Aircraft fuel system, ensuring proper aircraft fuel system performance, Kitfox service letter #45.

SERVICE LETTER #45

DATE:                    September 21, 1995

SUBJECT:              Ensuring proper fuel system performance

APPLICABILITY:     All Kitfox models with wing tanks

COMPLIANCE:       As required

FROM:                   SkyStar Aircraft Engineering Department

The fuel system on a Kitfox is a gravity feed design that will provide adequate fuel supply to factory-approved Rotax and Continental engines. This service letter is intended to emphasize the importance of the installation methods and operational practices that will ensure proper fuel system performance.

In a gravity feed fuel system, fuel passes through a finger strainer mounted in the tank(s), through fittings and hoses to the header tank and on to the shut off valve, through the gascolator or filter, and finally to the engine. It is vitally important that restrictions to the flow be minimized. The carbureted Rotax and fuel-injected Continental engines use a fuel pump to supply fuel to the engine. These pumps, however, are designed to push fuel and not pull it, and their presence does not in any way relieve the aircraft builder/operator from the responsibility of ensuring an unrestricted flow of fuel to the pump. Furthermore, the fuel that does arrive at the engine must be fresh, free of water and other contaminates, and of the proper grade and type for the engine.

Strict adherence to the following procedures will help keep your fuel system working properly.

FUEL LINE FABRICATION

1.             Be careful not to cut the inner lining of a hose when installing it over a barbed fitting. This is an easy mistake to make because pushing on the hose and tearing the lining doesn't feel very different from a proper assembly. If the hose tears internally, a small flap of rubber is raised that can act as a flapper valve and intermittently stop the flow of fuel. Check the barbed fitting for burrs or sharp edges before installing the hose, and use a few drops of motor oil on the barb to help prevent tearing the hose lining.

2.             All the aluminum flared fittings that are provided with the Kitfox kit are AN-standard aviation parts and have a flare angle of 37°. All non-aviation flaring tools and flared fittings (the inexpensive, heavy kind that you get at the hardware store or auto parts shop) have a flare angle of 45°; to mix the two flare angles is to build in a guaranteed fuel leak. To make a high quality flare, start by cutting the ends of the tube square and deburr the cut end. Clean and lubricate the cone of the flaring tool and, after flaring, inspect the tube flare for signs of galling (insufficient lubrication), splitting (too much flare), under flare, and non-concentricity (improper handling of the tool). If your flare fitting is properly tightened (70-90 in-lbs for ⁵/₁₆" aluminum tubing) and is leaking, find out what is wrong and fix it (the face of the fitting may be scored, for instance), and avoid the temptation to apply a sealant to the flare face. No sealant should ever be required on the flare, and it is bad practice to accept 'band-aid and bailing wire' quality workmanship anywhere on an aircraft.

3.             It is always best when bending aluminum tubing to use a bending tool. Successful bends can be made by hand, but care must be taken to prevent kinking the tube or creating a flat spot, both of which can restrict the flow of fuel or cause the tube to fail. When routing hoses, pay particular attention to the hose for signs of kinking (too tight a radius) or twisting. Make sure that any clamps or cable ties are not so tight as to pinch the hose shut.

4.             Fittings with pipe threads should always be installed with an aviation-grade, pliable (non-hardening) thread sealant (like Fuel Lube or Tightseal) on the pipe threads only. NEVER use Teflon tape-type products, as bits of tape often get into the system during installation or servicing and can cause blockages or mechanical failures. SkyStar sent out approximately 25 header tanks on which our vendor used Teflon tape on the fitting inserts. If you have one of these tanks and it is not leaking, it is best to leave it alone. If the fittings are ever rotated for any reason, the system should be drained, the fittings removed, and all traces of Teflon tape cleaned out (an old toothbrush works well to clean out the threads in the tank). Reassemble the fittings using the proper sealant, and check your filter and/or gascolator for contaminants. Also, avoid over-tightening the pipe fittings on the header tank, as this may damage the tank material itself.

FUEL LINE ROUTING

1.             Fuel lines and hoses must be routed free of conflict with moving parts and should be secured so they will not vibrate against the airframe. Also consider the human factors, i.e. will a passenger stretching their legs crush or tear open a line?

2.             Fuel lines should be routed below electrical wiring and the two should never be bundled together. This helps to minimize the possibility of fire due to leakage.

3.             It is best to secure fuel lines and hoses using cushioned clamps where possible.

4.             The fuel line from the wing tank to the header tank must be routed continuously down hill. If this is not maintained, air pockets or bubbles can get trapped in any high spots and severely restrict the flow of fuel. Be sure to check for this condition with the aircraft in a level flight attitude.

5.             The service loop in the fuel hose that allows the wings to fold deserves particular attention to ensure the hose does not kink from too tight a radius. It is possible that the hose will not kink initially, but in time the stresses imposed will slowly force the hose to collapse. For this reason, check this hose during each preflight inspection. Also, after unfolding the wings, be sure to check that the hose has not come to rest in a position that creates a high spot in the line.

FUEL TANK CAPS

1.             Before each flight remove the fuel tank caps and blow through the vent tube to verify that it is free from obstructions. While you've got each cap off, check the seal for signs of deterioration and the tank for proper fuel level, and then firmly replace the cap. A bad seal or missing or improperly tightened fuel tank cap can allow large quantities of fuel to be lost overboard in a very short period of time.

FUEL SIGHT GAUGE INSTALLATION

1.             If you are in the process of installing or have already installed one of the new style fuel level sight gauges (made from a piece of butyrate tubing which you must heat form), follow the directions given in the assembly manual carefully. When installing the spring washer over the end of the sight gauge tube, be certain that none of the tabs get bent. A bent tab on this washer can prevent the fitting from being secured properly to the tube. Also be careful not to over-tighten the fitting to prevent the threads from stripping. Once the gauge is installed on the tank, you should be able to firmly pull on the top and bottom of the tube simultaneously and it should not come out of the fittings. If it does move when pulled, it has been installed improperly and the fittings must be reinstalled correctly.

FUEL QUANTITY AND UNUSABLE FUEL

1.             It is very important to properly calibrate your fuel tank sight gauges in both a taxi attitude and a flight attitude. Use a measured container to incrementally fill the tanks with fuel, and mark the gauges accordingly.

2.             As of September 1 we will have revised the outlet port location on our 13 gallon fuel tanks to reduce the amount of unusable fuel in all flight attitudes. The previous wing tank designs (both 6 and 13 gallon) had the fuel outlet fitting at the rear of the tank, creating the potential for fuel starvation during long, steep, high-speed descents with the tanks low on fuel. During such a descent, fuel tends to run forward, away from the tank outlet, and during those periods fuel will be provided to the engine from the supply in the header tank. Because of variations from aircraft to aircraft, we recommend that each owner drain both tanks, set the aircraft up at a wings-level, 5° nose down attitude (measured across the head rack tubes), and then add measured amounts of fuel to each tank until the fuel just begins to flow into the header tank. This amount of fuel added is the unusable fuel quantity per tank in a steep descent. To help remind you of this limitation while flying, make a red mark on the fuel sight gauges at the unusable fuel level while in this descent attitude. If you find that you are required to make a fast and steep descent when you have little usable fuel remaining, check the header tank vent hose that runs to the top of the right wing tank. If the level of the fuel in this vent hose is lower than the outlet port on the wing tank, then the engine is running on header tank fuel only. Fly the descent in steps, leveling out every few minutes for a minute or so to replenish the fuel supply in the header tank. A retrofit kit is available for those with the old style tanks who would like to relocate the outlet ports to reduce their unusable fuel quantity. This retrofit will increase the usable fuel in each tank by approximately 2½ gallons in a cruise descent attitude. Ask our Customer Service department for kit number 10678.000, which is currently priced at $87.50 per tank.

LOW FUEL WARNING SYSTEMS

To help provide an additional measure of safety against fuel supply problems, SkyStar has developed (and strongly recommends) a low fuel warning system to alert the pilot of a low fuel level in the header tank. With this system a red warning light on the panel indicates that the fuel level in the header tank is dropping, giving you time to troubleshoot the problem (or land the airplane) before it gets quiet up front. Builders with the newer style Series 5 header tank already have the provision in the tank for the sending unit. This kit is P/N 10017.000 and costs $105.00. A kit that includes the header tank with sending unit provision and warning light system is also available (P/N 10637.000) and costs $219.95.