January 1, 2010
By Rod Machado
As a young flight instructor, if I wanted to know the amount of money in my account, I didn’t ask the bank president to bring out the bucks so I could get a good look at them. Instead, I paid $2 for an ATM receipt to show my balance. Unfortunately, this often meant that I’d have to close out my account since it was overdrawn.
My point is that you can often trust one thing (a receipt) to tell you how much of another thing (money) you have. The principle is sound, except when applied to fuel gauges as a means of gauging the amount of avgas in your tanks.
It seems that more than a few pilots today have lost a once-useful survival skill—skepticism about trusting fuel gauges. At least that’s my suspicion based on a recent exchange with several pilots. One fellow—an experienced pilot—suggested that there’s no way any airplane manufacturer would install fuel gauges that don’t provide accurate readings. That’s a good point. Over time, however, the point becomes irrelevant.
Let me explain it this way. Several years ago I gave away an old tube-type TV when it began showing transparent double images of the TV characters. As a new TV, the images were crisp and sharp. Since everything wears out over time (including an airplane’s fuel measuring system), the once-sharp images went fuzzy. I can only hope the person who now owns my old TV doesn’t watch Ghost Hunters, because he’ll go hoarse yelling, “Look behind you man! There are five ghosts back there. They’re all over. Run for your lives!” You get the point, right?
The airplane’s fuel measuring system (the gauges, fuel transmitter, rheostat, wiring) is also subject to wear and tear. Few would argue that fuel gauge readings become more accurate with time, which explains why you’ve never heard an airplane owner say, “In another 50 years, my gauges will be spot on.”
On the other hand, the FAA says that fuel gauge readings are not required to be all that accurate in the first place. Perhaps you’ve heard that fuel gauges are only required to read accurately when the tanks are empty of useable fuel. Some pilots even think that this statement is a myth, but I think they’re “mything” something if they do. On page 6-26 of the FAA’s Pilot’s Handbook of Aeronautical Knowledge, we find that, “Aircraft certification rules require accuracy in fuel gauges only when they read empty.” Isn’t that nice? The only time you need the gauge to be wrong is when it might be right (see FAR 23.1337). Now, this doesn’t mean that fuel gauges can’t, don’t, or won’t read accurately at other times. It just means that the standard for gauge accuracy at fuel levels other than zero useable fuel isn’t the FAA’s standard, nor is it entropy’s standard.
Fuel gauges can provide inaccurate readings for a number of reasons. Sometimes the gauge itself or its wiring goes bad. In about one-third of the cases, the fuel sensing/sending unit breaks down. Airplanes with bladder tanks have had tanks collapse, altering the fuel quantity, the movement of the fuel sensing unit, and the fuel gauge reading. The only reasonable conclusion to reach is that you can’t trust fuel gauges as the sole means of determining the fuel in your tanks.
If fuel gauges aren’t to be completely trusted, what good are they? Well, they serve a practical purpose of informing you when you have a fuel leak. Beyond that, the only way to know the amount of fuel in your tanks is to—if you’ll allow me a metaphor—have the banker show you your money. In other words, the only fuel you can count on is the fuel you measure by visual inspection of your fuel tanks. Even then, it pays to be a skeptic, because what you see isn’t necessarily what you get.
Some airplanes parked on a slanted surface (or with an abnormally inflated nose strut), might have a fuel tank that looks full, but is short several gallons. In fact, the only way to feel confident about the fuel capacity of a tank is by draining it then filling it when the airplane’s on a level surface. Just how are you supposed to do that as a rental pilot? Well, you aren’t.
Instead, it’s a good practice to avoid using more than three-quarters of your airplane’s useable fuel before refueling. This provides a safety buffer just in case your tanks hold a few gallons less than you think they do. As a general rule, the actual level of fuel in the tanks seems to closely parallel the level of wisdom in a pilot’s head. So be wise. Fill it up when it’s appropriate to do so.
OK, I know that you’re thinking, “Rod is ruining my buzz.” Yet I ask you to be skeptical about fuel gauge readings, mainly because I want your buzz to continue—the buzzing from your engine, that is. Feeling good when you see a sufficient fuel quantity on the gauges, without actually looking inside the tanks, is to use those gauges as mood stabilizers, and that’s a prescription for disaster. Instead, it’s always wise to think, “Show me the money,” before every flight. Counting the gallons in your tank is the only quantity you can count on.
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Aircraft Power and Fuel,
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