The Anatomy of a Fuel Quantity Indication System

January 2015

Few things are more important during a preflight check or while in flight than accurately knowing how much fuel is on board. Unlike an automobile, if you run out of fuel, you can’t just simply coast to a stop. Measuring the fuel on an aircraft can be a challenging task, with some larger models having as many as seven fuel tanks located in the fuselage, left and right wings, left and right tips and left and right auxiliaries.

Getting a specific amount of fuel into the proper tanks is the job of the Load Select System. Your aircraft may have a system similar to the following Honeywell example. It consists of the RG1017 Load Select Control and the JG1208 Load Select Indicator. They are typically housed in a panel, under the wing. The Load Select Control uses numbered thumbwheel switches to select the quantity of fuel to be pumped into the tanks. The Load Select Indicator has two LCD displays. The top display shows the quantity of fuel in the tanks and the bottom display shows the quantity of fuel selected to be loaded. The Load Select Indicator is solder sealed to reduce exposure to moisture and the possibility of the ensuing corrosion.

For the pilot, two other units are called into play for the reading of fuel quantity from the cockpit. The RG55 Selector Switch is used to select a reading of any individual tank or a total of all of the tanks. This switch selects which value is displayed on the JG911 Fuel Quantity Indicator. This fuel gauge has a pointer display, showing the fuel quantity in thousands of pounds or kilograms. This indicator is provided with a test function, which temporarily drives the pointer to the empty position from whatever position it is at to verify proper system operation.

COMMON SYSTEM SQUAWKS

What could possibly go wrong with such a system? Sometimes the problems are obvious, such as when the indicator reading is way off from the known quantity of fuel. Other times, the squawks may be intermittent. For example, differing readings are obtained when the selector switch is rocked back and forth to a single position. If there is a smaller discrepancy, the problem could go unnoticed. The following are common problems seen on these components.

The Selector Switch has a large wager switch that can become intermittent if the contacts get dirty or worn. Worn switches can lose that “detent” feeling, where the switch decisively goes into a chosen position. You may have to rock the switch back and forth through a position to get a solid reading. Worn switches will also feel loose.

The Fuel Quantity Indicator may exhibit issues such as a sluggish or sticking pointer, an oscillating pointer or simply an inaccurate reading.

The Load Select Control may also experience intermittent thumbwheel switch positions and it is often the victim of corrosion around the connector.

The Load Select Indicator problems can range from an improper or intermittent display to no display at all.

Corrosion on pins or connectors can cause inaccurate or intermittent readings throughout the system. Although a repair may be needed for major failures, a simple recalibration may cure an inaccuracy.