"Usable" fuel capacity???

[UnAnswerd]

In the PA28, if both tank were topped right up to the brim, the total fuel capacity would be 50 gallons. But what would be the "useable" capacity??? I couldn't find anything in the airplane manual.

Any information would be greatly appreciated.

 

[viper548]

If I remember correctly, all the fuel in PA-28's is usable.

 

[bigD]

It's been a long time since I've flown a Cherokee, but assuming you're talking about the PA28-140 you have listed in your profile, I remember it being 48 gallons.

 

[FlyChicaga]

48 gallons. 2 gallons unusable fuel.

 

[FOD]

now with that out the way, whats the purpose of having unusable fuel? just thought i would throw it out there, since i have had several DE's ask that.

 

[gern_blanston]

It's like a guarantee. You can probably use the unusable fuel, but you WILL be able use the usable fuel. Just a little pad to include fuel that might get stuck in a wrinkle in the bladder, inaccessable due to a little slip or skid, etc. Way back when, one of our instructors taxied in in a '152 (WITH a student), shut down, and put 25.6 gallons of fuel in it after a cross-country. Usable is 24.5.
The good news, from my point of view, was that it happened a week before my CFI checkride, so I had a job waiting!

 

[NoPax]

I always thought it was the fuel left in the lines, pump etc, after the tank was dry - no pressure etc., to feed the engine.

I also thought it was a consideration in weight and balance, Standard or Licensed Empty Weight.

When I fuelled at the FBO I eventually got a job instructing at I put more than the capacity into a Cessna 152, and a Seneca II.

I see you got a 1 star rating UnAnswerd?...someone's at it again!

 

[avbug]

now with that out the way, whats the purpose of having unusable fuel?

There is no purpose. That's why it's "unusable."

A certain amount of fuel remains which cannot be used due to fuel port positioning, and the inability of the lines to fully drain once the fuel pumps cavitate.

Fuel ports don't drain from the very bottom of the tank in order to prevent drafting any sediment or contaminants that may remain. What does remain is "ususable."

 

[Kream926]

look in the daaammnn POH. thats usually where we find these things. dude, seriously.

 

[avbug]

look in the daaammnn POH. thats usually where we find these things. dude, seriously.

I couldn't find anything in the airplane manual.

Seems he said he did that, doesn't it? Seriously.

 

[Kream926]

shouldnt it though?????????????????

 

[UnAnswerd]

look in the daaammnn POH. thats usually where we find these things. dude, seriously.

Is there a difference between a POH and this:

http://img.photobucket.com/albums/v224/TurbineSpeed/BK005.jpg

 

[FN FAL]

When I fuelled at the FBO I eventually got a job instructing at I put more than the capacity into a Cessna 152, and a Seneca II.

REALLY? Were the tanks dry because of mx?

 

[avbug]

The Type Certificate Data Sheet (Cert #2A13, Revision 45) for the Cherokee 140 indicates a fuel capacity of 50 gallons at 95 degrees F. It also indicates in Note 1 of the same TCDS that an unuseable capacity of 2.2 lbs exists for the same, regardless of serial number. Therefore, the actual amount of unuseable fuel varies with temperature. The total volume in gallons for the tank remains constant due to physical dimensions, but the volume of a pound of fuel varies with it's temperature.

You can split hairs all you want wandering how much useable fuel you have, but from a practical point of view, if you never burn off the bottom half, as the saying goes, you won't run out of fuel. Never burn down that low, and you don't have to worry about how far you can push the airplane.

 

[NoPax]

REALLY? Were the tanks dry because of mx?

No, just a couple of idiot pilots coming back from long cross countries. It wasn't much over the max fuel capacity, and I didn't spill any, nor did any leak out a vent...

 

[Dr Pokenhiemer]

Maybe your pump needed to be calibrated. Maybe they paid for more than they actually got? I had an FBO try to tell me I bought more than my capacity once--turned out the fueler forgot to reset the meter before pumping my fuel.

 

[TDTURBO]

I had a guy put in 94 gallons in my 182, it only holds 92 with 88 useable. There is gouging going on at FBO's, needless to say I contacted the Department of weights and measures to investigate. Always watch what they put in, a lot of places will rip you off. I was told this was one of many calls they had on this guy, he advertises the lowest fuel prices in my area, now I know why.

 

[avbug]

Fuel volume, and consequently gallonage, varies with fuel temperature. Rated capacity for a fuel cell occurs at a specific temperature...change the temperature, the actual amount of fuel that goes into that tank will vary from the published numbers.

 

[TDTURBO]

Fuel volume, and consequently gallonage, varies with fuel temperature. Rated capacity for a fuel cell occurs at a specific temperature...change the temperature, the actual amount of fuel that goes into that tank will vary from the published numbers.

I understand that, but by 10 gallons or approximately 10%?

 

[avbug]

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.

 

[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.