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Measuring Fuel: How Do You Measure Up?

by H. Dean Chamberlain

Reprinted with permission of FAA Aviation News

A while ago, the Short Wing Piper Club's Internet chat room had an interesting discussion about how some of the members checked fuel quantity. Some used wooden sticks or dowels to measure (dip) their tanks. Others used pipettes to measure the fuel level. Pipettes are narrow diameter tubes used to collect fluid either by suction or by being inserted into a liquid and then closing the upper end of the tube to retain the fluid. In the case of measuring fuel, one end of the tube is inserted vertically to the bottom of the tank, and the open end is blocked with a finger so that the tube retains the amount of fuel that corresponds to the level of fuel in the tank.

Some commercial aircraft fuel measuring tubes have been calibrated for special aircraft and their respective size tanks. For example, there are calibrated pipettes for the Cessna 182 with standard tanks and another for the C 182 with long-range tanks. That same company also makes a generic tube with a corresponding chart that pilots or owners can calibrate to their specific aircraft.

Some Club members used their fingers or just looked into the tanks to see if they were full. Some trust their fuel gauges. Others check their fuel level and keep good records of their aircraft's fuel burn per flight hour. They base their flight planning on both a known fuel quantity and engine run time.

What made the discussion interesting were the possible errors and limitations of the various methods. For example, many of us have flown aircraft where a wooden stick was used to measure fuel level. The technique is simple. Take the "calibrated" fuel stick, stick it in the tank, and rapidly remove it and note the wet area. The assumption is the height of the wet area corresponds to the depth of the fuel. If the stick has been properly calibrated and marked either in inches or gallons, the data can be used to figure the amount of fuel in the tank. Sounds simple enough. But wait, as pointed out in the chat room, what happens if the wing rocks during the test? Do you get an accurate test? What if the fuel is sloshing in the tank? Is that test valid? Or what if the stick is absorbent and the fuel level indicated on the stick is higher than the level in the tank because of capillary action? What seemed so simple a concept has now become suspect.

A common problem pointed out by one person applies to both sticks and pipettes is whether the stick or pipette was inserted vertically in the tank and did the stick or tube in fact go all the way to the bottom of the tank. A problem unique to the pipette is the proper way to read the tube. For example, liquids in such tubes are subject to gravity. Some liquids will form a convex surface while other liquids will form a concave surface. Then the question becomes where is the actual level that corresponds to the level in the tank? In some cases, the manufacturer's directions tell how to read the tube.

Although such attention to detail may seem trivial to some people, one must consider the design of some fuel tanks. In a perfectly square tank, the fuel is proportional to the depth. In a long, wide tank with minimal depth, a small error in reading the fluid level can make a significant difference in the quantity in the tank over such a wide surface area. The same is true of not holding the measuring device vertical.

As you can see, how one takes the sample is as important as how one reads the sample.

Some tanks have tabs installed by the manufacturer that indicate a certain fuel level. In such cases, one fuels to the tab for a certain level of fuel. The question here is if the wings were perfectly level at the time of fueling.

Then there is the question of cross-feeding fuel tanks and uneven fuel burn. Some aircraft have fuel selector values with only a right or left tank selector value. The selector is either "on" one of the tanks or it is in the "off" position. This is the simplest system. Some aircraft have, in addition to a "left," "right," or "off" position, a "both" position. When fueling or measuring fuel in the "both" position, if the wings are not perfectly level, there is the risk of fuel from one tank transferring to the "lower" tank. This can result in an erroneous fuel reading. To prevent this problem, one can make sure the fuel selector switch is not in the "both" position when fueling or measuring the fuel level.

In aircraft with multiple or complex fuel systems, the pilot must understand the overall system to avoid any potential problems. For example, does a wing tip tank fill a main tank? Or does a main tank fill a header tank? Such questions determine how much fuel is onboard and how is it used.

Knowing a complex system's design and fuel flow determines how much fuel is available for use. The only thing worse than running out of fuel is running out of fuel and having an accident with fuel in the tanks because you did not know how to access it or control it. One is the result of bad planning. The other is not knowing your aircraft's systems. Both can be deadly.

So, why did we have this brief discussion about fuel management? The reason is contained in 14 Code of Federal Regulations (CFR) '23.1337(b), Powerplant instruments installation. It reads:

(b) Fuel quantity indication. There must be a means to indicate to the flight crew members the quantity of usable fuel in each tank during flight. An indicator calibrated in appropriate units and clearly marked to indicate those units must be used. In addition:

(1) Each fuel quantity indicator must be calibrated to read "zero" during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply determined under 14 CFR '23.959(a);

(2) Each exposed sight gauge used as a fuel quantity indicator must be protected against damage;

(3) Each sight gauge that forms a trap in which water can collect and freeze must have means to allow drainage on the ground;

(4) There must be a means to indicate the amount of usable fuel in each tank when the airplane is on the ground (such as by a stick gauge);

(5) Tanks with interconnected outlets and airspaces may be considered as one tank and need not have separate indicators; and

(6) No fuel quantity indicator is required for an auxiliary tank that is used only to transfer fuel to other tanks if the relative size of the tank, the rate of fuel transfer, and operating instructions are adequate to -

(i) Guard against overflow; and
(ii) Give the flight crewmembers prompt warning if transfer is not proceeding as planned.

As you can see, the regulations only require that the aircraft fuel gauge read "zero" during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply determined under 14 CFR '23.959(a). Therefore, the gauge cannot be depended upon for checking the fuel quantity in a tank. This is especially true of the smaller, less sophisticated general aviation aircraft. Visual or physical checking or both are the only safe means of determining the actual quantity of fuel onboard such aircraft.

How do you measure fuel? Do you measure up?