EASA ATPL Package (former JAA) Airframe/ Systems/ Power Plant

Fuel System

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Sample Question: Fuel System

Question 3717
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The diagram shown in annex represents a jet fuel system. The fuel-flow measurement is carried out:
A
after high pressure pump first stage (item 2).
B
after high pressure valve (item 4).
C
after low pressure valve (item 1).
D
in the fuel control unit (item 3).

Fuel System Fundamentals for ATPL Students

An aircraft fuel system stores, conditions, measures, and delivers fuel to engines and auxiliary power units (APUs) safely and reliably across the full flight envelope. On most transport-category aeroplanes, fuel is carried in integral wing tanks (sealed wing structure) with baffles to restrict fuel movement and a collector/feed box to maintain a locally higher fuel level at the boost pump inlet. This mitigates unporting during high angles of attack, sideslip, or turbulence and ensures positive pressure at the pump.

Fuel supply is driven by electrically powered, 115 V AC centrifugal low-pressure (LP) booster pumps that feed an engine-mounted high-pressure (HP) pump. LP booster pumps prevent vapour lock—a blockage from vapour bubbles—and protect the HP pump from cavitation. In cold conditions, engine-mounted fuel heaters (often fuel–oil heat exchangers) raise fuel temperature primarily to prevent icing of the LP filter. For accurate performance monitoring, fuel flow is measured downstream of the HP shutoff valve, so the indication reflects actual engine consumption. The fuel vent system maintains tank pressure near ambient, preventing overpressure or vacuum; many large aircraft vent via inlets on the underside of the wing. During pressure refuelling, an automatic fuelling shut-off valve stops refuelling when a preset level is reached to avoid overfill.

Operationally, ATPL procedures emphasize balanced feeding and contingency management. If the centre tank is empty and the APU is operating, a detected lateral imbalance can typically be corrected by selecting crossfeed OPEN, turning the left main tank pumps OFF and the right main tank pumps ON to transfer fuel as per the AFM/FCOM. In an engine fire, the pilot isolates the fuel supply by operating the fire handle or fuel shutoff controls, closing the LP/HP fuel valves to the affected engine. Knowledge of fuel grades and temperature limits is essential: the freezing point hierarchy is Jet A (highest), Jet A-1, then Jet B (lowest). Managing fuel temperature, using heaters as required, helps avoid waxing or filter icing at altitude while remaining compliant with aviation regulations and aircraft systems limitations.

What the Fuel System question bank covers

  • Fuel tank design and management: integral tanks, baffles, feed/collector boxes, surge and venting systems.
  • Pumps and valves: LP booster pumps (115 V AC centrifugal), HP pumps, LP/HP shutoff valves, crossfeed architecture.
  • Fuel conditioning and measurement: engine-mounted heaters to prevent LP filter icing, fuel-flow measurement location and rationale.
  • Refuelling and ground operations: pressure refuelling, automatic shut-off valves, contamination and temperature considerations.
  • Operational procedures: APU supply, fuel balancing/crossfeed techniques, vapour lock prevention, anti-cavitation practices.
  • Emergency actions: fuel isolation in engine fire scenarios, system protections per ATPL procedures and aviation regulations.
  • Fuel types and properties: Jet A, Jet A-1, Jet B, with emphasis on freezing points and temperature management in flight.