Types of Aircraft Fuel Systems: Gravity-Feed and Fuel-Pump

Gravity-Feed Systems

A gravity-feed system uses gravity to move fuel from the tanks to the engine. On high-wing airplanes the tanks are in the wings, above the carburetor, so fuel flows down through the system. That's why many high-wing trainers have simple fuel systems.

Fuel-Pump Systems

If gravity can't do the job, pumps are added. On low-wing airplanes the wing tanks are below the carburetor, so a pump is needed. A fuel-pump aircraft has two. The main one is engine driven. An electrically driven auxiliary pump, also called a boost pump, is used for engine starting and in case the engine pump fails. A switch in the flight deck controls it, and a fuel pressure gauge shows the pressure in the lines, with normal range in the flight manual or color coded on the gauge.

Fuel injection adds its own parts. A fuel-injected engine usually has six basic components: an engine-driven pump, a fuel-air control unit, a fuel manifold, discharge nozzles, an auxiliary pump and pressure or flow indicators. The auxiliary pump supplies pressure for starting and emergencies, and after start the engine pump takes over.

The Parts a Pilot Handles

Fuel selector

A common selector valve has four positions: left, right, both and off. Left or right feeds only from that tank, and both feeds from both. Pilots use the left and right positions to balance fuel between the wings, and placards can show limits such as "level flight only" or "both" for takeoffs and landings.

Whatever the selector type, watch consumption closely. Running a tank dry doesn't only stop the engine, because it can also let air into the system and cause vapor lock, which makes the engine hard to restart. On fuel-injected engines the fuel can become so hot it vaporizes in the line, so it doesn't reach the cylinders. Running a long time on one tank unbalances the load.

Fuel primer

Both system types may include a primer. It draws fuel from the tanks and vaporizes it directly into the cylinders before starting, which helps in cold weather when there isn't enough heat to vaporize fuel in the carburetor. The FAA says to lock the primer in place when it isn't in use, because a loose knob can vibrate out of position in flight and cause an excessively rich mixture. Read the priming instructions to avoid overpriming.

Tanks, vents and overflow

Tanks are normally inside the wings, with a filler opening on top covered by a cap. They're vented to the outside, through the cap or a tube in the wing surface, so the pressure inside stays at atmospheric. An overflow drain lets expanding fuel escape without damaging the tank. If the tanks were filled on a hot day, it isn't unusual to see fuel coming out of the overflow drain.

Gauges

The fuel quantity gauges show the amount measured by a sensor in each tank, in gallons or pounds. The FAA says certification rules require accuracy only when they read empty, so any other reading should be verified. Don't depend solely on the gauges: check each tank visually on the preflight, then compare that with the indication.

Strainer, sumps and drains

After leaving the tank, fuel passes through a strainer that removes moisture and sediment before it reaches the carburetor. Contaminants are heavier than aviation fuel, so they settle in a sump at the bottom of the strainer. The system can have a sump, a strainer and tank drains, and they may be together. Drain the strainer before each flight.

Fuels and Their Colors

Aviation gasoline (AVGAS) is identified by an octane or performance number that gives its antiknock value, its resistance to detonating in the cylinder. Higher grades are for higher-compression engines. Piston aircraft use AVGAS 80, 100 and 100LL, and the "LL" means low lead. Turbine aircraft burn Jet A, Jet A-1 or Jet B. Jet fuel is basically kerosene and has a distinctive kerosene smell. Because the right fuel is critical, dyes and labels help to identify it.

The color coding extends to decals and fuel handling equipment. AVGAS is identified by name in white letters on a red background, while turbine fuels use white letters on a black background.

• Never use a lower grade than specified. It can push cylinder head and oil temperatures out of their normal range and cause detonation. If the right grade isn't available, use the next higher one.
• Find the grade in the flight manual, on placards in the flight deck and next to the filler caps.
• Automobile gasoline must never go in an aircraft engine unless the aircraft has a Supplemental Type Certificate for it.
• 100VLL. The FAA cites a safety information bulletin saying 100VLL is acceptable for aircraft and engines approved for grades 80, 91, 100 and 100LL. It has about 19 percent less lead than 100LL, and it's blue and virtually indistinguishable.

Water and Contamination

Water is the principal fuel contaminant. In cold weather it can freeze and block fuel lines, and in warm weather it can run into the carburetor and stop the engine. The FAA says accidents from fuel contamination have often been traced to servicing with improperly filtered fuel from small tanks or drums, and to storing aircraft with partly filled tanks.

How to check for it

1. Drain fuel from the strainer quick drain and from each tank sump into a transparent container.
2. Look for dirt, water and cloudiness. Suspended water makes fuel look cloudy, and settled water separates clearly from the colored fuel.
3. Drain enough from the strainer that you're sure you're sampling fuel from the tank. The right amount depends on the length of fuel line from the tank to the drain. If the first sample has contamination, drain more until none shows.
4. Drain the tank sumps too. Residual water in a tank can remain after the strainer drain stops showing any.
5. Don't take off until all water and contaminants are gone.

To limit condensation, fill the tanks after each flight or after the last flight of the day, and avoid refueling from cans and drums. A chamois skin and funnel can be the only option in a remote place, but the FAA says they aren't a guarantee. Worn-out chamois don't filter water, a new one already wet doesn't either, and most imitation chamois don't.

Frozen water

Water in suspension can freeze into ice crystals big enough to block screens, strainers and filters, and it can feed carburetor icing when conditions are otherwise not conducive to it. Free water that froze on the bottom of the tanks or filter can make drains useless and melt later into the system. If you find that, the handbook says a warm hangar can re-establish proper draining, and all sumps and drains should be checked before flight.

Some aircraft are approved for anti-icing additives. The handbook says small concentrations of certain compounds inhibit fuel system icing, with EGME at a maximum of 0.15 percent by volume substantially reducing it. Too little or too much additive makes it less effective, and additives are in no way a substitute for carburetor heat.

Static Electricity and Refueling

Static electricity builds up from air passing over an aircraft in flight and from fuel flowing through the hose and nozzle. The FAA says to attach a ground wire to the aircraft before the fuel cap comes off. Because the aircraft and the refueler carry different charges, they need to be bonded to each other so the difference is equalized. The nozzle stays bonded to the aircraft throughout, and a fuel truck should be grounded before the nozzle touches the aircraft.

Passing fuel through a chamois increases the static charge, so the aircraft must be properly grounded and the nozzle, chamois and funnel bonded to it. Under no circumstances should a plastic bucket or other nonconductive container be used.

Fuel and the Rest of the Airplane

The boost pump runs on the electrical system, covered in electrical systems. What the fuel feeds is in aircraft engines, and wing tanks are part of the structure discussed in fuselage types. To work out how long your fuel lasts, try the calculators.

Frequently Asked Questions

What are the two types of fuel systems on small aircraft?

Gravity-feed and fuel-pump systems, according to the FAA's Pilot's Handbook of Aeronautical Knowledge.

Do high-wing airplanes use gravity feed?

Often, yes. With the tanks in the wings above the carburetor, fuel flows by gravity. If gravity can't be used, as on low-wing aircraft, pumps are installed.

What's a boost pump?

An electrically driven auxiliary fuel pump used for engine starting and as a backup if the engine-driven pump fails.

Why are fuel gauges not fully trusted?

The FAA says certification rules require accuracy only when they read empty, so pilots verify the fuel level visually before flight.

What color is 100LL?

Blue. AVGAS 80 is red, AVGAS 100 is green, and Jet A is colorless or straw colored.

Can you use car gas in an airplane?

Only if the aircraft has a Supplemental Type Certificate that approves it. Otherwise automobile gasoline must never be used in aircraft engines.

How do pilots check for water in the fuel?

They drain samples from the strainer and each tank sump into a transparent container before every flight and look for water, dirt or cloudiness. They keep draining until none shows.