion of the outer cover, with the braids not exposed is a minor defect. Minor adjustment of the hose clamps to avoid chafing is recommended, (unless otherwise approved clamping of pipes should be in accordance with aircraft type design)
d. Corrosion Light local corrosion of braids and end fittings, due to oxidation or chemical attack, may be a minor defect to be monitored in subsequent inspections.
e. Brittleness High temperatures and long service may harden hoses and make them brittle and should be replaced. This may also point to incorrect type of hose for the application and should be investigated.
f. Contamination Instances of significant hardening, discoloration or sponginess of the outer rubber cover may indicate chemical contamination and could require replacement of the hose.
g. Leakage Any leakage from fittings that retains the flexible element is a major defect and the hose should be replaced.
h. Damaged fire sleeve Localized cuts and abrasions where the hose is not exposed is a minor defect for further monitoring.
i. Blisters Puncture the blister and if the operating fluid leaks out, it is a major defect.
Hose Manufacture
6.1. Manufacture of hoses in the course of maintenance is permitted. FAA Advisory Circular AC43.13-1B CHG 1 “Acceptable Methods Techniques and Practices – Aircraft inspection and Repair” is an approved document per Instrument Number CASA 06/03 and can be used for the assembly of hoses. UK CAA Leaflet 5-5 “Civil Aircraft Airworthiness Information and Procedures; Hose and Hose Assemblies” is also an acceptable standard when data from the hose manufacturer is not available. Hose certification standards identified in Paragraph 4 of this AWB also should be referred to regarding the design requirements.
Engine Compartment hoses are required to be fire resistant. One way to achieve this is to use fire sleeves. Addition of fire-sleeve that the original manufacturer of the hose did not install is to be in accordance with approved data. It has to be noted that addition of fire-sleeves can mask defects and should not be carried out on old hoses with minor defects.
All manufactured hoses should be identified with a unique part number different from the original equipment manufacturer’s part number, and tested per paragraph 7 requirements of this AWB and operationally checked after installation.
Testing of Hoses
During overhaul or major refurbishment of the engines, all hoses should be tested. A test interval of six years is recommended for airframe hoses.
Before testing hoses should be inspected per paragraph 5.1 of this AWB. In general the serviceability criteria for hose assembly when not installed (ie at piece part level) should be more stringent than when it is installed on the aircraft. Removable fire sleeves should be removed during this inspection.
Hoses should be tested for internal restriction or kinking by passing a steel ball in both directions. Refer FAA AC 43.13-1B CHG 1 for the size of the ball to suit various hose sizes. In general a steel ball the diameter of which is approximately 90 percent of the bore of the end fitting will be adequate for this test.
Hoses should be hydraulic pressure tested at 1.5 times the maximum system pressure without leakage.
Vacuum hoses should be tested to 28 inches of mercury vacuum and the hose should not collapse after it has been tested for leakage per paragraph 7.3.
Hoses with Teflon lining, or hoses that have undergone a permanent set should be restrained in their preset shape using lock wiring during the test to ensure that they do not flex.
After test, the hose should be cleaned, ends blanked and marked with the date of test, and test pressure, along with direction of fluid flow and type of fluid where applicable. Data may be stenciled on the external surface, or impressed on a metal band secured to the hose.
Hoses should be suitably preserved. Restraint per paragraph 7.5 should be retained during transport and storage.
Handling and Storage of Hoses
Hoses with Teflon lining, or hoses that have undergone permanent set should be restrained during handling and storage.
Suitable sealing blanks should be fitted.
Protective coating should be applied to the end fittings to prevent corrosion.
Rubber and synthetic rubber hoses where storage life is specified by the manufacturer and should be adhered to.
Excessive bending should be avoided during handling and storage.
Hoses should be stored in individually sealed plastic wraps below 26 ºC.
Summary
All hoses in service must be inspected at regular intervals.
Replace all flammable fluid carrying hoses in the engine compartment per manufacturer’s recommendations or at intervals of five years when such recommendations are not available. Flammable fluid carrying airframe hoses are to be replaced at ten-year intervals when manufacturer’s recommendations are not available.
Inspection, manufacture and storage of all aircraft hoses must be carried out in accordance with approved data.
Bibliography
- Federal Aviation Administration (FAA), Technical Standards Order (TSO) TSO C42 Propeller Feathering Hose Assemblies.
- FAA TSO-C75 Hydraulic Hose Assemblies
- FAA TSO-C53a, Fuel and Engine Oil System Hose Assemblies.
- FAA TSO -C140, Fuel, Engine Oil and Hydraulic Fluid Hose Assemblies.
- FAA Advisory Circular (AC) 43.13-1B CHG 1. Acceptable Methods, Techniques and Practices – Aircraft Inspection and Repair.
- FAA AC 20-7N General Aviation Inspection Aids Summary.
- SAE Aerospace Recommended Practice (ARP) ARP 1658 REV B, Hose Assemblies, Installed, Visual Inspection Guide.
- SAE AIR 1569 REV A, Handling and Installation Practice for Aerospace Hose Assemblies.
- United Kingdom (UK) Civil Aviation Authority (CAA) Leaflet 5-5 “Civil Aircraft Airworthiness Information and Procedures, Hoses and Hose Assemblies”.
- Aeroquip Bulletin AA91 “7 Quick Ways to spot hose line problems before they cause trouble” 1998
