Thank you Marty. Very informative and makes me wonder how some technologies don't seem to advance as much as others. You would think that in today's world we could hope for better.
The problems of measurement of any quantities of any kind whatsoever is
an art and science all to itself.The problems of reliably indicating the amount of fuel present in tanks of varying sizes and shapes under wildly varying conditions of temperature, vibration, and turbulence are not trivial.
Besides designing some kind of ideal sensors that are reasonably priced, that can survive in a hostile environment, and that accurately model the amount of fuel in a tank of a particular shape, that system needs to be calibrated and maintained in calibration for a particular aircraft:
"Aircraft Fuel Gauge Accuracy."If we imagine a perfectly uniform container (a cylinder or a cube) on a laboratory table, all you need is a visual ruler (a float, a set of lines etched on the class) to know with a reasonable degree of precision how much liquid is in the container.
Now make that container into an airfoil shape so that it will fit into a wing section. Or cast it into the form of a tip tank. Set the contents in motion in that irregularly-shaped tank. Let the fuel foam and slosh forwards and backwards and undergo plus and minus G forces not only up and down but from side-to-side in various trajectories (a coordinated turn versus a slip, for example).
How many of these multiple forces and conditions will the model used by your system take into account? With you equip it with accelerometers and gyros (more parts that can fail!) to smooth out errors caused by these variables? How often will you update the display if it is digital? By what means will you transmit the information if the system is analog? What kinds of errors are introduced into the system by the methods of transmission of information that you choose?
What kind of safety margins do you want to build into your system? Do you want to deliberately err on the side of caution, so that when the gauge reads "E" there is still a little bit of usable fuel left? Or do you want the prop to stop turning at the same time the needle hits bottom?
Gasoline expands when it is hot and contracts when it is cold. Will you add a temperature correction to your system so that it accurately reflects fuel by weight instead of by volume?
Suppose you decide to use a float as your basic gauge. When the tank is full, the float is highly accurate. No float can keep floating when the gas gets low. At some point, the float hits bottom while there is, perhaps, some usable fuel still in the tank but not enough to keep the float up. How much gas is that? Should you put the float into its own cylinder that goes deeper than the bottom of the tank? Should you try to isolate it from the wave action in the tank? By what means do you measure and report the height of the float? Do you average a set of readings periodically or simply let the gauges shiver with the changing conditions?
I am not a pilot.
I fly radio-controlled airplanes.
I read a lot about aviation.
I'm not sure that I've indicated all of the variables correctly.
I imagine that today's gauges are, on average, better than those of 1937. Persuading pilots to use all of the information at their disposal to make sure that they don't run out of gas until after landing seems to be one of the best ways to improve aviation safety, but there seems to be room for improvement.
A friend of mine--an A&P and a CFI--took off on a short hop in a Taylorcraft to reposition it for delivery to a customer. He ran out of fuel very shortly after takeoff, tried to turn back to his runway, and destroyed the plane among some trees. To this day, he swears that there
should have been five gallons in the aircraft and he is convinced that someone stole the gas. Nevertheless, he should have stuck a stick in the tank before he took off--the most primitive and effective method of making sure that there is gas available at the start of a flight (as a general rule--I guess that even sticks can be deceived).
I am an armchair pilot. I serve in the battalion of Captain Hindsight. I survey the results of others' mistakes. I know what they should have done differently. I'm not putting my money where my mouth is. You may take the things I say with as much salt as your doctor allows in your diet.