Got Gas?

I left off a discussion of the fuel containment system noting that the fuel indication system was a topic for another blog entry. You may have been sceptical about the idea of an entire blog entry on the gas gauges. It's somewhat more complex than a little needle over an arc with an F at one end and an E at the other, like in my car.

It starts even more simply than that, with an aluminum dipstick. I'm not sure why "dipstick" is such an insult, this is a state-of-the-art, custom made piece of high technology. When the airplane is on level ground and the boost pumps have been turned off for at least 15 minutes the levels of the different fuel cells can be assumed to be equal. The dipstick is calibrated so the wet mark shows the total weight of JP-4 or Jet B fuel in the tank. (Other grades are denser, so the same level equates to a greater weight of fuel. You'd have to multiply by the relative density to convert.) It's tricky to actually read the level of jet fuel that shows on an aluminum dipstick. Another aviatrix taught me to breathe heavily on the dipstick, creating a sheen of condensation, before putting it in the tank. The fuel level shows up clearly after you remove it. I may have to come up with another system in hot climates.

There is a fuel capacity probe in each of the eight fuel cells that determines the fuel quantity presence based on capacitance. Not capacity: capacitance. A capacitor is a device that stores energy as an electrostatic field between two conducting plates separated by a non-conducting material. Its ability to store energy is called its capacitance. The property of an insulating material that increases capacitance over that of a vacuum is called its dielectric constant. So change the material separating the plates and you change the capacitance. In a fuel capacity probe, the size and separation of the plates remains constant, and the capacitance varies with the fuel or lack thereof in the space between them. So a change in the amount of fuel in a tank changes the capacitance of the probe-fuel-probe capacitor and induces a current, which drives the fuel gauge.

I wouldn't have thought that this would serve as any better a method of measuring quantity than a level detector, but I would have been wrong. Remember the airplane may be fuelled with any of a number of different turbine fuels, of different densities, and the density of the same fuel will change with temperature. Apparently, the dielectric constant is related to density, such that the fuel gauges are calibrated in pounds and read accurately regardless of the grade ortemperature of fuel used. I understand that this may not be completely linear and that some temperature corrections may be required. I lack some information here.

The fuel gauges run off the 115V AC bus. If power to the gauge is lost, the needles do not fall to zero, but remain at the same indication. Pilots say "DC dies and AC lies" to remember the behaviour of failed instruments. You can test the calibration of the gauges by pressing the IND TEST button near the fuel selector. The needle should fall to the zero indication and then return to the same fuel indication.

The AC system is backed up by DC-powered low level warning lights that illuminate when the fuel level falls below 75 pounds in the forward tank or 110 pounds in the aft. If the ejector system has failed and the collector cell has the same level as the other cells, the low level sensors will give false indications and trigger at 330 pounds in the forward tank or 440 pounds in the aft tank. Good test question, that.

There's also a fuel flow meter for each engine, calibrated in pounds per hour. Each is powered from the 26V AC bus, and is protected by a half amp fuse. Sometime I'll tell you a story about why I will never forget that it's a half amp fuse.