7.1 Airframe Fuel System Boundaries and Safe Servicing

Tank Types and the Airframe Fuel Boundary

Airframe fuel study begins with a boundary. On the Airframe knowledge test, the focus is the airframe side: tanks or cells, vents, drains, selector valves, sumps, strainers, boost and transfer pumps, crossfeed controls, fueling and defueling provisions, jettison plumbing where installed, and fuel quantity indication. Engine-driven fuel pumps, carburetors, fuel injection servos, fuel controls, and turbine fuel nozzles belong to powerplant fuel metering. The firewall fuel shutoff is the usual hand-off point. A safe mechanic can state where one system ends and the other begins before opening a panel.

Fuel tanks come in three constructions the test names directly. Integral (wet-wing) tanks use the sealed wing structure itself as the tank — no separate container — saving weight and maximizing volume on transport and high-performance aircraft; leaks are repaired by reworking fuel-tank sealant. Bladder cells are flexible reinforced-rubber containers snapped into a structural cavity; they resist crash rupture and are removable for repair.

Rigid removable tanks are riveted or welded metal containers (common on older light aircraft) secured in a bay. Every tank needs a filler, a vent, a sump/drain at the low point, baffles to limit fuel slosh, and a finger strainer at the outlet.

Venting is mandatory. As fuel is drawn down, the vent admits air so the tank does not collapse under suction; as fuel warms it admits expansion overflow. A blocked vent produces fuel starvation that mimics a pump or selector failure, and on the ground can collapse or burst a tank. Vents are checked for blockage, ice, and insect nests every inspection.

Pumps, Selectors, Crossfeed, and Quantity Indication

Boost pumps (usually electric centrifugal) pressurize the line from tank to engine, prevent vapor lock at altitude/high temperature, and provide engine-start and backup pressure. Transfer pumps move fuel between tanks — for example, from tip or auxiliary tanks into a main feed tank. Crossfeed plumbing lets one tank feed the opposite-side engine to manage lateral fuel imbalance on multi-engine aircraft. Selector valves route or shut off flow and must carry the correct placards and positive detents (OFF, LEFT, RIGHT, BOTH/CROSSFEED). A mispositioned selector mimics a pump failure, so verify position, rigging, and placards first.

Fuel quantity indication uses two main technologies. A float sender moves a variable resistor (rheostat) as fuel level changes — simple, but it reads volume and is sensitive to attitude. A capacitance system uses tank probes acting as capacitors; because the dielectric constant of fuel differs from air, capacitance changes with the mass of fuel between the plates. Capacitance systems measure fuel by weight (mass), are more accurate in unusual attitudes, and use a compensator to correct for fuel temperature/dielectric variation.

Before condemning a transmitter, separate sender, wiring, power, ground, compensator, and indicator faults.

Contamination Control and Safe Servicing

Sumping is the first defense against contamination. Water (heavier than gasoline, but Jet-A density is closer to water) settles to tank low points and is drained into a clear container after every fueling and after rain. Cloudy or hazy fuel indicates suspended water; a distinct lower layer or a cup that does not smell or color correctly indicates a problem. Sediment, rust, microbial growth (the "jet-fuel fungus" Hormoconis resinae at the fuel/water interface), sealant debris, and wrong fuel grade all foul strainers.

If multiple drains show contamination, treat the whole system as contaminated until proven otherwise — never accept one clear sample as proof the system is clean.

Fueling, defueling, spills, and tank entry are high-risk. Static electricity generated by fuel flow can ignite vapors, so the aircraft is bonded to the fuel source and the nozzle is bonded to the aircraft before flow begins. Control ignition sources, have fire protection staged, use correct PPE, and follow environmental handling for spills. Tank entry adds confined-space, vapor, lighting, tool, and rescue hazards and is governed by the aircraft procedure. The Airframe ACS treats fuel servicing, contamination, spills, defueling, and tank entry as risk-management items, so the exam can test judgment as much as component names.

Know the fuel grades too, because servicing the wrong grade is a contamination event. Reciprocating-engine avgas is dyed for identification — 100LL (low lead) is blue, 100/130 is green, and 80/87 is red — while turbine Jet-A is clear-to-straw and is not interchangeable with avgas. A piston engine fueled with Jet-A will detonate or fail; the exam expects the mechanic to verify grade, color, and the correct filler placard.

Defueling is performed when changing fuel grade, before certain maintenance, or for weight reduction, and it reverses every fueling hazard while adding the risk of drawing fuel back through a contaminated path — so it uses approved equipment, bonding, and a clean receiving container.

Finally, the feed system architecture appears on the test. Gravity-feed systems (high-wing light aircraft) rely on tank height above the carburetor and need no boost pump but absolutely depend on open vents. Pump-feed (pressure-feed) systems (low-wing and all turbine aircraft) use boost and engine-driven pumps to lift fuel against gravity.

Fuel jettison (dump) systems on large aircraft let the crew dump fuel through wing-tip or dedicated nozzles to reach a safe landing weight; the dump valves, nozzles, and plumbing are airframe inspection items. Across all of these, the safe exam answer respects the boundary: trace the actual aircraft flow path, isolate supply, indication, venting, valve, and contamination causes, and use approved data rather than swapping parts.