Lesson 3.2 - Fuel System

Lesson 3.2 – The Fuel System

The fuel system is engineered to provide an uninterrupted flow of clean fuel from the storage tanks straight to the engine combustion chambers. This fuel must remain available under all configurations of engine power, altitude, aircraft attitude, and approved maneuvers. 

Reference:

• FAA Pilot’s Handbook of Aeronautical Knowledge (PHAK), Chapter 7 – Aircraft Systems [1]

1. Gravity-Feed vs. Pump-Feed Systems

The FAA broadens fuel systems into two distinct structural design classes: 

Gravity-Feed Systems (High-Wing Aircraft)

• How it Works: Utilizes the force of gravity to transfer fuel from the wings down to the engine induction system.

• Aircraft Type: Standard on high-wing aircraft like the Cessna 172. Because the tanks sit physically higher than the carburetor, fuel naturally flows downhill, requiring no mechanical pumps to maintain pressure. [1, 2]

Fuel-Pump Systems (Low-Wing Aircraft)

• How it Works: Because the wing tanks sit physically below the engine carburetor or fuel-air control units on low-wing aircraft, gravity cannot be utilized to pull fuel. [1]

• Aircraft Type: Standard on low-wing models like the Piper PA-28 Cherokee. [1, 2]

• The Dual-Pump Setup: These systems integrate two independent fuel pumps:

  1. Main Pump: Driven mechanically by the engine accessory gears.

  2. Auxiliary Pump (Boost Pump): Driven electrically from the cockpit master switches. It is utilized to prime the system for engine starting and provides added safety backup if the engine-driven pump fails in flight. 

2. FAA Fuel Grades, Colors, and Dyes

To prevent pilots from accidentally misfuelling their aircraft, specialized chemical dyes are added to denote the exact octane rating and anti-knock capabilities of the aviation gasoline (AVGAS). 

Please refer to the fuel grade chart at the top of this page.

⚠️ The Critical FAA Exam Rule: If your recommended grade of AVGAS is completely unavailable at the ramp, you can legally substitute the next higher grade of fuel. However, you must NEVER use a lower grade than specified. Lower grades withstand less compression, which triggers severe detonation, explosive combustion waves, high cylinder temperatures, and rapid engine failure. 

🎨 The Mixing Rule Trap

If a student pilot mixes two separate grades of AVGAS together (for instance, mixing Red Grade 80 with Blue Grade 100LL), the chemical tracking colors cancel each other out. The fuel will turn completely clear/transparent. If the fuel sampling cup looks clear like water, it is either contaminated or mixed with conflicting octane formulas. 80 octane has become rare during the past 20 years and is largely a special order fuel delivery for those of us that operate aircraft with older valves and need a bit of lead in our gas. 

Aircraft engines designed for 100LL will not function well with 80 octane in their tanks. Pre-ignition, detonation, and and an unintended very short post take-off climb are all things to avoid. 

So, please be certain that your 100LL engine isn't polluted with 80 octane. Definitely watch out for a problem with Jet-A if you are not fueling the airplane yourself. Jet A is finely prepared kerosene. It freezes at -40 degrees C. Jet A1 freezes at -47 degrees C.

3. Fuel System Management & Preflight Hazards

The Empty Gauge Rule

Per FAA certification guidelines, aircraft fuel quantity gauges are legally required to be perfectly accurate only when they read empty. Any reading above empty should be treated by the pilot as a loose estimate. Never trust a gauge panel alone—always visually verify fuel levels inside the tanks during your preflight walkaround and compute your fuel burn logs against time. 

Water Contamination

Water is heavier than AVGAS. Because of its density weight, water settles directly to the absolute lowest points of the fuel system: the tank sumps and the main fuel strainer assembly bowl.

• The Hazard: In warm weather, water entering the carburetor throat will immediately starve the cylinders of combustion fuel, stopping the engine. In winter weather, water freezes inside the lines, blocking all fuel flow. 

• Preflight Action: Pilots must use a transparent sampler cup to drain the quick-drain valves before every single flight, inspecting the sample visually for suspended water droplets or cloudiness. Occasionally you will find a small amount of water which condensed in your tanks. 

• *Action: Keep sumping until 100% pure colored fuel without any water streams out.

Static Electricity & Refueling Bonding

Friction from fuel pumping through a rubber hose line builds up a major static differential charge between the fuel truck nozzle and the aircraft airframe. To prevent a spark from igniting fuel vapors, a ground/bonding wire must be mechanically clipped to an exposed metal point on the airframe before the fuel tank filler caps are removed.