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Adding unusable fuel in aircraft profile


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Mike@EDTG
Mike@EDTG
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Unfortunately there is no option to define the amount of dead fuel within the aircraft profile. The only way to do this right now is adding an additional w&b station including weight&arm of the unusable fuel. Reduce amount of max fuel capacity in the profile to usable & everything will be fine. Any plans to exclude dead fuel in future releases?

P.s. please ad gasoline/diesel to fuel type options

-mike
psb777
psb777
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There is often confusion as to whether, when an aircraft is weighed, the empty weight includes unusable fuel. My understanding is that it does. (How would they get the unusable fuel out of the tanks for the weighing? Similarly the oil is included in the empty weight.) The thing to do is to enter the USABLE fuel as the capacity. I think the best way for this to be fixed in SD is to note this standard meaning of "empty weight" and to change the narrative on the fuel tank prompt from "capacity" to "usable fuel". The help on the field should provide or link to the empty weight definition.

Some fuel tanks have a considerable amount of unusable fuel. A related but off topic discussion is this: How is your dipstick calibrated? Usable or unusable?
InfrequentFlyer
InfrequentFlyer
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Sorry to revive an old thread, but as a new kid on the block I was also wondering about this.  As far as I can tell, the SkyDemon calculated endurance does not consider unusable fuel, which can be as much as 15 minutes flying time.  For example in a PA28 there's 2 US gal unusable, which can be more than 15 minutes at cruise, depending on the model.  In a 152 it's around 1.5 US gal unusable, or 14 minutes at cruise.

"Unusable fuel" is not the same as "fuel that can't easily be drained from the tank".  According to FAR 23:

"The unusable fuel supply for each tank must be established as not less than the quanitty at which the first evidence of malfunctioning occurs under the most adverse fuel feed condition occuring under each intended operation and flight maneuver involving that tank."

In one weighing schedule I looked at, it explicitly stated that empty weight includes no fuel - not even unusable fuel. 

On SkyDemon, if you include total fuel in the aircraft data then the endurance will be slightly optimistic.  If you put in only usable fuel then the endurance should be more accurate, but the aircraft weight will be out by a few pounds (maybe 15-20lb). 

Not a big deal (and no one has any business getting within 15 minutes of fuel exhaustion), but assuming one uses actual fuel loading, perhaps worth noting that the estimated endurance will be optimistic by a quarter of an hour or so. 

Unless, of course, I've totally misunderstood what's happening - which is quite possible!

Edited 8/5/2015 8:25:07 AM by InfrequentFlyer
ckurz7000
ckurz7000
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The empty weight of an aircraft includes unusable fuel as well as all other fluids required to operate the aircraft. The fuel gauges are required to read "zero" when all USABLE fuel is gone. So why don't you define the tank volume as the amount of usable fuel and include the weight of unusable fuel in the empty weight of the aircraft? That's how it is supposed to be done.

-- Chris.
InfrequentFlyer
InfrequentFlyer
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ckurz7000 (8/5/2015)
The empty weight of an aircraft includes unusable fuel as well as all other fluids required to operate the aircraft. The fuel gauges are required to read "zero" when all USABLE fuel is gone. So why don't you define the tank volume as the amount of usable fuel and include the weight of unusable fuel in the empty weight of the aircraft? That's how it is supposed to be done.

-- Chris.


Chris, do you have a definitive source for that?  The Weight and Centre of Gravity Schedule in front of me includes:
"The basic (empty) weight includes the weight of NIL gals unusable fuel and FULL oil" (plus a basic equipment list). 

I've done some Googling, and I won't bore you with the details, but in various sources (including various parts of FAA 8083) empty weight may or may not include oil, and "unusable fuel" included in basic empty weight may mean "The unusable fuel supply for each tank must be established as not less than the quanitty at which the first evidence of malfunctioning occurs under the most adverse fuel feed condition occuring under each intended operation and flight maneuver involving that tank."  Alternatively, unusable fuel is sometimes defined as the same as "undrainable fuel".  Or, as in the case I've been looking at, it may not be included at all.

The long and short of it: don't assume theoritical endurance figures allow for unusable fuel unless you've checked. 

GO

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