10.1 Engine Fuel System Flow, Contamination, and Vapor

The Fuel Path From Tank to Combustion

Before fuel can be metered it must arrive as clean liquid fuel at the correct pressure. The typical reciprocating path runs: tank outlet, fuel selector valve, finger strainer/gascolator, boost (auxiliary) pump, firewall shutoff, engine-driven pump, then the carburetor or fuel-injection servo, the flow divider, and finally the discharge nozzles.

A turbine path adds a tank boost pump, a low-pressure (LP) engine pump, a fuel/oil heater or heated filter, a high-pressure (HP) pump, the fuel control unit (FCU) or FADEC, a fuel flow divider/dividing valve, and the spray nozzles. A restriction or air leak early in either chain can look exactly like a metering fault unless the mechanic follows fuel from source to flame.

Fuel pressure and fuel flow are related but distinct. Pressure is the force available in the line; flow is the rate of fuel movement. A plugged filter can drop both at high power. A partially blocked injector nozzle can leave upstream pressure normal or even high while that one cylinder runs lean. A leak downstream of a flow transmitter can indicate flow without producing power. Always ask where the gauge senses, what it measures, and whether the symptom is engine-wide or cylinder-specific.

Contamination, Vapor Lock, and a Diagnostic Sequence

Contamination determines cause and effect. Water settles in low points, can freeze in lines and filters at altitude, corrodes parts, and may pass as a slug that interrupts combustion. Sediment/dirt can hold a valve open, clog a screen, scour a pump, or distort metering. Microbial growth thrives at the fuel/water interface and builds sludge that plugs filters and accelerates corrosion. Deteriorated hoses shed liner particles internally. Corrective action is not just draining a sample; it may require finding how contamination entered and whether it reached precision components.

Vapor lock occurs when fuel changes state in the line. Heat, low pressure (a suction-side restriction or high altitude), and volatile fuel let liquid flash to vapor. Pumps are designed to move liquid; vapor compresses, causes cavitation, and produces fluctuating pressure and power loss. A boost pump suppresses vapor by raising inlet (suction-side) pressure, but only when system instructions call for it - that is why the correct exam answer is system-specific, not a blanket rule.

A disciplined fuel-supply sequence:

1. Verify correct fuel grade, quantity, tank selection, and tank venting.
2. Drain sumps and inspect samples for water, debris, color, and odor using approved procedures.
3. Check strainers, filters, boost-pump operation, engine-driven pump output, and line/hose condition.
4. Compare fuel pressure, fuel flow, RPM/manifold pressure, EGT, and power response.
5. Inspect downstream metering only after proving the supply path delivers clean liquid fuel.

For knowledge-test items, watch the timing clue. A restriction may not appear at idle because demand is low; at takeoff, climb, or acceleration the same restriction starves the engine. A blocked vent can run normally for minutes until tank vacuum limits flow. Cause-effect timing usually separates a tank, vent, pump, filter, or metering problem - the gauge reading alone rarely does.

Pumps, Pressure Sensing, and Fuel-Type Differences

Most certificated engines use two pumps in series. A boost (auxiliary) pump - usually electric and submerged or near the tank - primes the system, supplies fuel for start, and provides backup and vapor suppression by raising suction-side pressure. The engine-driven pump is the primary source in flight. On a gravity-feed high-wing aircraft the boost pump may be omitted because head pressure feeds the carburetor.

A failed engine-driven pump shows as a pressure loss recoverable by selecting the boost pump; if the boost pump masks the fault, the discrepancy can be missed until the next flight, which is why pressure readings are compared with and without the boost pump running.

Fuel grade and type change failure behavior. Reciprocating engines burn avgas (100LL is the common grade, dyed blue), which is volatile and prone to vapor lock and carburetor icing. Turbine engines burn Jet A / Jet A-1 kerosene, which is far less volatile (low vapor-lock risk) but more prone to holding water and supporting microbial growth, so turbine systems add heated filters, fuel/oil heat exchangers, and water-scavenge provisions.

Misfueling a piston engine with jet fuel is catastrophic - it cannot vaporize properly, causes severe detonation, and destroys the engine - which is why fuel selector placards, nozzle sizes, and color codes are treated as airworthiness items.

Key supply-system facts to retain:

• Gravity feed needs no boost pump; low-wing designs require a boost pump.
• The gascolator/sump is the system low point where water and debris collect for draining.
• Compare fuel pressure with and without the boost pump to isolate engine-driven pump faults.
• 100LL is dyed blue; Jet A is straw/clear - color and smell help catch misfueling.
• Microbial growth and water are bigger threats in kerosene systems than in avgas.