A gravity-feed fuel system, with the tank above the engine and a line running down through a filter and shut-off to the carburettor, has no pump to fail, but fuel can stop flowing when the tank outlet is uncovered in a steep climb, when the vent blocks, or when water or dirt settles in the line. Keep a visible gauge, a water drain at the lowest point, a filter, and a measured burn rate so the endurance is known before take-off. Running out of fuel is a pilot-preventable way to end a flight, and it ends many.
Part of Aerodynamics and Flight
Tank, lines, filter and vent that deliver fuel to an aircraft engine in every attitude it will fly.
Why Gravity Feed, And Where It Fails
The carrier prefers a tank above the engine because there is no pump to fail. The same carrier lists the classic fault: in a steep nose-up attitude the fuel runs to the rear of the tank and uncovers the outlet (unporting), so it recommends moderate climb angles of 10-15 degrees and a sight gauge. Other faults:
- Vent blocked — the tank cannot draw air, the flow stops and the engine quits after a short while.
- Water and dirt — water is heavier than fuel and collects at the lowest point; it stops an engine at once. The corpus’s Gasoline article describes a clear-jar check for water and gum, and Ethanol in Gasoline Engines notes that ethanol blends need dry ethanol and fresh fuel lines for rubber parts.
- Vapour lock and heat — a line near the exhaust boils the fuel.
- Leaks — a leak near a hot engine is a fire.
Worked Example: Endurance From A Measured Burn Rate
Endurance = usable fuel ÷ consumption. The two inputs below are placeholders; the real consumption is measured by running the engine at flight power on the ground and timing the fuel used. With 30 L usable and 15 L/h, endurance is 30 ÷ 15 = 2.0 h. If the tank cannot be fully used in a climb, the unusable amount must be subtracted: measure it too. Plan to land with a reserve whose size you decide in advance and write in the aircraft’s log; flying until the engine stutters is not a plan.
Method: Building And Checking The System
Step 1: Put the tank where it feeds in every attitude of the flight envelope, and outside the cockpit and away from the exhaust.
Step 2: Use clear or strong fuel line, short, with no tight bends, supported against vibration and joined to the engine with a flexible length (Engine Mounting).
Step 3: Fit a shut-off valve within the pilot’s reach.
Step 4: Fit a filter and a drain at the lowest point. Drain a sample before each flight and look for water.
Step 5: Vent the tank so that the vent cannot freeze, clog with insects or leak into the cockpit in a roll.
Step 6: Add a sight gauge that can be read in flight and calibrate it by filling in measured amounts.
Step 7: Earth the tank and filler to avoid static when fuelling, and fuel outdoors only.
Common Mistakes
- Trusting an uncalibrated gauge.
- Flying on the last quarter of the tank in a nose-up attitude.
- Using old fuel — gum and water.
- Storing fuel in an unsuitable container — see Fire & Vapor Safety.
Safety
Fuel starvation and fire
Petrol vapour is heavier than air, ignites from sparks, hot surfaces and static, and collects in low places. Fuel only outdoors with the engine off, and never fly until you have tested the system in the attitudes you will use. Light no flame to look for a leak; find one by sight and smell and by wiping joints dry and watching.
What’s Next
- Learn next: Cooling Systems, the other thing that stops engines
- Combine with: Ground Testing, where fuel flow is tested