Scale changes everything except the job. A transport aeroplane's fuel system does what the previous lesson's did, for a hundred tonnes of kerosene: more tanks, pumps in multiples, a refuelling system that loads it all in minutes, and, on the largest aircraft, a system for throwing fuel away on purpose. The fuels change too, and their freezing points start to matter.
The jet fuels
Three fuels serve the turbine fleet:
JET-A1 — the worldwide standard kerosene: density about 0.8 kg per litre, flash point at least +38°C, freezing point −47°C.
JET-A — the same kerosene family, mainly a United States fuel: identical handling qualities, but a higher freezing point of −40°C, which matters on long polar sectors.
JET-B — a wide-cut blend of kerosene and gasoline for very cold climates: lighter than the straight kerosenes at about 0.76 kg per litre, it stays liquid to below about −50°C, at the price of gasoline-like volatility and a very low flash point, which makes it the hazardous one to handle.
Cold is the operational thread: hours of cruise at −55°C outside air chill the fuel toward its limit, which is why fuel temperature is monitored and why additives exist against freezing: fuel-system icing inhibitors, blended in to stop the water dissolved in the fuel from freezing in filters and small passages before the fuel itself is anywhere near its own freezing point.
The system, at transport scale
The function of the transport fuel system is delivery under all the piston lesson's rules, implemented at transport scale with:
Lines and pressure valves — the plumbing between every tank and every engine, and the valves that keep its pressures within limits.
Centrifugal boost pumps — two per tank, immersed in the fuel, pushing it to the engines under positive pressure.
The fuel shut-off valve — one per engine, closed by the fire handle, starving an engine on command.
Filters and strainers — catching contamination before it reaches a pump or an engine.
Tanks — in the wings, the wing tips, the fuselage and the tail.
Bafflers — perforated walls that calm the fuel, detailed below.
Sumps, drains and the vent system — the water traps and the tanks' breathing, doing the piston lesson's jobs at scale.
Quantity and temperature sensors — feeding the gauges and the fuel-temperature monitoring the first section demanded.
Four items get the syllabus's special attention, asked for by name:
Trim fuel tanks — a tank in the tailplane on some long-haul types, filled and emptied in cruise to move the centre of gravity aft: that reduces the download the tailplane must carry to balance the aircraft, and with it the trim drag that download costs. The fuel does double duty as ballast, transferred automatically by its own pumps and lines.
Bafflers (baffles) — perforated walls inside the tanks that let fuel seep between bays but stop it surging in waves during acceleration, braking and manoeuvre: without them, tonnes of liquid would slosh hard enough to upset balance and hammer structure, and outlets would uncover in every turn. Baffle check valves also trap fuel inboard, keeping the boost pumps fed as the level falls.
The refuelling/defuelling system — a single pressure connection (usually under one wing) feeding all tanks through a refuel gallery, with a control panel that shows quantities and shuts each tank's valve at its preselected load: tonnes aboard in minutes, no ladders, no open filler caps. The same plumbing runs backwards to defuel.
The fuel dump (jettison) system — pumps and dump valves feeding nozzles at the trailing edges, for throwing fuel overboard in flight.