07-28-2013, 05:43 PM
In small planes you use a dipstick or sight tube before you take off. Then you monitor fuel flow. Newer planes have totalizators of the actual fuel used but unless you have dipped the tanks before takeoff these are not helpful. You have to know what's actually in the tanks before starting your flight.
Turbine aircraft usually receive a slip showing how many pounds of fuel were pumped into the tank. But older ones still rely on dipping. More than one passenger jet has run out of gas because the ground crew miscalculated a conversion from metric to English or vice verse.
Gentlemen,
For the last 35 years I have been a commercial/instrument rated pilot in both fixed wing and helicopter aircraft. I also worked as the airshow demonstration pilot for the Stoddard-Hamilton Aircraft Company. Over the last 3.5 decades I have flown everything from hot air balloons to supersonic jet fighter trainers, well over 100 different types of aircraft and I have never used a dipstick nor seen anybody use a dipstick to check fuel levels in modern (post-World War II) light aircraft. 95+ percent of aircraft carry the bulk of their fuel in the wing (the wings are the main fuel tank) and because of it's dihedral (the shallow V shape) it is not even possible to use a dipstick to check fuel levels in most aircraft wings (the straight top wing on an Aviat Husky or a Cub would be an exception here). It is possible to use a dipstick in the fuselage tank of an old biplane like the Great Lakes or the early model Pitts Special. Perhaps this is where Stretch observed such a procedure.
A sight gauge is a clear plastic tube attached to fittings on the top and bottom of an aircraft fuel tank. They can only be used on a fuselage tank mounted directly in front of the pilot like later model Pitts Special aircraft or on high wing aircraft like the Aviat Husky that do not have any wing dihedral. As the fuel level in the tank drops the level in that visible plastic tube drops. The sight gauge does not give a precise reading of fuel on board but it does give a reliable indication of approximately how full an aircraft fuel tank is as there is nothing mechanical to fail.
The vast majority of light aircraft have a fuel filler cap on both the left and right wings. Descending down below that gas cap into the wing tank itself is an aluminum tab that sticks partway down to the bottom of the tank, generally with a slot cut into it somewhat higher up. The maximum amount of fuel the wing will hold is clearly labeled outside that cap. The amount of fuel held in the wing if it is filled to the slot cut in that tab is also labeled, and the amount of fuel held in the wing if it is filled to the bottom of the tab is labeled as well.
In my Glasair III for example, when filled to the top each wing will hold 32 gallons. When filled to the slots each wing will be carrying 25 gallons, and when filled to the bottom of the tabs each wing will be carrying 15 gallons. This exact same system is used on Cirrus, Beechcraft, Piper, and Cessna aircraft. Many turbine and jet aircraft use this system as well.
When instructing line personnel on how you want your aircraft filled you tell them you want it filled either to the tabs, to the slots, or to the top. Before taking off the pilot will always visually verify that the aircraft was filled per his instructions. In my case, I fuel my aircraft myself to eliminate any chance of line personnel doing it incorrectly.
I have never flown an aircraft with a fuel gauge that I would trust. Even moderate turbulence has fuel sloshing around in the wing and it's not uncommon to fly with one wing slightly low which results in an inaccurate reading on the fuel gauge. After spending a ton of money trying to get the fuel gauge in my current aircraft to work with an acceptable degree of accuracy, I finally gave up and removed the gauge and the sending units. I now use a fuel flow meter (the totalizator that Stretch described) that very accurately measures the amount of fuel used. These are also called fuel computers. At the start of a flight the pilot must program in the number of gallons in the tank being monitored. Almost every aircraft has more than one fuel tank so if you run out of fuel in one you can simply switch to the other. So if both of my wing tanks are full I program in 64 gallons. As I fly the fuel computer will tell me how many gallons I have burned, how many gallons remain, and how much time I have until I run out at the current fuel flow rate. So if I'm flying along burning 18 gallons per hour and there is 36 gallons left in my wing, the fuel computer will tell me that I will run out in two hours.
This device is incredibly accurate, to within a fraction of a gallon, BUT..... as Stretch pointed out, you must know how much fuel you are starting out with before a fuel computer will do you any good.
There are a lot of older aircraft without fuel computers that still have fuel gauges. No pilot relies upon them. When I started flying back in the 1970s I would make note of how much fuel is on board by opening the caps and looking at those tabs, then I would keep track of hours flown, and fuel burn rate per hour to determine how much fuel I had remaining at any given time. The fuel computer newer planes have does that calculation for you.
The bottom line is, if fuel gauges in million dollar aircraft aren't worth a damn and are not relied upon, it's probably too much to expect for a fuel gauge to be super accurate in a $10,000 motorcycle. After a few tanks of fuel a rider will know pretty accurately how many miles per gallon he can expect. With that knowledge the trip odometer on your motorcycle is almost as accurate as the fuel computer in a modern light aircraft. A gauge reading is only a relative indication. At least it will tell you if your neighbor siphoned your tank dry last night while you were asleep. If you know that your bike gets 45 miles to the gallon, and your tank holds 3.9 gallons, you know you'll run dry in 175 miles. If your trip odometer shows you have traveled 150 miles since your last fill up, you know that you will run out in almost exactly 25 more miles. So if the next station is 15 miles up ahead you're probably in pretty good shape.
Using your trip odometer will keep you from having to push your beautiful CB1100 and lower your stress level when that last bar starts to flash. All the best.
Chip