Sylvie Willis
Fuel gauges are necessary to indicate the quantity of fuel remaining in the tanks during an aircraft's operation. The four general types of fuel gauges are: (1) Sight glass, (2) mechanical, (3) electrical, and (4) electronic. The type of fuel gauge installation depends on the size of the aircraft and the number and location of the fuel tanks. Aircraft fuel tanks are commonly located in the wings instead of the main body or fuselage of the aircraft. This improves the overall structural efficiency by counter-balancing the wing's lift and the fuselage's weight, and improves passenger safety by locating fuel away from the passenger compartment. Some manufacturers use float-operated, direct-reading fuel gauges mounted on the lower surface of the wing.
| Characteristics | Values |
|---|---|
| Types of fuel gauges | Sight glass, mechanical, electrical, and electronic |
| Installation | Depends on the size of the aircraft and the number and location of the fuel tanks |
| Location of fuel tanks | Inside the wing, in a soft bladder in the wing, or the wing itself |
| Advantages of wing-based fuel tanks | Improves overall structural efficiency, reduces aircraft fuel system complexity, and improves passenger safety |
| Disadvantages of wing-based fuel tanks | Higher risk of catastrophic damage to wings in case of in-flight fuel ignition |
| Examples of aircraft with wing fuel tanks | Mooney, Cessna 172, King Air series, Piper PA-28, Cirrus SR-2X, TBM |
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What You'll Learn
Fuel gauges in the cockpit
Fuel gauges are necessary in the cockpit so that the pilot may know the quantity of fuel remaining in the tanks during the operation of the aircraft. The four general types of fuel gauges are:
- Sight glass
- Mechanical
- Electrical
- Electronic
The type of fuel gauge installation depends on the size of the aircraft and the number and location of the fuel tanks. Sight glass and mechanical fuel gauges are not suitable for aircraft where tanks are located a significant distance from the cockpit, so larger aircraft use either electrical or electronic fuel quantity gauges.
Some aircraft are provided with a means to determine the fuel quantity in each tank when the aircraft is on the ground. This can be done through float-operated, direct-reading fuel gauges mounted in the lower surface of the wing, or under-wing bayonet gauges. There are two types of under-wing bayonet gauges: drip gauges and sight gauges. The drip gauge requires the user to proceed slowly, using a trial-and-error method to find the exact fuel level. A long, hollow drip tube is used, and it takes time to drain once filled with fuel. A substantial error in reading will be made if the diminishing drainage drip is mistaken for the steady drip that signifies that the tube is properly positioned.
The sight gauge is simpler in construction than the drip gauge and offers unmistakable visual evidence when it is properly positioned for reading. The sight gauge is a long lucite rod, protected by a calibrated tube, which terminates at the top in an exposed quartz tip. When the tip is above the fuel, it acts as a reflector. Light rays travelling up the lucite rod are deflected at right angles by the 45° surface at one side of the tip and deflected 90° again by the 45° surface.
The electronic-type (capacitance) fuel quantity gauge differs from other types in that it has no movable devices in the fuel tank. Instead of floats and their attendant mechanical units, the dielectric qualities of fuel and air furnish a measurement of fuel quantity. The tank transmitter is a simple electric condenser, with the dielectric (or non-conducting material) of the condenser being fuel and air (vapour) above the fuel. The capacitance of the tank unit at any one time will depend on the existing proportion of fuel and vapours in the tank.
It is important to have accurate fuel gauges in the cockpit for safety reasons. Fuel exhaustion accidents make up a significant fraction of the total accident rate, and they are almost entirely preventable. Pilots should insist on having accurate fuel gauges in any plane they fly.
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Float-operated gauges
The float-operated gauges are simple and reliable, providing an accurate indication of the fuel level. They are easy to install and maintain, making them a popular choice for aircraft fuel gauges. In some cases, the gauges may be calibrated in gallons or have a metal scale to indicate the fuel quantity.
In addition to their use in aircraft, float-operated gauges are also commonly used in boats and other vehicles with fuel or oil tanks. For example, the Rochester Gauge company offers replacement fuel floats for mechanical fuel gauges in boats and older vehicles. These replacement floats are made from nitrile rubber and are designed to fit specific fuel tank configurations.
Overall, float-operated gauges are a crucial component of aircraft fuel systems, providing accurate and reliable fuel level indications to ensure safe and efficient operations.
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Under-wing bayonet gauges
Drip gauges use a long, hollow drip tube that is drawn out from the lower wing surface. When the tube reaches the level of the fuel, it fills with fuel and a steady drip from a drip hole indicates that the tube is correctly positioned. The drip gauge tube may be calibrated in pounds or inches. When calibrated in inches, the reading is compared with a chart to give a reading of the fuel quantity in gallons. However, it is necessary to proceed slowly when using drip gauges, as a substantial error in reading will be made if the diminishing drainage drip is mistaken for the steady drip that signifies the correct positioning of the tube.
Sight gauges are simpler in construction and offer unmistakable visual evidence when they are correctly positioned for reading. They consist of a long lucite rod, protected by a calibrated tube, which terminates at the top in an exposed quartz tip. When the tip is above the fuel, it acts as a reflector, with light rays travelling up the lucite rod being deflected at right angles by the 45-degree surface on one side of the tip and then deflected 90 degrees again by another 45-degree surface.
The use of under-wing bayonet gauges and other fuel quantity gauges is important for the safe operation of aircraft. The FAA has emphasised the importance of preventing fuel exhaustion accidents, which are almost entirely preventable. Fuel gauges provide flight crew members with the necessary information to monitor the quantity of usable fuel in each tank during flight and help ensure a safe journey.
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Drip and sight gauges
Drip gauges use a long, hollow drip tube that is drawn out from the lower wing surface. When the tube reaches the level of the fuel, it fills with fuel and a steady drip from a drip hole signifies that the tube is properly positioned. The drip gauge tube may be calibrated in pounds or inches. When calibrated in inches, the reading is compared with a special chart to give a reading of fuel quantity in gallons. However, it is necessary to proceed slowly when using drip gauges, using the trial and error method to find the exact fuel level. In large area tanks, a large amount of fuel is represented by a fraction of an inch variation in fuel level. The long drip tubes also require some time to drain once they are filled with fuel, and an error in reading will be made if the diminishing drainage drip is mistaken for the steady drip that signifies the correct positioning.
Sight gauges are simpler in construction and offer unmistakable visual evidence when they are properly positioned for reading. They are made of a long lucite rod, protected by a calibrated tube, which terminates at the top in an exposed quartz tip. When the tip is above the fuel, it acts as a reflector, with light rays travelling up the lucite rod being deflected at right angles by the 45° surface at one side of the tip and then deflected 90° again by the opposing side. Any portion of the tip submerged in fuel will not act as a reflector, so when the fuel level is part way up the taper, a light pattern is created that is visible at the lower end of the rod. This pattern has the dimension and shape described by the intersection of the tip and the fuel. When the reflected light is reduced to the smallest perceptible point or hairline, depending on the type of gauge, the rod is properly positioned. The fuel tank quantity can then be read on the tube where it emerges from the recessed guide housing.
Sight gauges have been the subject of several service bulletins, including one regarding improper draining that led to incorrect fuel quantity indications and another about the material of the sight gauge itself. Discolouration and cracking of the plastic tube sight gauges have been reported, which may be due to incompatibility with certain types of fuel. This can lead to an inability to determine fuel quantity in the tank, possible fuel leakage, and fires inside the cockpit.
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Plastic tube sight gauges
The plastic tube sight gauge is a clear, thick, formed Tygon tubing that is placed on the same level as the tank. It operates based on the principle that a liquid seeks its own level. When the fuel reaches the level of the tube, it enters through the open top, and the steady drip from a drip hole indicates that the tube is properly positioned. The drip gauge tube may be calibrated in pounds or inches, and when calibrated in inches, the reading is compared to a special chart to determine the fuel quantity in gallons.
While plastic tube sight gauges are commonly used, there have been reports of issues with discolouration and cracking. These issues appear to be caused by the use of automotive fuel or an auto fuel/AVGAS mixture, as the original gauge plastic tubing is not compatible with automotive fuel. Discolouration and cracking can lead to an inability to determine fuel quantity accurately, potentially resulting in fuel leakage and fires inside the cockpit.
To address these issues, it is recommended to visually inspect the plastic tube sight gauge regularly, including the connection area behind the cockpit trim panels. If discolouration or cracking is observed, the gauge should be replaced with a compatible type of plastic tubing to ensure accurate fuel quantity readings and maintain safety.
Overall, plastic tube sight gauges are a simple and effective method for determining fuel quantity in aircraft tanks, but proper maintenance and compatibility checks are necessary to ensure their accuracy and safety.
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