ATPL Fuel Questions

Fuel tank booster pumps, which feed fuel from the tanks toward the engines at low pressure, are typically:

• Centrifugal pumps, operating at relatively low pressure
• Centrifugal pumps, operating at relatively high pressure
• Spur gear pumps, operating at relatively high pressure
• Spur gear pumps, operating at relatively low pressure

Explanation: Booster pumps are normally centrifugal pumps, chosen because they can run safely even with some vapor present in the fuel (unlike a positive-displacement gear pump), operating at a comparatively low pressure sufficient to feed the engine-driven pump, which then raises pressure much further for injection.

Fuel - Booster Pumps

The purpose of a surge box (surge tank baffle box) inside a fuel tank is to:

• Prevent fuel from sloshing away from the pump inlet during abnormal aircraft manoeuvres
• Ventilate the tank during high-pressure refuelling
• Allow fuel to move freely between tanks while refuelling
• Collect sediment and contaminants at the bottom of the tank

Explanation: A surge box surrounds the boost pump inlet with a perforated housing that keeps a reserve of fuel around the pump pickup even when the aircraft is pitched, rolled, or manoeuvring abnormally and the bulk of the fuel has sloshed elsewhere in the tank, helping prevent the pump from momentarily running dry.

Fuel - Surge Boxes

The function of baffles fitted inside a fuel tank is primarily to:

• Reduce fuel surge/sloshing during aircraft manoeuvring
• Prevent fuel from migrating toward the wingtip specifically
• Prevent booster pump cavitation directly
• Reduce fuel flow rate at high altitude

Explanation: Internal baffles partially subdivide a fuel tank, restricting how freely fuel can slosh back and forth or surge as the aircraft manoeuvres. This helps keep the fuel more evenly distributed within the tank and reduces the dynamic loads that sloshing fuel could otherwise place on the tank structure.

Fuel - Tank Baffles

A vane-type (or similar mechanical) fuel flowmeter commonly works on the principle of:

• Measuring the fuel's quantity of motion (momentum) as it passes through the meter
• Measuring fuel volume and viscosity together
• Measuring fuel pressure and temperature together
• Measuring changes in capacitive dielectric, as in a fuel quantity probe

Explanation: A common mechanical fuel flowmeter design uses a rotating vaned element in the fuel stream; the torque needed to spin a second element against a calibrated spring relates directly to the momentum (mass flow rate) of the fuel passing through, giving a reading proportional to fuel mass flow rather than volume alone.

Fuel - Fuel Flowmeters

The purpose of a refuelling Volumetric Top-Off (VTO) unit is to:

• Automatically close the fuelling valve for that tank once it reaches full
• Keep the feeder box within the tank full of fuel at all times
• Close the tank vent system once the tank is full
• Close surge check valves in the outboard tanks to keep them full until centre-tank fuel is used

Explanation: A VTO unit senses when its associated tank has reached a preset full level during pressure refuelling and automatically shuts the fuelling valve for that tank, allowing multiple tanks to be filled simultaneously from a single pressure refuelling point without requiring a person to manually stop each one at the right moment.

Fuel - Volumetric Top-Off Units

An aircraft fuel dumping (jettison) system is generally designed to dump fuel down to:

• A predetermined, safe minimum fuel value, not all the way to zero usable fuel
• The absolute unusable fuel quantity
• A fixed reserve of a set number of gallons remaining in each tank regardless of aircraft type
• Exactly the maximum landing weight figure, regardless of remaining reserves

Explanation: A fuel jettison system is designed to stop automatically once a predetermined safe minimum fuel quantity remains, ensuring the aircraft retains adequate fuel reserves for a safe approach, landing, and any required diversion or go-around, rather than being able to dump fuel down to an unsafe or unusable level.

Fuel - Fuel Jettison System

The fuel tanks of a modern passenger airliner are normally filled during ground refuelling by:

• Fuel being pumped in under pressure by the refuelling truck/bowser
• Fuel being drawn in by suction from the aircraft's own booster pumps
• Gravity feed from an elevated fuelling point
• The volumetric top-off (VTO) unit acting as the fuel source itself

Explanation: Modern transport aircraft are normally refuelled by pressure refuelling: the ground bowser's own pumps push fuel into the aircraft's tanks through one or more single-point connections, with the aircraft's VTO units automatically stopping flow into each tank as it reaches full, rather than relying on the aircraft to draw fuel in itself.

Fuel - Ground Refuelling Method

The function of baffle check valves fitted within a fuel tank's internal structure is primarily to:

• Prevent fuel from migrating out toward the wingtip during flight
• Reduce overall fuel flow rate at high altitude
• Prevent surge/sloshing during aircraft manoeuvring
• Prevent booster pump cavitation directly

Explanation: Baffle check valves are one-way valves built into the internal rib/baffle structure of a wing fuel tank; they allow fuel to flow inboard toward the pump/collector area but resist it flowing back outboard toward the wingtip, helping keep the usable fuel supply concentrated where the booster pumps can reach it.

Fuel - Baffle Check Valves