"Usable" fuel capacity???

[TDTURBO]

Ten gallons difference due to temperature, yes. Ten percent, I don't know...it may be possible, but that's for an intellectual type to sort out. However, in your case it was two gallons difference...you said 94 were put in, with a 92 gallon capacity. That's not ten gallons, but two, and it's only 2.2% of the total capacity, not ten percent.

When considering the ammount of fuel put into the tank, it's the total capacity, rather than the useable ammount that's relevant.

Why were your tanks that low to begin with?

A typical light aircraft fuel system is a float activated rheostat, and is purely volumetric in nature. A fuel gauge calibrated at zero degrees C will experience approximately a five percent error when going to fuel temp extremes of plus or minus 55 degrees, as suggested by AC 27.1337. Bear in mind that's strictly error on the fuel gauge. The variation by temperature is approximately 1% of fuel volume for each six degrees off the calibration temp, for some fuels.

You may have less useable fuel than you think if you've just flown at altitude or from a colder location, if you're reckoning fuel burn by gallons instead of pounds.

Also of note is the fact that virtually all gas pump calibration measurement is done using a Seraphin can, which measures strictly volume, and doesn't take into account temperature.

I should have mentioned I was at IFR minimums, 1 hr of fuel with no need for an alternate. This meant I had 10 gallons left, verified by dipping the tanks and my Shadin FF before filling up. I know the guy ripped me off, I just didn't make it clear as to how I new. Sorry.

 

[DGdaPilot]

Unusable fuel due to fuel being in the lines? No, if it's in the lines, then it's usable. If anything, that could increase the amount of usuable fuel you have (if you fill your tanks while also having some fuel in the lines). Don't you think that with today's technology, we could make all the fuel in the tank usable? Unusable fuel is there for a specific purpose. It's because of the low points designed into the tanks, which just happens to be where the fuel strainers are. The fuel, heavier sediment, and water sinks to these low points and therefore cannot be sucked into the engine. They can only be removed by straining the fuel valves.

 

[NYCPilot]

I'ts 50 Total, 48 useable.

As others mentioned, more than 48 gals. may be useable under the right conditions. The maximum of 2 gallons that are unusable are only inaccessible under certain extreme aircraft attitudes. Slips, skids, etc.

 

[avbug]

Just because you have fuel in the lines doesn't mean you can use it. Never, ever assume that the published unuseable fuel is the maximum amount of unuseable fuel. The truth is, you may have far more fuel that's unuseable.

I fly one type airplane that recently has experienced several incidents involving fuel starvation when at least an hour's fuel was on board (didn't involve me, for the record). One fuel tank ran dry, the lines were full, one tank half full, and the turbine engine failed due to fuel starvation as both tanks were necessary to feed the header tank that fed the engine driven fuel pump. The aircraft type features an emergency wobble pump which was utilized in each case, which failed to flow the adequate amount of fuel due to air being drafted from the empty tank.

Never assume that you can use everything that's published, or that the published unuseable numbers are the upper limit of what's unuseable.

Fuel pumps fail, selectors fail, valves stick, etc. Systems may make the fuel unavailable. Never plan for minimum fuel, because eventually it will bite you. Hard.

 

[NYCPilot]

According to the POH, under normal circumstances the amount listed as usesable should be a very close approximation. Mismanagement of the fuel system can cause this figure to change as avbug stated. Stipulations such as running one tank completely dry as in the aforementioned incident, will cause a deviation from this useable figure. It is also important to remember that these figures were derived from a brand new airplane, perfectly rigged and balanced with an engine, tanks and fuel system free of imperfections. After years of use and abuse, clogging, repairs, replacements, fittings, etc., it would be unwise to assume that this figure of maximum amount of useable fuel is a reliable one. It is also worth noting that fuel bladders can change in size over time and may not actually be capable of holding the maxium amount of fuel as described by the manufacturer. Temperature also has a very limited role in determining your total quantities as well. One reason that jet fuel is measured in pounds rather than in gallons as is the case with avgas has to do with the expansion coefficient of jet fuel as the types of aircraft that rely on this means of fuel normally travel across a broad range of altitudes, and hence temperature differences, which affect the volume of the fuel. To account for the amount of jet fuel on board by metering it in gallons would be highly inaccurate.

At any rate, for your checkride or for whatever the impetus for the question was, suffice it to say that you should give the book values. You stated that it was not listed. You must not have probed deep enough as this figure will be contained in there. You can impress your examiner as to why this may not be such a reliable figure, beyond providing the raw numbers.

Hopefully, you are aware of the various fuel reserves required during VFR flight. Keep in mind though, that these are only FAA mandated minimums and you should have at least a conservative figure of 1 hour derived from your flight plannning calculations. This takes into account winds, especially unforecast ones or ones taken from reports hours old. Keep in mind that you may also be vectored around, asked to fly extended traffic patterns, have difficulty locating your X-CTY destination, may not follow a precise ground track as planned on your sectional, etc. This will all casue more of a fuel burn than you may have planned for. Don't just use the point to point distances added to determine the total fuel burn.