January 1, 2006
Steven W. Ells
It was a simpler world when the Bureau of Air Commerce, a division of the U.S. Department of Commerce, issued Aeronautics Bulletin No. 7-A. This report, titled "Airworthiness Requirements for Aircraft," contained numerous pearls of wisdom about hoses: "Flexible hose connections shall have metal liners. Flexible fuel lines shall be suitably supported." Another telling sentence in this 70-year-old document reads, "All piping shall be made of materials, including their normal or inherent impurities, which will not react chemically with any fuel, oils, or liquids that are likely to be placed in the tanks." There's also a statement requiring that hoses and pipes installed forward of the firewall "be critically inspected for possible fire hazards." Those are the guidelines from 70 years ago and they still work well today.
The vast majority of flexible hoses manufactured and sold for installation on certified airplanes are built to the standards outlined in documents called "technical standard orders," or TSOs. Those that apply to flexible hoses are TSO C42 for propeller feathering lines, TSO C75 for hydraulic lines, and TSO C53a for fuel and oil lines. It's logical to think that this means that every airplane at every airport around this great country of ours is sporting TSOed hoses from spinner to tail cone — but it isn't necessarily so.
In the past, many original equipment manufacturers (OEMs) used their parts manufacturer approval to approve the installation of hoses built to their own in-house standard, as Cessna did during the 1970s and 1980s for the flexible hoses used in the landing-gear systems of its single-engine retractable-landing-gear airplanes. Cessna used non-TSOed materials because of a requirement for a very small bend radius that could not be obtained using TSOed hoses. Many maintenance shops still buy hose-making materials in bulk and fabricate perfectly airworthy hoses for installation on private-use customer airplanes. Hose assembly instructions, as well as part number, hose size, bend radius, and pressure specification tables, are in chapter 9 of the A&P technician's bible, Advisory Circular 43.13-1B. Both TSOed hoses and field-built hoses are constructed out of the same materials.
There are two basic types of hoses in general use today — rubber and Teflon. All hoses consist of three parts: the inner tube, the reinforcement, and the cover. Sometimes the reinforcement does double duty as the reinforcement and the cover.
The first type, the rubber hose, has an inner part — the part that's in contact with the fluid — that is made of either Buna N synthetic rubber or a compound called HSP, which tolerates slightly higher temperatures.
Rubber hoses have performed well over the years but as engine-compartment temperatures increased, hoses with Teflon tubes were developed. Teflon-tube hoses can handle much higher temperatures than rubber hoses and are compatible with nearly every substance and agent. That's important but the more important factor is that — unlike the rubber tube — the Teflon tube is not age sensitive so it has an unlimited shelf life. Does that mean its on-airplane life is unlimited? No, and we'll delve further into the subject of hose life and other pluses and minuses later in this article. Stratoflex and Aeroquip are two well-known hose manufacturers and both make rubber and Teflon-tube hoses. Titeflex is less well known in GA circles and limits its production to Teflon hoses.
Both the rubber and Teflon tubes are surrounded by a reinforcement to prevent tube deformation from pressure and/or suction, and to protect the tube. This reinforcement can be woven stainless steel, woven fabric, woven carbon steel, or combinations of these materials. The cover, or outer layer, can be either the exposed woven stainless-steel reinforcement or a braided fabric layer impregnated with a synthetic oil-resistant coating.
Modern Teflon-tube hoses with molded integral (external) fire sleeves are wrongly termed "lifetime hoses." These hoses are very durable but no hose is immune from the wear and deterioration that take place over time. Teflon hoses cannot always be substituted for rubber hoses because they won't bend in as tight a radius as rubber hoses. Teflon hoses with integral fire sleeves are easy to identify because the integral fire sleeve is molded onto the hose. There aren't any clamps and the sleeve is rust brown and has a smooth surface.
There are two factors to consider when buying replacement hoses. The first is where the hose is going to be installed, and the second is the cost.
The fire zone is defined as the area on the engine side of the fire wall. Although there is no written regulation requiring that private airplane owners — those operating under Part 91 rules — install fire-resistant hose assemblies forward of the fire wall, it's a darn good idea.
Many OEMs installed non-fire-resistant hoses in the fire zone. This is especially true for airplanes that were built in the 1960s, 1970s, and 1980s. Today's new airplanes are equipped with fire-resistant hoses in the fire zone. If original hoses are still installed in older aircraft, they should be replaced simply because of their age. If they're located in the fire zone, replace them with fire-resistant hoses.
What's the difference between fire-resistant and non-fire-resistant hoses? A fire sleeve. There are two types of fire sleeves — the slip-on and the molded-on, or integral. The minute a woven fiberglass slip-on fire sleeve is properly installed over a rubber tube or Teflon tube hose, that assembly becomes fire resistant. Slip-on fire sleeving has a bright-orange exterior while the integral sleeving is colored a rust brown. TSO C53a requires that fire-resistant hose assemblies be able to withstand a 2,000-degree Fahrenheit flame applied from one-quarter inch away for 5 minutes.
Hoses used aft of the fire wall do not have to be fire resistant. Therefore, they don't need fire sleeves. This is a general rule and all hose installations must comply with the airplane manufacturer's guidelines, or regulatory data that supersede the manufacturer's guidelines. In other words, look in the parts manual first, and then check for airworthiness directives (ADs).
Based on the number of airworthiness directives that pop up when the word hoses is typed into the search window on the FAA's airworthiness directive home page, hoses haven't been getting much respect from the maintenance community. But there's another factor at work. Since there are very few mandated hose replacements, owners and maintenance technicians aren't sure when to replace hoses. Replacing hoses on a regular basis should be part of every owner's continued airworthiness program.
All hoses have a finite life. The maintenance of continued airworthiness requires that all flexible hoses be replaced at regular intervals. What is the life of a flexible hose? That's a difficult question. It depends on the installation. The most important of these parameters include temperature (both ambient and operating), pressure, and cyclic loads. There's some guidance provided in AD 95-26-33. This AD applies to Piper PA-28 and PA-32 airplanes and is a good general rule of thumb for fire-zone hose replacement intervals.
This AD requires that the original engine-to-oil-cooler flexible hoses be visually inspected every 100 hours. The AD also mandates that the original-style hoses be replaced every eight years or 1,000 hours, whichever comes first. When the original hoses are replaced with hoses manufactured in accordance with TSO C53a type D, the repetitive 100-hour inspections are no longer required but the eight-year/1,000-hour replacement interval remains in effect. The applicability paragraph states that the AD applies to oil-cooler hoses that are not built to TSO C53a type-D standards.
What's the difference between the original hoses and the type-D standard hoses? A hose built to a type-D standard (of TSO C53a) is a fire-resistant, high-temperature hose assembly designed for use in the fire zone and is suitable for use in areas where air temperatures don't exceed 450 degrees F. Teflon-tube hoses must be used in high-temperature applications. Rubber-tube hoses can be built to type-C standards — they're termed "fire-resistant, normal-temperature assemblies" and are still approved for fire-zone installations but are restricted to areas where the air temperatures don't exceed 250 degrees F.
Obviously this means that the FAA has data that support the conclusion that both type-C standard hoses and type-D standard hoses can be expected to remain airworthy for 1,000 hours or eight years in this particular application. Many owners believe a proper replacement interval for fire-zone hoses is at every engine change, but this assumption is dangerous because this interval could stretch to beyond 20 years for light-usage airplanes.
Although it's legal for maintenance facilities that have the correct tools and the ability to pressure test the hoses for leakage to make their own hoses for installation on an owner's airplane, most shops buy hose kits from one of the hose-manufacturing facilities around the country. There are two reasons for this, described below.
With all the emphasis on parts traceability over the past decade, almost every inspector and repair station owner knows that an approved parts paper trail is not only very comforting, but also makes good business and safety sense. Therefore, most hoses installed today on GA airplanes are built to TSO standards by fabrication shops. Shops that build TSO hoses are trained and audited continuously by both the factory and the FAA to ensure ongoing compliance with established quality and FAA standards. Since these TSOed parts are identical to the original parts in fit, form, and function, the installation of new hoses constitutes a minor alteration. After all, the hoses are built to an FAA standard.
That said, there's no rule that private aircraft owners — who do not use their airplanes for hire — have to install TSOed hose assemblies, but unless the local labor is cheap it's almost always less expensive and faster to buy TSOed hose assemblies. TSOed products must be identified by a permanent marking that includes the name and address of the TSO holder, the equipment name or part number, and the date of manufacture. With TSOed hoses, this is a data tag secured around each hose that guarantees the hose was assembled from modern materials by shops trained in the assembly and inspection of hoses.
The cost of an engine change-hose kit ranges from $250 for a low-horsepower light airplane to $1,400 for a turbocharged, high-performance engine.
Fire-sleeve material is sold in different diameters to fit over a completed hose assembly. There's one catch — unless the sleeve is properly installed, sliding a fire sleeve over a hose assembly does not make it a fire-resistant hose assembly.
Each end of the sleeve must first be sealed by dipping or coating it with high-temperature silicone gasket material before a stainless-steel clamp made by the Band-It company is installed to hold the fire sleeve firmly in place. Securing the end of the fire sleeve by wrapping a couple of turns of safety wire around the end is a no-no. A Band-It clamp installation tool sells for between $130 and $175 — there is an economy tool for around $25 — while individual clamps range from $1.50 to $1.75.
Costs for hose stock vary by size. A common Buna-N rubber-tube hose with a three-eighths-inch inside diameter — the same-size hose that's required to comply with the AD mentioned previously — sells in parts supply houses for $5.10 a foot. The end fittings cost $6.90 each and two are needed for each hose. The appropriate fire sleeving sells for $7.60 a foot and the Band-It clamps for this size of hose cost $1.65. The materials cost for two 48-inch pieces of hose is $132.50 at today's prices.
In summary, hose maintenance is really simple: Keep a sharp eye on hoses in the fire zone and replace hoses at regular intervals. If they're more than 10 years old, don't even think about it — just replace them.
Hoses either work or they don't. Hose failures can cause anything from an unexpected ride through the pucker brush if a brake hose fails to a life-threatening, in-flight engine fire if an oil or fuel hose fails. Hoses are like almost everything on an airplane — an ounce of prevention is worth a pound of cure.
E-mail the author at firstname.lastname@example.org.
Links to additional information about aircraft hoses may be found on AOPA Online.
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