There are a few reasons to use heat resistant 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.