Usable Fuel

cocolos

cocolos

Pre-takeoff checklist
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cocolos
So I understand there's a set amount of fuel that you cannot account for as being usable due to the tank configuration and the tubes that run from the tanks to the engine.

If I want to fill my two tanks on a 152 half way:
(Assuming each tank holds 13 gallons, with a total of 24.5 usable gallons)

Do I just keep putting gas in to the tank until I measure 6.5 gallons on each tank with a direct fuel level indicator? How do I account for the unusable fuel?
 
The first 3/4 gallon of fuel in each tank is unusable. If you had an accurate fuel stick that showed 6.5 gallons total in a tank, 5.75 would be usable. I have not found fuel sticks to be very accurate but they get you in the ballpark. I usually fill the tanks and time the running time and plan on a one hour reserve just to be safe.
 
so probably the best bet is to assume you have 5 when the meter says 6.5 and then give yourself an hour reserve.
 
Measure the fuel in the tank. Subtract unusable amount for the aircraft in question. Add to that figure the number of gallons needed/desired to make the flight with sufficient reserves.

In other words, if you measure 4 gallons of fuel in the tank and want to take off with 6.5 gallons of usuable fuel in that tank, you would subtract 0.75 gal from 4 and add 3.25 gallons at the pump. Personally, I'd round up to whole numbers to keep it simple and add some extra cushion.
 
Witmo said:
I have not found fuel sticks to be very accurate but they get you in the ballpark. I usually fill the tanks and time the running time and plan on a one hour reserve just to be safe.
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Good point and good reason to round up on the numbers.

I have seen some fuel sticks that are better than others. I generally trust my stick to within a half gallon due to the fact that I made my fuel tables after several fuelings with a universal stick.
 
Depends on the measuring device you use, and whether it's designed to show total fuel or usable fuel. If you're using a Fuel Hawk or similar commercial product, it should tell you that on the device. If you make one yourself, it depends on how you mark it when you construct it.
 
I actually ignore completely any reference to usable and unusable fuel in my Cardinal. It holds 25 gallons per wing with 1/2 gallon per side unusable. If I'm flight planning and am concerned about the unavailability of 1 gallon of gas, I'm operating way to close to the edge and violating my 1.5 hour reserve limit.

Also, as Ron suggests, I use a Fuel Hawk (generic model) and when I first bought the plane, I marked it off at every five gallons as I added fuel to the empty tank(s). Now, I religiously stick each tank before each flight.
 
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Fearless Tower said:
Measure the fuel in the tank. Subtract unusable amount for the aircraft in question. Add to that figure the number of gallons needed/desired to make the flight with sufficient reserves.
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The only way to accurately measure the fuel in any tank is to totally drain the tanks, which means removing all the fuel that wouldn't leave via the outlets. And that means draining the last of it out of the tank sumps. Unusable fuel is that fuel that will not leave via the outlets when the airplane is in a climbing or descending attitude.

And then you meter fuel into the tanks, five gallons at a time for example, and make markings on a dipstick as you go. The airplane must be level. The unusable fuel is put in first, and if it shows on the dipstick, that is Zero Fuel. You zero the pump and start calibrating the stick. If the unusable doesn't show on the stick, you zero the pump anyway and the first five gallons is marked as five gallons.

We did that for all our airplanes, and it's a lot of work. We found that the ready-made sticks weren't too accurate at all.

Dan
 
Dan Thomas said:
The only way to accurately measure the fuel in any tank is to totally drain the tanks, which means removing all the fuel that wouldn't leave via the outlets. And that means draining the last of it out of the tank sumps. Unusable fuel is that fuel that will not leave via the outlets when the airplane is in a climbing or descending attitude.

And then you meter fuel into the tanks, five gallons at a time for example, and make markings on a dipstick as you go. The airplane must be level. The unusable fuel is put in first, and if it shows on the dipstick, that is Zero Fuel. You zero the pump and start calibrating the stick. If the unusable doesn't show on the stick, you zero the pump anyway and the first five gallons is marked as five gallons.

We did that for all our airplanes, and it's a lot of work. We found that the ready-made sticks weren't too accurate at all.

Dan
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Did that too and confirmed it with several fuelings since.

I use the universal stick and calibrated it to my tanks (the ready-made sticks don't work too well for the '48 170).
 
I do not know a single aircraft you can't fly the tanks dry.
 
Just fill the right tank full. Then you KNOW you have 12.25 gallons usable.

Airplane will fly just fine, and the fuel will even out in about 1/2 hour.
 
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Tom-D said:
I do not know a single aircraft you can't fly the tanks dry.
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Meet my plane.

I often dry tank on long XC flights. Doing so gives me almost 30 minutes add'l reserve.

I have 5 gallons "unusable" per side.

In straight and level flight it'll burn all but the last 1/2 gallon.

BUT...

If I'm into the last 5 gallons in cruise flight and pitch down to start my descent, then the engine will die immediately.

That'll wake you up.

Well, okay, the engine doesn't quit "immediately" since there is a reserve in the gascollator and carb bowl, but the fuel flow goes to zero immediately and the silence follows shortly there after if corrective action isn't taken.

I can say with certainly that my unusable is truly unusable. I can't say it's exactly five gallons, but it's pretty close.
 
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Generally, I'm in the camp that says if you're trying to get it that close, you're operating with too little margin. Way too many accidents are blamed on fuel starvation.
 
denverpilot said:
Generally, I'm in the camp that says if you're trying to get it that close, you're operating with too little margin. Way too many accidents are blamed on fuel starvation.
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I agree, no reason to test the engineers calculations on that one.
 
denverpilot said:
Generally, I'm in the camp that says if you're trying to get it that close, you're operating with too little margin. Way too many accidents are blamed on fuel starvation.
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If you're referring to my "dry tanking", Nate, then I guess I don't understand the concern. I've done it dozens of times and only twice have I not caught the tank going dry on the fuel flow before the engine starts sputtering. In every other case I've seen the flow start "wagging out" in plenty of time to switch tanks. Even when the engine sputters it's a non-event (even though it DOES get your attention). Just switch the tanks and everything is fine.

Here are my stats:

I carry 55 usable (by the book, but in reality I can squeeze just shy of 68 gallons in my tanks with 58 being usable). By dry tanking, I can add 4.5 to 5 gallons of usable to the total (this has been confirmed by numerous fill ups after landing with a dry tank). That's over a 9% increase in my safety margin and does not increase the amount of time I'll spend in the air.

I flight plan at 11 gph so, with 55 gallons usable, that gives me 5+00 until I'm bone dry.

In reality, I typically burn more like 10.7 @ 22/2300.

My personal limits are 4+00 IFR and 4+30 VFR.

So, by dry tanking (typically my left tank), I increase my safety margins from 1+00 and 0+30 to 1+24 and 0+54.

I have an EI fuel flow, it's dead accurate and I have it calibrated at 5% over actual fuel flow just for a little more safety margin. I also have gauges that actually work.

In summary, dry tanking is just one more tool that allows you to increase your safety margin.

Now, if your concern is that a pilot might "freak out" when he hears the engine sputter and forget that (s)he's "dry tanking". Then my response would be...well...that person shouldn't be dry tanking in the first place. In fact, if their short term memory and awareness is that bad then maybe they shouldn't be piloting a plane in the first place. (maybe that comes off as overly harsh but, my god, pay attention).
 
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I saw a 152 that ran out of fuel turning base to final. There was 1 quart left in the system, when the FAA drained it... Had he not been so low, it may have actually restarted...
 
ja_user said:
I saw a 152 that ran out of fuel turning base to final. There was 1 quart left in the system, when the FAA drained it... Had he not been so low, it may have actually restarted...
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The unusable fuel could easily been undrainable.
 
kgruber said:
The unusable fuel could easily been undrainable.
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There is no undrainable fuel. There are drains at low points on any airplane I've ever worked on.

Unusable fuel is unusable because it can't get out of the tank outlet because the outlet is higher than the bottom of the tank. The sump drain is at the lowest possible point in the tank.

Dan
 
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