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Anchor 8

Fuel Tank Unit

 

• The capacitance tank unit serves as the sensing element

   for each subsystem

 

• Each unit consists of two metallic cylinders, a terminal

   block, and two mounting brackets

 

• The cylinders form a capacitor to provide a capacitance

   that varies with fuel level between the cylinders

 

• The length of each tank unit is determined by the height

   of the fuel tank at its particular location

Capacitance Probe in Action

Most modern aircraft use one or more fuel capacitance probes hooked up to a computer to produce a number that gets reported to you as fuel capacity. The technology has improved greatly over the years but leaves most of us with a standard pilot answer when asked how it all works: "it's magic."

 

 

 

The fuel quantity probes are capacitors. The capacitance of the probes changes as the dielectric constant of the insulating medium between the capacitor electrodes changes. The dielectric in the probes is either:

 

 

 

  • Liquid fuel

  • Mixture of air and fuel vapor

  • Combination of the above two

 

When the fuel tanks are full, the capacitance of the probes is greater than the probe capacitance when the fuel tanks are empty. Changes in fuel level cause a change in probe capacitance. This capacitive signal is supplied to the [Fuel Quantity Signal Conditioner].

 

 

 

A single capacitance probe is basically a hollow metal tube with another tube in the middle. They are held in place by insulators to ensure they do not touch. A hole at the bottom allows fuel to flow inside; a hole at the top allows air to enter as well. Small, known electrical charges are placed on these tubes and the capacitance is measured. The value changes with the ratio of fuel to air.

In practice, several capacitance probes are placed throughout the tank so a better sampling can be taken. Then a computer checks these against known values to come up with a fuel quantity. The probes can be fooled by the density of the fuel and some aircraft include density measuring devices to adjust the reported figure.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

FUEL TANK INDICATOR 

Tank content is displayed on gauges and these should be reasonable correct when the tanks are empty. Compare the indications of the gauges when dipping the tanks so accuracy can be checked. Remember that during some maneuvers, slipping skidding and in turbulence, tank indications can and will vary accordingly. If the aircraft is equipped with a flow indicator its indications should be checked against the level of fuel in the tank after the flight and the expected calculated consumption.

 

 

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