Fuel storage, feed, transfer and venting

Separate the tank, feed path and transfer path

Distinguish fuel storage, engine feed, crossfeed and tank transfer, including the roles of pumps, vents and valves.

Mechanism and relationships

Fuel storage retains usable and unusable fuel within structural and venting limits. The feed system delivers fuel to an engine with adequate pressure and flow; a transfer system moves fuel between tanks. Boost pumps can provide positive inlet pressure, reducing vapour formation and supporting feed arrangements. An engine-driven pump then meets the engine fuel-system requirement. Crossfeed usually connects feed manifolds so an engine can be supplied from another tank or side; it does not necessarily move fuel from one tank into another.

Reason through the evidence

Vents admit or release air as fuel quantity, temperature and ambient pressure change. A blocked vent can allow tank pressure to diverge from ambient, impairing feed or stressing the tank even when quantity is adequate. Baffles and collector cells help retain fuel at pickups as the aeroplane moves. A full indication is therefore not proof of an uninterrupted engine feed. Identify the selected tank, pickup, pumps, valves and delivery path rather than treating all onboard fuel as one freely available pool.

Apply the explanation to a different case

A tank may contain fuel while its pickup is uncovered in a manoeuvre or while a required valve remains closed. Conversely a low tank can still feed normally within its designed conditions. Usability depends on geometry and delivery, not just the arithmetic total. A collector cell is intended to support pickup supply, but its refill flow and reserve are finite. For a system sketch, mark where air can enter the tank, where fuel is drawn, how pressure is supplied and which valves actually separate tanks. That turns an unlabelled fuel box into an explainable feed system.

Keep this distinction clear

Fuel pumps and shutoff valves may depend on different electrical buses or mechanical drives. An electrically dead cockpit indication does not automatically mean that all engine-driven feed pressure is lost, while normal engine rotation does not prove that every transfer pump still has electrical power.

Use mass accounting to reason about imbalance

Apply a tank mass balance and compare quantity trends with consumption and transfer to investigate an apparent imbalance.

Mechanism and relationships

A tank mass balance is initial mass plus inflow minus outflow. Feed and transfer flows must be assigned to the correct tank. An imbalance can result from unequal consumption, transfer logic, a leak or a quantity-indication fault; these explanations predict different total-fuel and rate trends. Burning from a heavier tank changes distribution without moving fuel across a transfer line. Fuel jettison, where installed, is a separate controlled discharge with defined limitations.

Reason through the evidence

A calculated fuel remaining value integrates measured or assumed flow, while a tank gauge estimates what is present. Agreement is useful but can be undermined by common wrong inputs. Compare expected consumption, measured quantity and known transfer actions. A decreasing total that exceeds measured engine consumption is evidence to investigate, not proof that a particular tank or pipe is leaking. The lab offers a closed bookkeeping system with explicitly labelled transfer and engine flows.

Apply the explanation to a different case

Suppose left and right gauges change in opposite directions while total fuel follows expected engine consumption. A commanded transfer could explain that pattern because it redistributes quantity without adding or removing total mass. If both gauges fall faster than the measured engine-flow total predicts, the same transfer explanation is insufficient by itself. Quantity-sensor error, flow-measurement error and an actual loss remain different possibilities. The exercise is to test conservation against the stated evidence, not to identify a leak location from one indicator. Real fuel checks retain their required timing, tolerances and procedures.

Keep this distinction clear

A fuel-temperature indication concerns the sensor’s location and thermal response. It is not necessarily the coldest point in every tank or line. Comparing a temperature with a fuel limitation requires the applicable fuel specification and aircraft monitoring assumptions rather than treating one sensor as a complete thermal map.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.