Best Practices for Hose Replacement and Procurement
Every aircraft operates constantly reliant on the concert of thousands of parts working together. Not just individual parts, but dozens of systems and subsystems that are constantly operating under the most physically stressful and demanding conditions in the world. One the one most consistently unsung heroes of healthy aircraft operations, but when they fail, the consequences are sometimes catastrophic.
Let’s talk about the importance of aircraft hoses, and hose assemblies to the lifespan and safety of flight for modern aircraft.
Understanding Aircraft Hoses
The hose is a simple mechanism: it allows the pressurized or unpressurized movement of fluids through the systems, but in the capacity of supplying pressure, fuel, or lubricating oils to a system, and then returning the fluid back to a reservoir.
But these are no ordinary houses. In flight, a hose has to hold up to the harshest conditions on the planet. There is no room for failure. To ensure the highest standards of reliability and durability, aircraft hoses are made of specialty materials and are often braided in stainless steel or nylon, depending on the location and type of hose.
Aircraft hoses are a critical component of an aircraft’s engine compartment, and their condition can impact flight safety. If a critical hose (oil supply or return) rupture in flight, it can or will render the engine out of commission.
Aircraft hoses and fittings have followed the same standardization for many years now, which is the AN standard, which stands for Army-Navy Standard. The AN standard was designed to standardize fittings and hoses between aircraft of other services. This was to make the supply chain more reliable and simple, and also to reduce costs and increase supplies of common items.
There is absolutely no reason to have more than one type of fitting, so this made sense, much in the same vein as creating a MIL-SPEC standard for crating and shipping parts, components, and military materials in general.
Original hoses can remain in service for decades, potentially going unnoticed during annual inspections. This is something that all service techs need to be aware of because there are so many different hoses used for many different things.
Careful maintenance can help prevent unexpected issues with fuel and oil lines, which are the show stoppers (along with hydraulic lines).
Hydraulic Hose Assemblies
Speaking of hydraulic hoses, let’s take a look at hydraulic hose assemblies, hose fittings, and what can lead to hydraulic hose failure.
Hydraulic hose assemblies require proper routing to minimize stress on the hose and fittings. This is probably one of the most common failures maintenance techs see on a regular basis, regardless of hose types. When a hose is routed, it will have chafe guards in place to keep sensitive hoses from chafing or fraying. Hydraulic hose assemblies are particularly sensitive because they operate at such high pressures, and high pressures equal high temperatures.
Proper hydraulic hose routing is crucial for minimizing stress on the hose and fittings. But first, make absolutely sure that it is either the correct hose for the job, or if one has been manufactured it is the correct pressure rating and correct length for the job at hand.
High-pressure hoses like hydraulics require proper routing to prevent hose failures, which can cause severe injury to operators. Not only will it dump the entire hydraulic system in a matter of moments (the reservoir unloads very quickly when it is under several thousand PSI).
OEMs should utilize proper hydraulic hose routing in their fluid power systems to ensure optimal performance and safety. You have to know that the hoses are either a direct OEM replacement hose procured specifically for the MDS, or that the replacement hose is manufactured to the exact specifications as the OEM. Routing is extremely important for hydraulic hoses, and a rupture can mean not only a depleted hydraulic system but also a serious fire hazard.
This incident from Dyess AFB is a great example of how basic, common hydraulic system maintenance can become deadly, and why hydraulic hose maintenance is so important. Thankfully everyone was alright, but it resulted in the total loss of the aircraft valued at nearly half a billion dollars.
Hose Materials and Compatibility
Alright, so let’s take a look at materials. When choosing a hose material, consider compatibility with common aviation chemicals, pressure rating, temperature rating, service life, and cost. Of course, if you are looking to procure an OEM replacement, you can work with us and rest assured we will replenish your supply of OEM hoses, lines, and fittings from trusted sources.
Some of the most common materials used for aircraft hoses are rubber, Chlorinated Polyethylene (CPE) synthetic rubber or Polytetrafluoroethylene (PTFE) known as Teflon. This is completely dependent upon what the specs for the specific hose in question; different chemicals have properties that interact very differently with hose materials.
Research suitable hoses and fittings per the technical data for that aircraft and that component. Even items that appear identical may use different materials and techniques, or are even rated for different pressures for the same system. Using a hose rated for 2,000 PSI when the system runs at 3,000 PSI creates a ticking time bomb and it will go off.
Hose Fittings and Connections
Just as important as the hose itself are the fittings used. The fittings used are standardized fittings which we touched on a little earlier. You cannot accept any substitutes here. AN fittings are the standard and they are standard for a reason, so make sure you have genuine AN fittings and not inferior knock-offs. While these may appear identical and feel the same, too, they are most likely not made of the same grade of aluminum as AN fittings. Here a few things that make these fitting special and specific to aviation and aerospace applications (although they are also commonly used for high-performance automotive applications):
- AN fittings are often used in aviation and high-performance vehicles.
- AN uses a 37-degree flared fitting to form a metal-to-metal seal.
- Crimp type fittings require a hydraulic press with specific dies to properly crimp the collar onto the hose ends.
- Reusable hose ends use a two-part system to hold the hose in place and come in different sizes, angles, and colors.
Hose Routing and Installation
This part of the maintenance process is so important that we’ll cover it again: hose routing is imperative to the overall lifecycle and reliability of every hose. Route hoses carefully to avoid heat and excessive use of angle fittings. There are specific ways that hoses are to be routed as designated by the manufacturer, so make sure to follow the technical data precisely for installation details.
Avoid twisting hoses during installation and use two wrenches if necessary. This is a fairly standard practice, so make sure to use them. Also, make sure to use any special tools like flare nut wrenches to prevent marring or damage on the fittings.
From AC 43.13-1B:
“All flexible hose installations should be supported at least every 24 inches. Closer supports are preferred. They should be carefully routed and securely clamped to avoid abrasion, kinking, or excessive flexing. Excessive flexing may cause weakening of the hose or loosening at the fittings.”
Refer to AC 43.13-1B for more information about general guidance on hose installation practices.
Maintenance and Inspection
There are a few main considerations for maintenance and inspection of aircraft hoses. First, regularly inspect hoses for signs of wear, damage, or deterioration. Hoses should not be stretched tight between two fittings, per AC 43.13-1B. According to the circular, this will lead to premature breakdown and eventual failure
Check for abrasion, heat damage, or fuel/oil soaking. Like most rubber items, when a hose is showing signs of being soaked through with oil or fuel (or anything else; the hose should be dry), it is due for replacement.
Consider using Teflon hoses with a stainless steel covering for a longer service life. These are kind of the industry standard for certain but not all applications. Again, refer to the technical data specific to your aircraft. If you can provide a national stock number for the item, all OEM and suitable substitutes can be procured by Greenwood Aerospace. Even the hard to find ones.
Factors Affecting Hose Life
The FAA provides limited information on hose life, and there is no specific guidance on in-service life of elastomeric hose. This is mostly due to the fact that you cannot accurately predict the lifecycle of a hose.
The FAA recommends replacing a hose if any damage is found, as stated in FAA AC43.13 -1B, and each branch of the military’s technical data will say the same thing. Most aircraft include hose inspection guidelines and timetables during their inspection cycles. Military aircraft always have hose inspection as part of phase or isochronal inspection, and usually have them as part of the hourly postflight inspection process, too.
The most common factors affecting hose life include pressure, temperature, and exposure to chemicals. Hoses that are in high-stress or high-wear areas will sometimes have a time-compliance technical order for inspection, especially if there has been a history of premature wear or sudden failure.
Hose Assemblies: Selection and Replacement
You should always use OEM hoses procured from reputable sources if available. If not, make sure that your internal shop is experienced with making them to OEM spec. Follow manufacturer’s instructions and use new material and fittings for homemade hoses, take accurate measurements to get the correct hose length.
Troubleshooting Common Issues
The most common issues with hoses include leaks, damage, and deterioration. If your hoses are exhibiting these symptoms, you should make sure to replace them with OEM-approved hoses immediately.
Fittings should also be inspected for damage often, particularly when the hose is replaced. Fittings are fairly inexpensive in the grand scheme of things, so replacing them should be a priority when you have the opportunity (i.e., doing work that is already being done in the area). Hoses are one of those things that techs should always look at if they are in the area and replaced if there is any question about them at all.
If there are any obvious chafe points or kinking hazards in the routing, you should consult the engineering team for the airframe in question and see about alternate routing solutions or other options to decrease premature wear on the hoses.
Greenwood Aerospace Procurement Solutions
The best course of action is to replace the hose with an OEM hose, and we can help with that! Not only can we procure them, but we are also your kitting solution specialists to provide complete kits of common hoses that need replacement. We can also kit contingency packages for just about any maintenance action, and we can store them in our on-site warehouse if need be.
You can rest assured that we are MIL-SPEC shipping and packaging specialists, so your parts and materials will be sent exactly as specified. Give us a call to get your procurement request started at (580) 762-2580, or you can start an online quote right now!
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