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Aircraft Maintenance: Healthy hoses

On the list of aircraft components that can ruin your day, flexible hoses rank pretty high.

The data tag includes the information about the hose, including date of manufacture, length, and hose-end type.  Photos courtesy of Jeff Simon.

Throughout the engine and airframe, these hoses carry hydraulic fluid, oil, fuel, and even air. They perform at pressures ranging from zero to over 2,000 psi (or more) at fluid temperatures that can vary from subzero at startup on the coldest winter day to over 200 degrees Fahrenheit on a hot summer climbout. And, they do all of this in an environment where they are subject to vibration and, in the case of brake hoses, constant flexing and motion.

Given all of these demands, it would make sense that aircraft owners and mechanics would pay very close attention to flexible hoses—inspecting them carefully, replacing them as they age, and treating them gently during maintenance…except they don’t. The reality is that the average general aviation aircraft is full of hoses that are well past their useful life, aged to the point of cracking, and manhandled during maintenance.

Having seen many cases of failed hoses that grounded aircraft (or caused unplanned landings), I decided to embark on a complete brake and engine hose update on our Bonanza. Modern hoses have many options to choose from, so I took some time at EAA AirVenture Oshkosh in Wisconsin to meet with Randy Herber, president of Herber Aircraft Service. Herber Aircraft does much more than just aircraft hoses, but hoses are a core competency, and the company is the industry’s go-to for just about any hose question you could have.

Understanding hose types

Photos courtesy of Herber Aircraft Service.

There are two basic types of modern flexible hoses for GA aircraft: rubber and Teflon (for the sake of simplicity, we will use Eaton/Aeroquip model numbers, but other manufacturers are similar). Rubber hoses such as Eaton Aeroquip 303 have been around for ages and are the least expensive option. They are usually black with a rubber core, a carbon steel reinforcing braid, and a black nylon outer layer for chafe resistance. They are the least expensive hose option, and they are life limited because they degrade both internally and externally as a result of age and heat exposure. Rubber hoses should be replaced every five years and inspected carefully for signs of failure as they age.

Teflon hoses, on the other hand, are considered “lifetime hoses” because they are not expected to fail internally simply as a result of time in service (TIS). Teflon hoses such as 666 (medium pressure) or AE246 (high pressure) are composed of a blue Teflon inner hose, coated with a black lining to provide electrical conductivity between the end fittings. They are wrapped in stainless steel (versus carbon steel), and use stainless fittings as well. The durability of the Teflon, coupled with the corrosion resistance of the stainless wrap and stainless fittings, provides the “lifetime” of service that these hoses are known for. That said, the concept of “lifetime” is a bit of a misnomer because even Teflon hoses must be routinely inspected for damage caused by mishandling, wear, or other issues.

Fire protection

When using hoses in the engine compartment, it’s imperative to protect against fire in the case of a hose failure. This is where the “firesleeve” comes in. The FAA requires an additional layer of protection around hoses designed to contain flammable fluids in the event of a hose failure. The most common form of firesleeve is the orange sleeving that is commonly applied over hoses, using crimped steel bands at each end. Teflon hoses, on the other hand, offer an upgrade to molded-on integral silicone firesleeves that are attractive and easy to clean. They are available in brown or blue and really spruce up an engine.

Hose tips for every owner

Randy Herber knows hoses like no one else, so I asked him for a list of things that every owner should know:

  1. Listen to your hoses. Healthy hoses should be flexible and quiet. If you move a rubber hose and hear a cracking sound, it’s failing inside and should be replaced. You’re hearing the rubber breaking, and rubber pieces could be flaking off and contaminating your engine.
  2. Treat hoses with respect. Never bend a hose beyond its minimum bend radius, twist it, or intentionally clamp it off. Too often, mechanics will take off only one end of a hose in order to get access to something in the engine, then bend the hose back dramatically or kink it, just to keep it out of the way. This is bad for all hoses, but in the case of Teflon hoses a kink can damage the inner core, leading to a failure. Once a Teflon hose has been kinked, it must be replaced.
  3. Avoid hose kits. It can be tempting to try to order a “complete hose kit” for your make and model of aircraft. However, this often leads to problems because most aircraft are unique as a result of many modifications and equipment additions over the years. It’s easier and far more accurate to simply write down the hose information from the data tag on each hose and order what you need. The tag will contain the part number with the length, material, fittings, and other relevant information. If you need to measure a hose yourself, measure the length from the center of the fitting B-nuts on either end of the hose. Hoses are measured in inches, with the last digit representing the number of eighths of an inch (0151 = 15.125”).

Too many aircraft in the GA fleet are flying with decades-old brake, fuel, and oil hoses. It’s a recipe for disaster that’s easily remedied. If you’re curious about the age of your hoses, just check the tags. Every hose has a data tag that includes the part number, parts manufacturer approval information, and the date of manufacture. Just a few minutes of inspection will give you either piece of mind or a new project to tackle. If you have questions, contact a hose expert such as Herber Aircraft to get answers and make decisions that are right for you. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.

Jeff Simon
Jeff Simon
Jeff Simon is an A&P mechanic, IA, pilot, and aircraft owner. He has spent the last 22 years promoting owner-assisted aircraft maintenance and created the first inspection tool for geared alternator couplings available at Jeff is also the creator of SocialFlight, the free mobile app and website that maps more than 20,000 aviation events, hundred-dollar hamburger destinations, and also offers educational aviation videos. Free apps are available for iOS and Android devices, and users can also visit
Topics: Aircraft Maintenance
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