What It Looks Like

The smooth, sleek nose and engine cowls on an airplane hide a very crowded and busy chunk of real estate from public view. A good time to see exactly what's beneath the cowls is during a 100-hour or annual inspection of the airplane. The shop removes the cowls to expose the engine and accessories for detailed examination and repair of any problems.

One of the first things you'll notice when viewing an exposed aircraft engine is the confusion of hoses snaking over, under, around, and through the engine and its components. Why so many hoses? And why do some have a bright orange covering?

Like a human heart, an aircraft engine is the basic power center for all of the aircraft's functions. The engine produces the horsepower to turn the propeller; it generates the electricity to power the avionics and keep the battery charged; and it turns the gears in the pumps that supply vacuum and/or pressure to power certain instruments and controls.

Also like a human heart, an aircraft engine both controls and depends on fluids to do its work. It needs avgas to power the internal combustion process, which runs the pump that draws the gas from the tanks. But unlike the human heart, which deals exclusively with one fluid � blood - an aircraft engine employs several different kinds of fluids to do its work. Along with aviation gasoline, an aircraft piston engine pumps oil and, in some cases, hydraulic fluid. Like major blood vessels transporting blood to and from the heart, the profusion of hoses you see in an uncowled engine carry fuel, oil, air, and hydraulic fluid to and from the engine and the various pumps it drives.

Why the orange coating on some of the hoses? The orange is a special fire-resistant silicone rubber. If a fluid leak were to cause a fire somewhere in the engine bay, the fire sleeves shielding the flexible hoses and metal tubes carrying flammable gas, oil, or hydraulic fluid to and from the engine and accessories would prevent the fire from burning through the hoses or tubes. Without the fire sleeve protection, the heat could quickly rupture the hoses or tubes, and the escaping fluid would feed the fire.

Although in-flight engine fires are very rare, it's important to make sure that critical hoses and lines are well protected by an insulting blanket of fire sleeve.