Aircraft Fuel Pump – All concepts you need to know and 5 Important Classifications

Aircraft Fuel Pump

Image Credits: Image by Aviavlad from Pixabay

The subject of discussion: Aircraft Fuel Pump and its various types

Aircraft Fuel Pump

Who doesn’t love to fly? A journey in an aircraft is an exhilarated experience, more so when you have an aerospace engineer within yourself. The subject that we will specifically discuss here is- What is an Aircraft Fuel Pump? Sounds intricate, but we will go step-by-step for all our readers to understand. Please put on your seat belts, and let’s start our journey.

The Aircraft Fuel Pump is a component specific to the aircraft fuel system. Hence, let us first learn what an aircraft fuel system is.

What is Aircraft Fuel System? | Aircraft Fuel System Design

The crew can use an aircraft fuel system to pump, manage, and deliver aviation fuel to the aircraft’s propulsion and auxiliary power units (APU). Because of the variable performance of the aircraft in which they are placed, fuel systems vary substantially. A single-engine piston aircraft has a primary fuel system; a tanker (such as the KC-135) can also distribute gasoline to other aircraft in addition to managing its own fuel.

Fuel is delivered to a fuel control valve via fuel lines (usually known as the fuel selector). This valve has several functions. The first is that it serves as a gasoline shutoff valve. The pilot can choose which tank feeds the engine in the second function. The pilot can choose between the left and right tanks in many aircraft.

The shutoff function is a separate valve situated after the fuel selecting valve in some aircraft. A gascolator is a fuel filter that may be opened on the ground and drained of gasoline contaminants heavier than petroleum, primarily water and silt, following the selector valve—located at a low position in the fuel stream.

Regardless of the aircraft’s operating conditions, each aircraft fuel system must store and distribute clean fuel to the engine(s) at a pressure and flow rate capable of sustaining operations. Due to the relative size and complexity of the aircraft in which they are installed, fuel systems vary substantially from one aircraft to the next.

A fuel system consists of a single gravity feed gasoline tank and the fuel line that connects it to the aircraft engine in its most basic form. The fuel system in a contemporary, multi-engine passenger or freight aircraft is likely to be made up of many fuel tanks situated in the wing, fuselage (or both), and, in some circumstances, the empange. Internal fuel pumps and accompanying valves and tubing will be installed in each tank to feed the engines, allow for refueling and defueling, isolate the various tanks, and, in some cases, allow for fuel dumping or the optimization of the aircraft center of gravity.

So let us propel back to our question…

What is an Aircraft Fuel Pump?

Aircraft Engine Fuel Pump

Aircraft Fuel Pump is the fuel feeding element of the aircraft fuel system in the case of low-wing aircraft designs, where the fuel tanks cannot be mounted high enough above the level of the carburetor and gravity cannot be utilized to develop a simple and efficient gravity flow system.

aircraft fuel pump
Aircraft Fuel Pump : NR-53B fuel pump and engine governor of the AL-21F-3 jet engine; Image Credits: LekseyNR53B 1967CC0 1.0

What is the use of Fuel Pump in aircraft?

Man’s most exemplary efforts will never be able to match nature’s simplicity and efficiency. As a result, it’s not unexpected that installing an aircraft fuel pump only partially solves the problem of delivering fuel to an engine without the assist of gravity. Gravity is impervious to failure, but aircraft fuel pumps are. As a result, you’ll need to add a backup pump of some sort to protect yourself against that possibility.

You now have two pumps, but how do you know if they’re working correctly? To get that information, you’ll need to install a fuel pressure gauge or a fuel flow meter (which is essentially the same as a gasoline pressure gauge). That’s it—just a smidgeon of the additional complexity that distinguishes a basic aircraft fuel pump system from a gravity flow gasoline system.

We shall learn about the fuel pressure gauge in other sections to come.

Aircraft Fuel Pump System | How does an aircraft fuel pump work ?

The gasoline pump system is quite similar to the gravity flow system in every way. The fuel tanks are the starting point for both systems. Fuel distribution begins when the fuel goes through a finger screen covered outlet in the fuel tank’s bottom. Fuel flows from the tank to a convenient fuel selecting valve in the cockpit via an aluminium line (at least 3/8″ in diameter).

After passing through the tank selector valve, the gasoline is sent to the primary filter, also known as the “gascolator,” which we’ve already discussed. The gascolator is usually found on the firewall and should be the fuel system’s lowest component. It is always equipped with a rapid drain valve, which allows the entire fuel system to be drained at once.

During the preflight examination, the gascolator also allows draining some fuel to check for the presence of water. Without removing any cowlings or covers, the rapid drain should be immediately accessible. After passing through the gascolator, the filtered fuel enters or bypasses a backup pump. Typically, this is an electric pump, although it might also be a hand-operated wobble pump.

Finally, the fuel reaches the engine-driven pump, which is the heart of the fuel pump system. This mechanical engine-driven pump is fastened directly to an auxiliary pad on the engine crankcase, from where it supplies fuel to the fuel injector or carburetor under pressure. Although the engine-driven pump is the primary fuel pressure source, aircraft produced under an Approved Type Certificate are required to have a backup auxiliary fuel pump installed.

Because these gasoline pumps must provide enough pressure to transport fuel from the tanks to the carburetor or fuel injector, you must have some way of determining if the appropriate pressure is being produced. As previously stated, this minor issue is resolved with the installation of a fuel pressure gauge.

Fuel Pressure Gauge Installation

Do jet engines require really high pressure fuel pumps and if so how much pressure do they need?

The fuel pressure gauge to be installed should be calibrated to fit the system’s fuel pressure range. A fuel injector, for example, requires a standard fuel pressure of around 24 psi, whereas a pressure type carburetor requires no more than 15 psi. A fuel pressure gauge capable of reporting far greater pressures than those required for the installation, on the other hand, may not be as accurate as one calibrated for a smaller fuel pressure range.

There’s one more thing to think about. Individual fuel pressure gauges are typically 2-1/4″ or 2-1/16″ in diameter (automotive type). As part of an “instrument cluster,” fuel pressure gauges are also available. These are extremely common, and most aircraft builders employ them.

After the gauge is installed in the instrument panel or a more accessible location, if necessary, an aluminium tube with standard AN fittings connects it to the carburetor or fuel injector.  As a result, a 14″ or even a 3/16″ aluminium line should work. Connect the fuel pressure line to the port in your fuel injector or carburetor housing using a restrictor style fitting. The fuel pressure line within the engine compartment should be a flexible aircraft hose built with conventional metal fittings, rather than an aluminium line from the fuel pressure gauge to the firewall bulkhead fitting.

Aircraft Engine Driven Fuel Pump | Aircraft Mechanical Fuel Pump

During regular engine operation, the engine-driven (mechanical) aircraft fuel pump automatically distributes the correct amount of fuel to the nearby fuel injector or carburetor. The well-known AC diaphragm-type aviation fuel pump is regarded as the industry standard for most small aircraft engines. It’s a self-priming pump with specially constructed diaphragms that don’t appear to be influenced by the numerous unique chemical qualities that make up today’s fuels.

When the proper engine-driven fuel pump is fitted, it should be able to deliver a minimum fuel flow of 125% of what is needed for maximum take-off power. This surplus capacity will not be a problem since an internal relief valve prevents the development of excessive fuel pressure. The internal relief valve is factory calibrated to supply the fuel at the correct fuel inlet pressure for a certain carburetor or fuel injector installation.

Engine-driven fuel pumps have a fantastic track record of reliability, although they do fail. The diaphragm most usually ruptures, causing fuel to shoot out of the drain line. If such a failure occurs, it is most typically believed to be due to old age rather than a lack of resources. If the diaphragm ruptures and the vent is not attached to a line that is routed away from the hot exhaust pipes to a safe overboard area, a failing engine-driven fuel pump might provide a significant fire hazard. Furthermore, the engine will not start without the assistance of a backup fuel pump.

Even if the engine’s main pump fails, an auxiliary pump can keep your machine operating. This is feasible because AC-type engine-driven fuel pumps feature an internal bypass valve that permits fuel to flow through the pump even after it fails.

Aircraft Auxiliary Fuel Pump | Booster Pump in Aircraft Fuel System | Aircraft Electric Fuel Pump

An auxiliary pump, booster pump, electric pump, or even a wobble pump is a backup by any other name. They all have the same purpose: to assist the engine-driven pump or, in extreme instances, to replace it totally. A self-contained electric motor powers the auxiliary fuel pump- a pilot-controlled by a switch on the instrument panel.

Purpose of Booster Pump in Aircraft Fuel System

Auxiliary or boost pumps can be used for a variety of purposes, including:

  1. It is used in the application of priming a fuel-injected engine before starting.
  2. Whenever the engine-driven pump fails or cannot maintain enough fuel flow, it is used to restore fuel supply to the engine.
  3. It’s being utilized to combat vapor formation, especially at altitudes exceeding 10,000 feet.
  4. Assisting in the restart of the engine when the pilot of your jet has let one of your fuel tanks run dry.
  5. Using the boost pump during take-offs and landings as a safety precaution.

When fitted, the backup pump is usually connected in series to the engine-driven pump. The boost pump must have an internal bypass valve to allow gasoline to pass through it whether or not it is turned on in order to be installed in this manner. If an electric boost pump is connected to the engine-driven pump without an internal bypass valve, every time the boost pump is turned off, all fuel flow to the engine-driven pump is blocked because no gasoline can run through it unless it is switched on.

Internal bypass valves are not standard in small low-pressure electric pumps used by builders for carburetor-equipped engines. When used to augment an engine-driven pump, these pumps must be connected in series with the engine pump. A parallel system may additionally require one or more one-way check valves, depending on the installation, to ensure that the fuel flows only toward the engine and not back to the tank while the electric pump is functioning. In any case, a parallel system will always be more difficult to deploy than an inline system.

The Wobble Pump

Sport aerobatic pilots use the original wobbling pump as a backup aircraft fuel pump. It performs comparable services to an electrical auxiliary pump, such as assisting in starting the engine and maintaining fuel pressure on demand. However, it is operated manually by the pilot and does not require an electrical system. The installation of a wobbling pump is similar to that of any other internal bypass boost pump. That is, it, too, can be connected in series to the engine’s primary fuel line.

The improved new Christen manual gasoline pump replaces the outdated WW-II surplus D series wobbling pumps, which are becoming scarce. The Christen manual fuel pump installation is substantially lighter, with a single compact device containing a fuel valve, fuel filter, and gasoline pump.

Types of Fuel Pumps in Aircraft | How many types of Aircraft Fuel Pump are there?

There are typically 5 different types of Aircraft Fuel Pumps in Aircraft, namely- Hand-Operated Fuel Pumps, Centrifugal Boost Pumps, Ejector Pumps, Pulsating Electric Pumps, and Vane-Type Fuel Pumps; all of which have been described below:

Hand-Operated Fuel Pumps

Hand-operated aircraft fuel pumps were installed in some vintage reciprocating engine airplanes. They’re utilized to supplement the engine-driven pump and to move fuel from one tank to another. These are essentially wobble pumps are double-acting pumps that provide fuel with each stroke of the pump handle. They’re essentially vane-type pumps with bored channels in the center, allowing a back-and-forth motion to pump the gasoline rather than a full rotation of the vanes, as is customary with electrically or engine-driven vane-type pumps.

While straightforward and with minimal room for error, a hand-operated pump necessitates the installation of fuel lines from the cockpit to the pump, posing a risk that can be avoided by using an electrically powered pump. Modern light reciprocating-engine aircraft typically have electric auxiliary pumps. However, for priming the engine(s) at startup, they frequently use a bare hand pump. When the priming knob is pulled aft, these simple devices are single-acting piston pumps that suck fuel into the pump cylinder. Fuel is pumped through lines to the engine cylinders when the lever is pressed forward.

Centrifugal Boost Pumps

The centrifugal pump is the most frequent type of auxiliary fuel pump found on airplanes, massive and high-performance aircraft. It is powered by an electric motor and is usually immersed in the gasoline tank or slightly outside the bottom of the tank, with the pump’s inlet extending into the tank. A pump removal valve is often installed if the pump is situated outside the tank, allowing the pump to be removed without depleting the gasoline tank.

A variable displacement pump is a centrifugal boost pump. As the impeller revolves, it takes in fuel at the center and expels it to the outside. A check valve in the output stops fuel from returning to the pump. The pump outlet is connected to a fuel feed pipe. If the boost pump is not working, a bypass valve can be put in the fuel feed system to allow the engine-driven pump to pull fuel from the tank.

Depending on the phase of aircraft operation, some centrifugal fuel pumps run at multiple speeds, as set by the pilot. Fuel pumps with only one speed are also prevalent. Regardless of temperature, altitude, or flight attitude, centrifugal fuel pumps in fuel tanks maintain positive pressure throughout the fuel system, eliminating vapor lock.

Because the electric motor is submerged in the fuel, submerged pumps feature fuel-proof coverings. Centrifugal pumps positioned outside the tank do not need this, but they do require some type of input in the fuel. This could be a tube with a shutoff valve that allows the pump to be replaced without empty the tank. A screen covers the input of both types of centrifugal pumps to prevent extraneous debris from being ingested.

Ejector Pumps

Fuel tanks with in-tank fuel pumps, such as centrifugal pumps, are designed to keep a constant fuel supply at the pump input. This prevents the pump from caving and guarantees that the fuel cools it. Baffles, including check valves, also known as flapper valves, can be used to divide the section of the gasoline tank designated to the pump installation. This allows gasoline to flow inboard to the pump during maneuvers, but it cannot flow outboard.

Ejector pumps are used in some airplanes to ensure that liquid fuel is always present at the pump’s inlet. Pump outflow is circulated back into the tank area, where the pump is positioned via a small diameter pipe. A venturi, which is part of the ejector, directs the fuel. Low pressure is created as the fuel rushes through the venturi.

Fuel can be sucked into the ejector assembly and pumped into the fuel pump tank section through an intake, or line, that starts outside of the tank pump region. In conjunction with baffle check valves, Ejector pumps maintain a positive head of fuel at the pump’s inlet.

Pulsating Electric Pumps

Smaller, less expensive auxiliary fuel pumps are frequently used in general aviation aircraft. The pulsing electric pump, often known as the plunger-type gasoline pump, is widely used. On bigger aircraft, it is typically utilized in the same way as a centrifugal fuel pump, except it is positioned downstream of the fuel tank outlets.

The engine-driven fuel pump is routed in parallel with the pulsing electric fuel pump. It provides fuel before the engine-driven fuel pump comes up to speed during startup, and it can also be used as a backup during take-off. It can also be used to prevent vapor lock at high altitudes. A plunger draws fuel into and pushes gasoline out of the pulsing electric pump. The plunger is moved back and forth in a pulsing motion by a solenoid that alternates between being electrified and de-energized.

The pressure of the fuel at the pump’s outlet controls the single-acting pulsing electric fuel pump. The pump cycles quickly when gasoline is required, with low pressure at the pump outlet. The pump slows as the fuel pressure rises because the calibrated spring encounters resistance while trying to propel the piston upwards. The plunger’s motion is dampened by a spring in the center.

Vane-Type Fuel Pumps

The most prevalent types of fuel pumps seen in reciprocating-engine aircraft are vane-type fuel pumps. They can be used as primary gasoline pumps as well as auxiliary or boost pumps. On the other hand, the vane-type pump is a constant displacement pump that transports a continuous volume of fuel with each revolution of the pump. An electric motor rotates the pump shaft when it is utilized as an auxiliary pump. The auxiliary gearbox often operates the vane pump in engine-powered systems.

Aircraft Fuel Pump: Rotary Vane-type; Image Credits- Rainer BielefeldRotary vane pumpCC BY-SA 3.0

An eccentric rotor is driven inside a cylinder, as is the case with all vane pumps. Vanes glide in and out of slots on the rotor, which are kept against the cylinder wall by a central floating spacer pin. The volume gap generated by the cylinder wall, the rotor, and the vanes increases and then reduces as the vanes revolve around the eccentric rotor.

The gasoline is taken into the pump through an inlet port where the vanes produce an increasing volume space. The space generated becomes smaller as the rotation progresses. Fuel is pumped out of the cylinder by an outlet port positioned there. The engine’s fuel metering mechanism supplies more gasoline than it needs to run. A vane pump’s consistent volume, on the other hand, can be excessive.

Most vane pumps contain a pressure relief feature that may be adjusted to regulate flow. It works by using the pressure built up at the pump’s outlet to pull a valve off its seat, allowing excess gasoline to flow back to the pump’s inlet side. The correct fuel volume is given by placing the relief at a pressure above the engine fuel metering device’s air intake pressure.

Fuel must flow through the pump to the fuel metering device while the engine is starting or if the vane pump is not working. A bypass valve inside the pump is used to accomplish this. When the pump’s inlet fuel pressure is higher than the output fuel pressure, a weakly sprung plate under the relief valve overcomes spring pressure.

When the vane pump is the engine-driven primary fuel pump, compensated vane-type fuel pumps are employed. As the air entry pressure of the fuel metering device changes owing to altitude or turbocharger outlet pressure, the relief valve setting modulates automatically to provide the correct delivery of fuel. The inlet air pressure source is connected to a vent chamber above a diaphragm coupled to the relief mechanism. The diaphragm assists or resists the relief valve spring pressure when air pressure changes, resulting in proper fuel delivery for the current condition at the fuel metering device.

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About Esha Chakraborty

I have a background in Aerospace Engineering, currently working towards the application of Robotics in the Defense and the Space Science Industry. I am a continuous learner and my passion for creative arts keeps me inclined towards designing novel engineering concepts.
With robots substituting almost all human actions in the future, I like to bring to my readers the foundational aspects of the subject in an easy yet informative manner. I also like to keep updated with the advancements in the aerospace industry simultaneously.

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