6.fuel-systems. Fuel Systems
An aircraft fuel system stores fuel and delivers it from the tanks to the engine at the proper pressure and flow rate to support all approved operating conditions. Light, single-engine airplanes use one of two basic designs: gravity-feed or fuel-pump systems.
Gravity-Feed Systems
In high-wing airplanes such as the Cessna 172, fuel tanks are mounted in the wings above the carburetor or fuel injection servo. Gravity alone delivers fuel to the engine, so no fuel pump is required. A typical gravity-feed system includes:
- Two wing tanks, each with a filler cap and vent
- A fuel selector valve (BOTH, LEFT, RIGHT, OFF)
- A fuel strainer (gascolator) at the lowest point of the system
- A primer for cold starts
- The carburetor or injection servo
Because head pressure depends on fuel level and aircraft attitude, gravity-feed systems must be vented to outside air; a blocked vent will cause fuel starvation as a vacuum develops in the tank.
Fuel-Pump Systems
Low-wing airplanes (Piper Cherokee, Cirrus SR22, Beechcraft Bonanza) cannot use gravity feed because the tanks sit below the engine. They use an engine-driven fuel pump as the primary source of fuel pressure, plus an electric auxiliary (boost) pump for:
- Engine starting
- Takeoff and landing
- Switching tanks
- Backup if the engine-driven pump fails
- Operations at high altitude or in hot weather where vapor lock is a risk
The pilot's operating handbook (POH) specifies exactly when the boost pump must be on.
Fuel Selector Valve
The fuel selector allows the pilot to choose which tank feeds the engine. Common positions are LEFT, RIGHT, BOTH, and OFF. On airplanes without a BOTH position, fuel must be managed manually to keep the airplane in lateral balance — typically by switching tanks every 30 minutes or as recommended by the POH.
Fuel Grades
Reciprocating aircraft engines burn aviation gasoline (AVGAS), which is graded by octane and dyed for identification:
- 80 — red (largely unavailable)
- 100 — green (high lead)
- 100LL — blue (low lead, the most common today)
- UL94 — purple (unleaded, limited availability)
Never use a grade lower than the one specified for the engine. A higher grade may be substituted only if specifically approved by the manufacturer. Using automotive gasoline (MOGAS) requires a Supplemental Type Certificate (STC). Jet A (clear or straw-colored) is kerosene-based and must never be put into a piston engine — even small amounts can cause detonation and catastrophic engine failure. Misfueling is prevented in part by different filler-port sizes and color-coded placards.
Fuel Contamination
The three most common contaminants are water, dirt/sediment, and the wrong grade of fuel. Water enters tanks primarily through condensation when warm, moist air in partially full tanks cools overnight. To minimize condensation, fill tanks at the end of each flying day.
Before every flight, the pilot must:
- Sump each tank drain and the gascolator into a clear sampler.
- Check color (blue for 100LL) and smell.
- Inspect for water (a clear bubble on the bottom) and particulates.
- Continue draining until the sample is clean and free of water.
Fuel Gauges and Indicators
Fuel quantity gauges are required by 14 CFR 23 to read accurately only at zero (empty). Pilots must visually verify fuel quantity before flight rather than relying solely on gauges. A fuel pressure gauge (in injected and pumped systems) and a fuel flow gauge (gallons per hour) help monitor system health and verify proper leaning.
Fuel Primer
A primer pumps raw fuel directly into the intake manifold or cylinder intake ports for cold starts. After use, the primer must be locked; an unlocked primer can vibrate open in flight, causing an excessively rich mixture and rough running.
Vapor Lock
Vapor lock occurs when fuel vaporizes in the lines, blocking liquid flow. It is most common on hot days, at high altitude, or after shutdown of a hot engine. The boost pump increases line pressure and helps prevent or clear vapor lock.
Fuel Management in Flight
Good fuel management is a required pilot skill:
- Know total fuel on board and burn rate.
- Plan a landing with at least the reserves required by 14 CFR 91.151 (30 minutes day VFR, 45 minutes night VFR or IFR at normal cruise).
- Switch tanks per POH guidance to keep the airplane in balance.
- Lean the mixture per POH to extend range and prevent fouled plugs.
- Cross-check fuel gauges, fuel flow, and time against the flight plan.
A disciplined preflight, correct fuel grade, clean samples, and active in-flight monitoring are the pilot's defenses against the leading cause of preventable engine failure: fuel exhaustion or starvation.