Aircraft refueling: hydrant systems, fuel trucks and jet fuel logistics

The jet has barely reached its stand when a compact vehicle rolls under the wing, a worker flips open a small lid in the apron concrete and couples up a hose. A few minutes later, thousands of liters of jet fuel are flowing into the wing – without a fuel truck in sight. Aircraft refueling through a hydrant system is one of the least conspicuous procedures at an airport and at the same time one of the most elaborate. This guide shows you how hydrant, dispenser and fuel truck work together, how refueling goes, which safety rules apply and what logistics chain stands behind it before the jet fuel even reaches the apron.

Aircraft refueling: the short answer

At large airports, the jet fuel mostly comes out of the ground. A fuel farm on the airport grounds feeds an underground pipe network that reaches the parking stands on the apron. There, hydrant valves sit in small pits in the ground. A dispenser vehicle connects the valve to the fuel connection under the wing, filters the fuel and measures the quantity. Where there is no hydrant, a refueler truck takes over, bringing the jet fuel in its own tank.

That is exactly how a planning approval for Munich Airport describes it: a fuel farm plus a fixed airfield refueling system, underground lines to the aprons, hydrant valves in pits, dispensers without their own tank body – and fuel trucks for all stands that are not connected to the hydrant network.

The hydrant system: a filling station under the apron

A hydrant system consists of three building blocks:

• Fuel farm: large storage tanks on the airport grounds in which the delivered jet fuel is stored, settled and checked.
• Pump station: It pressurizes the network so that every hydrant has enough flow for quick refueling.
• Pipe network with pits: underground pipes to the aprons; at the stands they end in hydrant valves under a flush-mounted cover.

Frankfurt shows how big such a system can get. There, Hydranten-Betriebs OHG operates a fuel farm and lines totaling around 60 kilometers of piping; the control system monitors the fuel farm, the supply pipelines and the tightness of the network. According to the control technology supplier, the system secures the supply for over 500 takeoffs a day. At Frankfurt Airport, the hydrant network is thus one of the central but invisible pieces of infrastructure.

The advantage is obvious: instead of dozens of heavy fuel trucks constantly shuttling between the fuel farm and the apron, only small dispensers drive from stand to stand. That relieves the apron roads, shortens routes and reduces the risk of accidents involving large amounts of fuel on wheels. In return, the network costs a lot of money and only makes sense where many aircraft regularly stand at fixed positions – at gates with passenger boarding bridges and on large apron areas. You can find more on the stand types in the article Gates, boarding bridges and remote stands.

Dispensers and fuel trucks compared

At the fence, both vehicles look similar at first glance. The difference is in the build.

Feature	Hydrant dispenser	Refueler truck
Own fuel tank	no	yes, tank body or semitrailer
Jet fuel source	hydrant valve in the ground	own load from the fuel farm
Typical size	compact, often a truck chassis or small vehicle	large and heavy, up to a tractor-trailer depending on the design
Where used	stands with a hydrant connection	remote stands without a connection, smaller airports, general aviation
Identifying feature	ground hose to the cover in the concrete	no ground connection, large tank body
Reloading	not needed	drive back to the fuel farm or filling station

Both vehicle types have a lift platform or a boom to bring the hose to the connection under the wing. On big long-haul jets like the Airbus A380, the connection is so high that nothing works without a platform. For large quantities, two vehicles may also work in parallel. At small airfields with propeller aircraft and piston-engine planes, on the other hand, you often find fixed pumps – but those mostly dispense avgas, not jet fuel.

How refueling works

Refueling is part of the turnaround and often runs in parallel with catering, cleaning and baggage. Roughly, it always follows the same pattern.

Pressure refueling under the wing is the normal case for airliners. It is a closed system: no open filler, no vapors at the wing, and the aircraft itself distributes the fuel. Overwing refueling through a filler cap on top of the wing is more like filling up at a gas station and is common on smaller aircraft.

Safety: fueling with passengers on board

One of the most common questions from the terminal: is fuel being loaded while I’m already seated? Often yes – because it saves time on the turnaround. This is allowed with Jet A or Jet A-1 fuel, but not with wide-cut fuels such as Jet B. The exact requirements come from the authority, the airport and the airline, but these precautions are typical:

• Cockpit staffed: A crew member or another qualified person stays in the cockpit during the entire refueling.
• Doors staffed: Exits for a possible evacuation are staffed by cabin crew and ready for use, usually at least two doors.
• Belts open: The seat belt sign is off so that nobody is strapped in during an emergency; the no-smoking sign is on.
• Paths clear: Aisles, emergency exits and the areas under the escape slides stay clear – including of ground vehicles.
• Voice link: The cockpit and the refueling supervisor are directly connected, usually by interphone. If the cockpit calls out, refueling is stopped immediately.

Whether the fire service must additionally stand by at the aircraft depends on local regulations. The exact conditions are set by the authority, the airport and the airline.

Jet A-1, Jet A, JP-8: what’s actually flowing

Jet fuel is a middle distillate from crude oil, roughly in the range of hydrocarbons with about 8 to 16 carbon atoms. Additives are added too, such as antistatic agents, antioxidants and corrosion inhibitors. The common grades:

Grade	Use	Freezing point	Special feature
Jet A-1	civil aviation worldwide, standard in Europe	at most −47 °C	specifications include DEF STAN 91-091 and ASTM D1655, anti-static additive required
Jet A	mainly USA	at most −40 °C	ASTM D1655, slightly higher density than Jet A-1
Jet B	very cold regions	much lower	wide-cut fuel, not for refueling with passengers on board
JP-8 (NATO F-34)	military	same as Jet A-1	based on Jet A-1 with military additives

The low freezing point is no detail: at cruising altitude it gets very cold outside, and on long routes the fuel in the wing cools down a lot. Especially on ultra-long-haul flights with many hours at high altitude, the crew therefore keeps an eye on the fuel temperature.

How jet fuel is passed on in the air, that is, from aircraft to aircraft, is a topic of its own. The article Recognizing tanker aircraft fits here.

Jet fuel logistics: from the refinery to the wing

Before a dispenser can couple up, the fuel has come a long way. The chain looks roughly like this:

1. Refinery: Jet fuel is obtained by distilling crude oil, treated with additives according to the specification and certified.
2. Transport to the airport: Depending on location, by pipeline, inland vessel, rail tank car or truck – large airports usually use several routes in parallel.
3. Fuel farm at the airport: Receipt, testing, storage. This is where it is decided whether a delivery is released.
4. Distribution: via the hydrant network or by filling the fuel trucks.
5. Into-plane: the actual refueling at the aircraft by a service provider.

Checks are made at every stage, above all for water and particles. Water is unwanted in an aircraft tank because it can freeze and give microorganisms a habitat. Filters in fuel farms and vehicles hold back dirt and free water, and samples are taken and inspected regularly. Many operators follow the standards of the Joint Inspection Group (JIG), a set of rules that, by its own account, over 2,750 facilities worldwide adhere to.

When the chain stalls

How sensitive this logistics chain is showed in October 2026 in Frankfurt. According to media reports, Lufthansa took on more fuel at other airports as a precaution because the capacity of a supply pipeline was restricted; Fraport pointed to routine maintenance on that line as well as to other supply routes and large fuel farm capacities. In addition, reports mentioned low Rhine water levels, at which tanker ships can only sail partially loaded. Such situations are rare, but they show why airports spread their supply across several routes.

Tankering and the EU rules

Tankering means carrying more fuel than needed for the flight so that less has to be loaded at the destination – for example when jet fuel is more expensive or scarce there. The problem: extra weight increases fuel burn and thus emissions. The EU regulation ReFuelEU Aviation therefore requires airlines, in principle, to load the fuel needed for the flight at EU departure airports.

The same regulation prescribes minimum shares of sustainable aviation fuel (SAF) for fuel suppliers: 2 percent from 2025, rising to 70 percent in 2050, with a separate sub-quota for synthetic fuels. Airports must provide the infrastructure so that SAF can be stored and loaded. For you at the fence, little changes: SAF is blended into fossil jet fuel and flows through the same lines and hoses. Approved production pathways may be blended in at up to 50 percent, depending on the process.

Recognizing refueling while spotting

At the fence, refueling is often easier to see than you’d think. Here’s what to look for:

• Vehicle under the wing: A vehicle with a platform between the fuselage and engine, usually on the right or left under the wing, is almost always the refueling service.
• Ground hose: If a hose runs from the vehicle to a cover in the concrete, it’s a hydrant dispenser. If there isn’t one and the vehicle is large and bulbous, it’s a fuel truck.
• Position: At gates with boarding bridges, hydrants are common; at remote stands and for freighters on special areas, you see fuel trucks more often.
• Order in the turnaround: Refueling runs in parallel with other work and is usually finished before pushback begins.
• What runs alongside: While fuel is loaded, ground power or the auxiliary power unit supplies the aircraft – more on this in the article APU, ground power and PCA.

Your best chances are on observation decks overlooking the gates, such as the Hamburg observation deck at Hamburg Airport, or at viewpoints overlooking the apron like the Munich Airport visitors’ hill. You get really close on an airport tour or apron tour, where refueling is often explained specifically. If you want to work on the apron yourself someday, the article Working on the apron gives an insight into the jobs around ground handling.

What to keep in mind

• Keep your distance and follow the rules: Photographing from the public area is one thing; the apron and security area are off limits. What’s allowed at the fence is in the article Is plane spotting allowed?.
• Not every truck at the wing is refueling: De-icing vehicles also work with platforms at the wing – in winter, a second look is worthwhile. More in the article on de-icing aircraft.
• Treat numbers with caution: How much an aircraft loads depends on route, load and weather and varies widely. There are no blanket liter figures per flight.
• Procedures are local: Which stands have hydrants, who carries out the refueling and which conditions apply is regulated by each airport itself.

Conclusion

At the fence, aircraft refueling looks like a side issue, but it stands at the end of a long chain of refinery, transport, fuel farm and pipe network. At large airports, the jet fuel comes straight out of the ground through the hydrant system, and a compact dispenser only makes the connection to the wing. Where there is no hydrant, the fuel truck pulls up. Fuel is loaded under the wing by pressure refueling, with strict rules on bonding, escape routes and communication – even when passengers are already on board.