7 Facts On Aircraft Fuel Pressure Gauge and Pressure Sensor


What is Aircraft Fuel Pressure Gauge ?

Aircraft Fuel Pressure Gauge Definition

An aircraft fuel pressure gauge is a device that measures and reports the quantity of fuel remaining in a vehicle’s tank(s). Every gauge is made up of two parts: an indication and a detecting or sending unit. The indication in the aircraft fuel pressure gauge is visible on the dashboard, usually in the instrument cluster, while the sensory unit is in the fuel tank. There are several types of gas-gauge in general, the sensor unit will measure the fuel’s level, and the indicator will report accordingly.

Soviet Mig-15 Aircraft Fuel Pressure Gauge; Image Source: Łukasz Karolewski (Dozymetr)Soviet MiG-15 aircraft fuel gaugeCC BY-SA 3.0

It is convenient to ensure through aircraft fuel pressure gauge that the fuel system provides fuel to the fuel measuring device and pressure inside the emptying fuel injectors is proportional to the amount of fuel flowing through the injector. It is possible to accurately measure the fuel flow by monitoring the fuel pressure at the indicators. If an injector blockage develops, the values will be inaccurate since the fuel flow will be restricted, although the pressure level reading will be high. Pressure warning lights signal any abnormal activity within the fuel system in the cockpit, which we will learn about in the segment of fuel indicators.

What are fuel indicators in an aircraft?

Fuel Quantity Gauge in Aircraft

You’ll probably need to check your car’s gasoline levels as you get ready to go on a long car trip. Fortunately, your car’s dashboard has a convenient dial that displays your fuel supply. A pilot follows a similar routine inside the cockpit, but the miles flown are substantially more, and the consequences are far more costly. Furthermore, an aircraft’s fuel system is significantly more sophisticated than that of a car. Fortunately for the pilot, several fuel system indications provide a holistic picture of the system’s health.

aircraft fuel pressure gauge
A Fuel Quantity Gauge of a Light Aircraft; Image source: RobbieMcConnelFuel contents gaugeCC BY-SA 3.0

What are the four general types of fuel quantity gauges?

Types of fuel quantity gauges

The common examples of fuel quantity indicators or fuel quantity gauges in an aircraft are listed below:

  1. Simple and direct reading indicators
  2. Mechanical fuel indicators
  3. Electric fuel indicators
  4. Digital and electronic fuel indicators

Mechanical Indicators

The usage of a float in the fuel tank is the core premise of most quantity indicators. The float goes up and down in response to changes in fuel levels and a carbon materials rod is connected to the float and extends through the fuel cap to measure the level in a float type fuel indicator. The float system is used in mechanical aircraft fuel pressure gauge, but a mechanical element moves a pointer on a dial face in the cockpit. Mechanical fuel quantity indicators frequently use a magnetic connection mechanism.

If electrical power is off, aircraft with capacitance-type fuel indicator utilize a mechanical indicator system to cross checking the fuel quantities and for this system, every tank is generally equipped with many fuel measuring sticks, sometimes known as drip sticks.

The drip stick can be down by push and rotating process until the fuel start to exit from port exist on the bottom of every stick and that is the point where the top of stick and the height of the gasoline are same. The sticks are equipped with a calibrated scale. The quantity of gasoline in the tank can be determined by summing the signals of all of the drip sticks and converting them to pound or gallon utilizing the specific chart.

Electric Fuel Indicators

Direct current is used to power electric aircraft fuel pressure gauge (DC). Variable resistance in an electric circuit is used to drive a ratiometer-type indicator in these systems. The indicator moves when the float in the tank shifts. Changes in the resistance alter the electric current running through the indicator, displaying the fuel quantity on a calibrated dial.

Digital and Electronic Fuel Indicators | Aircraft Digital Fuel Gauge

Digital indicators function similarly to electric aircraft fuel pressure gauge, except they convert the signal to the displays on a cockpit instrumental head or a flat screen inside the cockpit. There are no moving parts inside the fuel tank with more sophisticated electronic fuel quantity systems, often known as fuel probes. Rather, variable capacitance transmitters are mounted in the gasoline tank’s bottom. The capacitance of each unit wavers as the fuel level occurrence and average fuel level is determined from each of the reading.

Valve In-Transit Indicator Lights

Multiple fuel tanks connected by a system of tube and valve may be found in more complicated airplanes. Each valve must work properly to minimize pressure buildup or leaks. The opening and closing of valves are their primary function and the system light will be on/off as per opening and closing of the valve.

Valves and pumps are used on aircraft with several fuel tanks to move fuel and direct it to specific destinations, such as the engines, a separate tank, or overboard during fuel jettison. Valve-in-transit lights are used on some aircraft to alert the crew if the valve is open/closed, during those light that turn off are controlled by contact in the valve.

Alternatively, annunciator lights that indicate whether the valve is open or closed are employed. Valve-in-transit and valve position indicator, or light, are situated next to the valve ON/OFF switches on the fuel panel in the cockpit and the annunciator light is  incorporated by the switch, and valve positions are graphically depicted on screen using digital display systems.

How does a Fuel Pressure Gauge work?

Aircraft Fuel Gauge Requirements

Sensors are an important aspect of most vehicles, especially when measuring the fuel level in cars and planes. A transmitting unit mounted in the fuel tank is the most frequent type of aircraft fuel pressure gauge. The sending unit has a float on an arm that permit it to move up/down as per the amount of fuel in the tank. The potentiometer, or the variable resistor, is then connected to the arm and resistance of the potentiometer change with the arm swings up with full tank and down with empty tank.

The sending unit receives an electrical charge when the ignition is turned on. Because the potentiometer’s resistance is proportional to the float’s level, and current flow is proportional to a circuit’s overall resistance, the amount of current flowing through the circuit is proportional to the amount of gasoline in the tank.  In most circumstances, a high current level indicates a full tank, whereas a low current level indicates an empty tank. This ensures that the indicator will always read empty if the sending unit fails.

Aircraft Fuel Pressure Gauge; Image Source: Image by Clker-Free-Vector-Images from Pixabay

Although newer sensor systems can also use microprocessors for faster and more precise measurements, the sensor system is relatively basic compared to other sensors currently available. The float switch, the variable resistor, and the wiper are all part of the sensing system, which allows it to determine how much fuel is in a tank. The wiper is likewise as far away from the resistor’s ground end is feasible at this stage. The indicator receives the change in current and alters the reading as a result.

Aircraft Fuel Pressure Sensor

Checking that the fuel system is giving fuel to the fuel metering device is of crucial importance. Fuel pressure monitoring can provide pilots access to an early warning of a fuel system issue.

This article talks about the common aircraft fuel pressure sensing devices below:

Bourdon Tube

A direct reading Bourdon tube is commonly used in simple light reciprocating engine aircraft. This is adjoining to the inlet of the fuel metre through a line to the backside of the aircraft fuel pressure gauge in the cockpit’s instrumental panel.

A sensor with a transducer situated at the fuel entrance to the metering mechanism that delivers electrical impulses to a cockpit aircraft fuel pressure gauge may be used in a more complicated aircraft. The fuel pressure measuring instrument depicts the auxiliary pump press. until the engine is started on aircraft equipped with an auxiliary pump for start up and backed up of the engine driven pump. The gauge shows the pressure created by the engine-driven pump when the auxiliary pump is turned off.

Diaphragm

The diaphragm is a corrugated hollow metal disc with a thin wall. Whenever pressure is applied to one side of the disc through an aperture, the entire disc expands. The movement of the compressed diaphragm can be communicated to a pointer that register the change against the scale on the device by placing linkage in touch with the other side of the disc.

Diaphragms can be sealed as well. Before sealing, the diaphragm can be emptied, leaving nothing inside. The diaphragm is known as an aneroid when this is done. Many aviation instruments contain aneroids.

Bellows

A bellows is a device that consists of several diaphragm chambers joined together. The movement of the side walls of the bellow coincides with variation of press, and a pointer connection and gearing is connected to alerting the pilot, just as it is with a single diaphragm and measurement the difference in press. with two gases, known as differential pressure, this accordion-like assemblage of diaphragms can be quite useful.

Solid-state Sensing Devices

Modern airplanes use solid-state microtechnology pressure sensors to detect the critical pressures required for safe operation. Micro-electric signals are sent by some sensors, which are translated to digital format for usage by computers. The main appliance of solid-state sensor having changing constant properties as pressure changed.

What is the gauge in the airplane used to set pressure?

Differential Fuel Pressure Gauge

The fuel metering devices compare the fuel inlet to the air inlet press and an aircraft fuel pressure gauge with bellows is generally utilized for this case. A differential aircraft fuel pressure gauge may be used on more complicated and larger reciprocating engine aircraft. Solid-state sensors and sensors with digital o/p signal or signals that are transformed to digital o/p may be utilized in modern aircraft. If the instrument gauge is equipped with a microprocessor, signal may be processed through PC and relay to the display device.

Manifold Pressure Gauge in Aircraft

The manifold aircraft fuel pressure gauge in reciprocating engine aircraft indicates air pressure in the engine’s induction manifold. This shows how much power the engine is producing. The more power an engine can create, the higher the pressure of the fuel-air combination entering it. This means that for normally aspirated engines, the maximum indication is approximately atmospheric pressure. Full power indications for turbocharged or supercharged engines are above atmospheric pressure because the air is mixed with the fuel is pressurized.

Hydraulic Pressure Gauge in Aircraft

Hydraulic systems are frequently utilized for lifting and lower landing gear, operate flying controls, apply brakes, and perform other tasks. Normal operation of hydraulic equipment necessitates sufficient pressure in the hydraulic system generated by the hydraulic pumps. On complicated aircraft, numerous different aircraft fuel pressure gauge is employed to show the status of various supports not seen on light aircraft.

Hydraulic aircraft fuel pressure gauges are frequently seen in the cockpit and at or near the airframe’s hydraulic system repair point. For indication, the system pressure is often sent electrically from sensors or computers to cockpit gauges. Maintenance employees almost always use direct reading Bourdon tube type aircraft fuel pressure gauges in remote locations.

Fuel Temperature Gauge

Whenever the fuel temp reach the value to create ice in the fuel system, especially at the fuel filter, the pilot must be alerted and resistance-type electric fuel temperature sensor is used. It can be read on a classic ratio meter aircraft fuel pressure gauge or entered into a computer for digital display and processing. If the aircraft is equipped with one, a low fuel temperature can be remedied with the use of a fuel warmer.

Fuel temperature can also be factored into fuel flow processing calculations, as previously indicated. Microprocessors and computers can rectify viscosity discrepancies that impair fuel flow sensing accuracy at different temperatures.

Oil Pressure Gauge in Aircraft

Pressure Switch

In aviation, it’s typically enough to monitor whether a particular operating system’s pressure is too high or too low, so that action can be taken if one of these scenarios occurs. A pressure switch is frequently used to do this. Whenever a specific press is reached in a system, a pressure switch will open/close the electric circuitry.

A common example of how pressure switches are used is a low oil pressure indication switch, fitted in an engine to allow pressurized oil to be applied to the diaphragm of the switch. Oil pressure will rise when the engine is started, and the press against the diaphragm is sufficient to keep the switch contacts open.

As a result, no current flows through the circuit, and there is no indication of low oil pressure in the cockpit. The pressure on the diaphragm becomes insufficient to keep the switched contacts open if there is a loss of oil pressure. When the connections close, the circuit to the low oil pressure indicator, commonly a light, is closed, alerting the pilot.

Aircraft Fuel Level Sensor

Fuel level sensors in some smaller aircraft employ the same sensing techniques as in automobiles; however, many sensors are required to cover the increased capacity of the fuel tank in larger aircraft. Because the stakes are significantly higher—fuel loss could have catastrophic consequences—aircraft must avoid the flaws found in some vehicle fuel level sensors. Furthermore, aviation fuel level sensors must be able to cope with fluctuations in altitude, making them more sophisticated than vehicle fuel level sensors. Ultrasonic or capacitance sensors are commonly used in aircraft.

Ultrasonic sensors send out ultrasonic waves that are measured by another sensor on the other side of the tank to determine the height of the fuel in the tank. However, when capacitance sensors are employed, the fuel is spent through particular vents, causing the capacitance in the sensors to vary, allowing the level of fuel in the tank to be calculated. The onboard computing system subsequently relays this information to the pilots.

Aircraft Fuel Gauge Repair

When an aircraft fuel pressure gauge fails, the transmitting unit is usually the culprit. A malfunctioning fuel gauge can be caused by the gauge, wiring, or other components. However, sending units are more likely to fail. It can take a long time to test or replace sending units because they are positioned inside fuel tanks. The fuel tank must usually be removed unless the vehicle has an access port.

If an aircraft fuel pressure gauge always reads empty, regardless of whether the tank is full or empty, the issue is most likely with the transmitting unit. Either the potentiometer is faulty, the float is trapped in the down position, the internal wiring is defective, or there is a problem somewhere else in the circuit. It may be required to get rid of the transmitting unit and examine its resistance while manually manipulating the float arm, depending on how the specific aircraft fuel pressure gauge system in a car works.

The fault could be with the transmitter unit, the dash indication, or the wiring if the fuel gauge consistently shows full. A float trapped in the up position, as well as certain wiring and indication faults, will cause an aircraft fuel pressure gauge always to read full.

Learn about Aircraft Fuel Consumption here.

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. Connect with me with LinkedIn - http://linkedin.com/in/eshachakraborty93

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