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Fire Sleeve

So, What is Fire Sleeve?
 
Fire sleeve is a thick woven tube of fiberglass covered with a high temperature iron oxide silicone rubber compound outer coating, manufactured under different brand names. The aviation style is typically thicker, denser inner glass fiber than the industrial grade and gives higher insulation values, increased strength and enhanced abrasion resistance withstanding continuous exposure of temperatures to 500° F (260°C); up to 2000°F (1090°C) for 15-20 minutes; and up to 3000°F (1650°C) for 15-30 seconds.
 
Why Use Fire Sleeve
 
There are a few reasons to use fire sleeve on your aircraft. The most obvious, of course, is for fire protection. Many aircraft builders elect not to use fire sleeve at all. For example, it would be uncommon to see it used on an ultralight type aircraft with two stroke engines operating in the open air. Most don’t even use fuel lines that are capable of withstanding any heat at all. Nor do they need to. The potential for a fire is relatively remote. For cost, simplicity and weight, designers may elect to use simple polyurethane fuel line or automotive type fuel line. A two stoke engine will usually quit after a failure, long before the potential for a fire (in this case the engine quitting may be a good thing).

Four stroke engines, on the other hand, have been known to have relatively catastrophic failures, such as a cylinder head breaking off. Yet, the engine may continue to run long enough to get the aircraft home or at least to a safe landing area, resulting in a greater potential for a fire. The combustion gasses are exposed to fresh fuel coming in through the induction system making a fire almost inevitable. Even though we might have the option to shut off the fuel, it may not be an optimal time or location for a dead stick landing. In IMC conditions, for example, shutting off the engine, or not, can be the difference between life and death.

Additionally, if the oil lines are not protected, there is the possibility that they may burn allowing the oil to contribute to the fire causing even greater damage. Here, we usually do not have the option to shut off the oil system as we do on larger commercial aircraft. This makes protecting the oil lines even more important. In the early days of aviation there were enough problems caused by fires burning through fuel and oil lines that the standards for hose protection were readdressed. Today, the decision to protect hoses to the fuel, oil and other systems is often based on the operational environment of the aircraft and or the regulations that apply to a certain category of aircraft.

Fire sleeve is also commonly used to insulate fuel hose and hard lines. In many applications the temperatures under the cowl have the potential to cause vapor lock in the fuel system, which could cause engine stoppage in flight. We could have a similar problem after shut down when the temperatures rise as a result of reduced airflow through the engine compartment.

In addition to fire and vapor lock, there are other benefits for using fire sleeve. Hose assembly failures can result from kinking, chafing, impact, and flexing but they also suffer failures due to temperature cycling. These temperature cycles accelerate the aging process of the hose on a molecular level. Over time this can cause a break down of the rubber. Small pieces may then flake off of the inner lining leading to contamination of the fuel or oil systems. A hose that is brittle and inflexible is usually a hose that has been exposed to these conditions or left in service for too long. The use of fire sleeve can reduce the exposure of these hoses to extreme engine compartment temperatures.

When Do I need to Use Fire Sleeve?
 
In order to discuss when we need fire sleeve, we should start by looking at a few regulations and the ASTM standards in place by aircraft category:
 
Type Certificated Aircraft
 
There are a few requirements that affect the way aircraft fuel and oil systems are constructed for Type Certificated Aircraft. FAR § 23.99: Fuel system lines and fittings. (e) No flexible hose that might be adversely affected by exposure to high temperatures may be used where excessive temperatures will exist during operation or after engine shutdown. And §23.1183 Lines, fittings, and components. (a..., each component, line, and fitting carrying flammable fluids, gas, or air in any area subject to engine fire conditions must be at least fire resistant, ... Flexible hose assemblies (hose and end fittings) must be shown to be suitable for the particular application...
 
Special Light Sport Aircraft
 
Excerpts from the ASTM standards for Light Sport aircraft have taken lead from FAR Part 23 and include the following: _ASTM F 2339: 6.2 Fire Prevention—the design and construction of the engine and the materials used must minimize the probability of the occurrence and spread of fire by:

6.2.1 Using fire-resistant lines, fittings, and other components that contain a flammable liquid when supplied with the engine.

F2339-06(2009) Standard Practice for Design and Manufacture of Reciprocating Spark Ignition Engines for Light Sport Aircraft/ ASTM International, West Conshohocken, PA, 2009, www.astm.org.

Experimental Light Sport Aircraft Kits

All ELSA kits are a mirror image of a Special Light Sport Aircraft. They must comply with all ASTM standards.

Experimental Amateur Built Aircraft

The decision to use or not use fire sleeve really comes down to the builder’s best judgment.

There are more nuances in the regulations and ASTM standards, however the common theme is that hoses carrying flammable fluids “must be at least fire resistant” meaning that the hose will perform its intended function while withstanding 2000 (± 150) ºF flame for 5 minutes while “fire proof” means that it will withstand that temperature for 15 minutes. There are also several Technical Standard Orders, TSO-C53a, TSOC75 and TSO-C140, that apply to the manufacturer of hoses. They define the following four types of hoses:

(i) Type A. Non-fire-resistant “normal” temperature hose assemblies which are intended to be used in locations outside fire zones where the fluid and ambient air temperatures do not exceed 250°F.

(ii) Type B. Non-fire-resistant “high” temperature hose assemblies which are intended to be used in locations outside fire zones where the fluid and ambient air temperatures do not exceed 450°F.

(iii) Type C. Fire-resistant “normal” temperature hose assemblies which are intended to be used in locations within fire zones where the fluid and ambient air temperatures do not exceed 250°F.

(iv) Type D Fire-resistant “high” temperature hose assemblies which are intended to be used in locations within fire zones where the fluid and ambient air temperatures do not exceed 450°F

Type C hose is the most common for small aircraft in locations firewall forward.

Steps for Installing Fire Sleeve

  1. Select proper size fire sleeve for the size hose and end fittings. Stratoflex hoses using Stratoflex fire sleeve. Use a larger size fire sleeve over the outside of the hose. Refer to the following chart:

                       Hose Fire Sleeve

                       AE303-4   AE102-10

                       AE303-6   AE102-12

                       AE303-8   AE102-16

                       AE303-10 AE102-18

 

  1. Cut the fire sleeve to the proper length. When selecting the length of fire sleeve take into account the amount of sleeve that will be taken up by any bending of the hose. Any curve will shorten the length of the fire sleeve possibly exposing the hose or the ends. Remember, if it’s important enough to install on you aircraft it’s important to do it correctly. The fire sleeve can’t do the job if it isn’t covering all of the hose “the chain is only as strong as its weakest link”.
  1. The ends of the fire sleeve need to be protected in some manner from oil and fuel contamination. The most common method is to use a material called fire sleeve dip. Several manufactures make the stuff and it is readily available along with the fire sleeve itself from many suppliers such as Aircraft Spruce and Specialty. Another option includes the use of a high temp fire proof silicone, which comes in a caulking tube for around $30.00. This method is a bit crude and takes a little work to get a clean well protected end, but the price comparison to the fire sleeve dip at $110.00/ qt. makes it much more reasonable. A third option is the use of high temp fire sleeve wrapping tape. This is a simple clean way to terminate the ends of the fire sleeve but doesn’t always keep the oil out in the case of a particularly oil location. This method makes for a nice clean installation especially when the end of the hoses are not uniform or when there is an unusual clamping method.
  1. Select a method of securing so that fire may not propagate around the end of the fire sleeve. The use of the stainless steel band clamp is the most common practice, but even manufactures, like Diamond Aircraft, specify the use of stainless steel safety wire as a means to secure the fire sleeve. When this procedure is used we recommend using .041” stainless steel safety wire with at least two wraps around the ends. The band clamp method is the most popular. The band clamps can be purchased separately or as a kit with the installation tool. The clamps are installed over the end of the fire sleeve and then inserted into the toll. As the nut is turned, it simply rolls up the end of the band material and cinches down around the end of the fire sleeve making a fire proof seal. Once the clamp is tight, bend up the end of the excess band to a 90 degree angle and trim off the extra material, taking care to eliminate any sharp edges. A cut off wheel works well, as long as you don’t cut into the hose or clamp material.
  1. Do not install or reinstall any fire sleeve that is fuel or oil soaked. The reasons are obvious and in fact there are service bulletins and airworthiness directive addressing this specific topic requiring the replacement of any such fire sleeve. For example, AD 95-26-13 states in part, “... The fire sleeve of the hose should not be soaked with oil or have a brownish or whitish color, and there should be no evidence of deterioration as a result of heat, brittleness, or oil seepage”.

Installation of fire sleeve is a relative simple process. With a band clamp tool and your basic shop tools you should be able to complete the installation on your own hoses, both improving safety and adding a bit of professionalism for your firewall forward installation.

For more information on the Light Sport Repairman Maintenance Training Courses contact Rainbow Aviation Services. Web www.rainbowaviation.com, email info@rainbowaviation.com , voice 530824-0644 or 877-7 FLY LSA.

Light Aviation Edition December 2009

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