Challenger Ultralight Fuel System

The fuel system is, of course, a critical system in the Challenger Ultralight. Anything we can do to ensure reliability and eliminate the possibility of problems is well worth the effort. Keep in mind that fuel system failure is one of the leading causes of sudden engine stoppage on ultralight aircraft. Sudden engine stoppage is one of the most common causes of ultralight aircraft accidents and off-field forced landings.

Much has been written about the Challenger fuel system and there are many ideas about how to make a bullet proof fuel system, Often these become quite complex with parallel fuel feeds extending from the fuel tank, one with the standard pulse operated Mikuni pump, and the second with an electrically operated fuel pump (Some elect to place the two pumps in series). Add to this the manually operated squeeze bulb pump and you actually can actually have three circuits supplying fuel to one very small engine. The plumbing gets complicated!

Other components must be added to complete the system including a fuel filter, a manual Primer pump which is usually located in the cockpit, and a drain valve on the bottom of the fuel tank. Some gadget happy pilots will also add a fuel flow monitor to send data to their Electronic Instrumentation System (EIS). Finally the system includes the fuel lines and fittings needed to connect all these components.

The fuel line and fittings are the subject of a separate article because of the many potential problems that are associated with them. This article will deal solely with the other major fuel system components:

• The Fuel Tank
• The Fuel Tank Drain Valve
• The Fuel Filter
• The Primer Pump
• The Primer Bulb
• The Fuel Pump

Challenger Fuel Tank

The fuel tank is the first place we start in the fuel system on the aircraft. The tank should be well restrained so that it will not come loose should the aircraft encounter heavy turbulence or an upset, either on the ground or in the air. Gasoline weighs about 6 lbs per US gallon, so the weight of a full 15 gallon tank is quite considerable. Therefore, many builders with the optional large fiberglass or aluminum tanks rivet a piece of 3/4” aluminum angle across the top of the tank to ensure that it stays in place. This piece of angle connects to the two vertical 1” diameter 2DT-3 down tubes.

The fuel tank should be protected from chafing with the aluminum mounting frame that it sits in. The frame should be snug and lined with heavy duty foam tape to protect the integrity of the tank. This includes even the aluminum tanks.

Challenger Fuel Tank Drain Valve

Most Challengers in Canada are being supplied with the largest optional fuel tanks. We pilots love to have lots of fuel on board, the more the better it seems. These tanks have a threaded port installed at the bottom, usually 1/8NPT size. A drain valve should be installed here to facilitate checking the quality of the gas before each flight. This is especially true for the aluminum tanks as they will sweat when the temperature changes and the water in the air will condense inside the tank. Over time this water will accumulate. The drain valve is located in the lowest part of the tank and allows the water to be drained away. Any other contaminants such as sand and rust will also drain through the valve.

Challenger Fuel Filter

The fuel filter is the first component the gas should see when it leaves the tank on the way to the engine. The fuel filter should be mounted in a place quick and easy to see during the pre-flight inspection. It should also be easy to reach for replacement. The Rotax Engine Maintenance Manual (page 10-2) recommends replacing the fuel filter every 100 hours. Later in section 11.18 the manual mentions that the fuel filter can become blocked due to a reaction between the detergents in certain two stroke oils and water in the fuel. The presence of alcohol in fuel aggravates this problem. It also states that this type of blockage can be difficult to detect visually.

With these points in mind, a good place to locate the fuel filter is in the space directly above the fuel tank behind the rear seat. The fuel line connected to the filter can be fastened to one of the vertical aluminum airframe tubes in order to hold the filter in place. The filter body itself should not touch the airframe as the vibration may break the filter. This area can also be used for locating the fuel filter on Challengers with the optional 10 gallon wing tanks.

The standard Challenger kit usually comes with a clear plastic fuel filter with a paper filter element. This paper element may become saturated with water entrained in the fuel. In cold temperatures this entrained water might freeze and cause a fuel blockage. Therefore, in Canada and the northern United States, Challengers should be equipped with a fuel filter with a metal or plastic filter element. Rotax engine technicians often recommend the Bosch filter part # 4077.

Manual Primer Line Pump

The Primer Pump injects gas directly into the throat of the carburetors to assist in starting. This is particularly helpful in cold weather starts. It also helps starting when the aircraft has not been run for quite some time and the carburetor float bowls are empty. Pumping the primer a few times will help get started and can keep the engine going until the fuel pump fills the carburetor bowls.

It is recommended that the Primer be located in the cockpit within easy reach of the pilot as opposed to near the engine. Many builders mount the primer onto the instrument panel or on a bracket in the front cockpit.

The manual primer does not come with the Challenger kit. It has to be purchased separately. There are manual primers sold that are made in some third world country and are of dubious quality. The O-ring on the primer piston has been known to fail. This not only makes the primer useless but it can provide a flow path for fuel to be sucked into the engine and thus flooding it. Purchase only the primer that is stamped “Made in the USA” on the black handle.

The fuel line running to the primer is smaller than the main fuel line. It is 1/4” OD x 1/8” ID. Tee off of the main line just after the fuel filter with a tee fitting that reduces to this smaller size. Run this line up to the inlet of the primer and then back to the carburetors. At the carburetors tee off in two equal lengths of primer line (about 10 inches works well) and push it onto the brass carburetors fittings found near the idle adjustment screws. These fittings will be tight and do not require clamps.

Here is a trouble shooting note: If you connect the primer lines backwards, it may not allow the main fuel pump to work. The fuel pump will just suck air. Therefore, pump the primer first and use it to fill the fuel filter and as well as the primer lines all the way back to the engine. This will confirm you have it installed correctly and you have to do this at some point anyway. If you have been pumping away for a minute and do not see any fuel rising from the tank, it means the primer is connected backwards. You have the line from the tank on the wrong barb.

Squeeze Bulb Pump

The squeeze bulb pump is included in the Challenger kit. It is the same type of pump used on many marine two stroke engines. A couple of squeezes of this pump will draw fuel from the tank and fill the carburetor bowls. More than two squeezes will likely flood the engine.

Many Challenger builders do not include the squeeze bulb in their fuel line circuit. There have been reports of squeeze bulbs failing in service and introducing contaminants into the carburetors. In addition, it is one more component that the fuel pump has to suck fuel through.

The alternative to using the squeeze bulb is to open the throttle all the way (with the magnetos in OFF and the key removed!) and turn the propeller by hand. Turning the prop activates the pulse line that drives the Mikuni fuel pump. This will fill the carburetors with fuel. In fact, this method will allow you to confirm that your fuel pump is working as you can visually see the fuel moving through the fuel lines to the carburetors.

The Fuel Pump

The standard fuel pump supplied with the Challenger is a small pulse line operated pump about the size of a hockey puck. It has 4 connection barbs for 1/4”ID fuel hose, 3 on the side and one in the bottom.

There is one inlet line and two outlets that supply the two carburetors. These inlet and outlet fittings are indicated by arrows on the body of the pump. The one pointing in is the inlet. The outward pointing arrows go to the carburetors.

The pulse line comes in through a fourth connection in the bottom of the pump. The pulse signal comes from a brass 1/4” barb on the carburetor side of the Rotax engine. It operates a diaphragm that pumps the fuel. The pump is best suited to a horizontal mounting arrangement.

These fuel pumps are designed for years of trouble free service. The Rotax maintenance manual advises to check the fuel pressure of the pump every 75 hours of service. It should produce between 3 and 7 psi depending on whether it is pumping with flow or dead headed.

What Fuel System Design Should You Use?

Most are familiar with the KIS principal – Keep It Simple. This principal applies to fuel systems. The more complex any system becomes, the more problems the user will encounter. Complexity on an ultralight aircraft adds weight and cost. It often also increases the time required to trouble shoot the initial start up problems and the likelihood of a failure of one of the many components.