What it Looks Like - Aircraft Fuel Sender - Mark Twombly AOPA

What It Looks Like Fuel sender - Variable Potentiometer  By Mark Twombly How do you know with confidence how much fuel is in the tanks when you taxi for takeoff? The same way you do in your car: Check the fuel quantity indicators. As every new pilot eventually comes to realize, however, fuel quantity indicators in the typical light aircraft are notoriously inaccurate except, perhaps, when they read Empty. Fuel quantity needles bounce in turbulence, often lag in indicating a depleting supply of fuel, and then seem to speed up their descent as they near the big E. This is why we visually check the fuel quantity as part of our preflight inspection. A big reason for the temperamental behavior of analog fuel quantity gauges is the fuel sender, the mechanism buried within the fuel tank that measures fuel level. Although there are several different types of analog and digital fuel senders, the type found in most light single-engine aircraft is based on a simple float system. The lightweight float is attached to one end of a thin metal arm, the other end of which pivots inside a sealed housing. The housing, which is submerged in the fuel, contains a resistance element and contact system. The float arm contacts a coil or winding inside the housing that is energized with current sent from the fuel quantity indicator on the instrument panel. Different positions on the winding represent different resistance levels. As fuel is consumed and the float descends in the tank, the float arm contact slowly moves along the winding, gradually changing the resistance in the current read by the gauge. The current reading is translated into an analog needle indication. So what happens if the electrical current somehow shorts and "leaks" into the fuel tank? No worries -- the current is too weak to ignite the fuel or even fuel vapors. One manufacturer of resistance type float fuel senders says the design life is 100,000 empty-full-empty cycles but includes this important caveat: "Actual service life may be less because of wear caused by vibration and fuel movement." Add to that the ravages of time and infrequent use that no doubt accelerate the aging process of a fuel sender . So, along with learning to take off with sufficient fuel in the tanks to complete the intended mission plus some, we soon learn to treat fuel gauges with a healthy skepticism. The best bet is to back up the fuel quantity indicators with another simple measuring device: a watch.