Adding unusable fuel in aircraft profile

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Adding unusable fuel in aircraft profile

Adding unusable fuel in aircraft profile

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grahamb	grahamb posted 5/18/2026 5:50:16 PM Post Details Share Post
Group: Forum Members Posts: 659, Visits: 32K	+x miaelch - 5/18/2026 4:58:44 PM +x grahamb - 5/18/2026 4:55:12 PM +x miaelch - 5/18/2026 4:38:24 PM +x grahamb - 5/18/2026 4:28:01 PM +x miaelch - 5/18/2026 3:56:46 PM +x grahamb - 5/18/2026 3:50:44 PM +x miaelch - 5/18/2026 3:38:31 PM +x grahamb - 5/18/2026 12:14:55 PM +x miaelch - 5/18/2026 11:11:41 AM The issue we are talking about is that the empty weight in SkyDemon does not include the unusable fuel, but the empty weight in real life does. I posted two possibilities how to deal with this. Maybe I didn't make clear enough the disadvantages of my method No. 2 (that is, to manually calculate the empty weight without fuel and the empty CG without fuel and to enter these values into SkyDemon). 1. I assume that most SkyDemon users are not aware of the fact that they have to do such calculations at all. 2. It introduces a source for calculation errors. The purpose of the W&B feature is that we do not have to calculate manually with moments. 3. It is impossible to check the correctness of the aircraft profile without calculating. If the aircraft profile is provided by a flight school or a flying club then you need to explain to everybody why it is indeed correct. And yes, the error may be not so big. But at some point in time in the past it was decided that the unusable fuel is important enough to be included as a feature into SkyDemon. And in my opinion, if there is a feature then it should be correct. Surely the empty weight in SkyDemon is whatever you want it to be? It doesn’t decide for you whether unusable fuel is included or not. It is not. In SkyDemon the take-off mass is calculated as empty weight plus fuel on board (plus crew plus luggage). Analogously for the CG. This leads to the consequences that I described earlier (you have to work-around with the empty weight or with the fuel definition). So you declare the empty weight and CoG to SkyDemon as including the unusable fuel. I just can't see how that is a problem. I just updated my previous post. Please think about it in detail. (Sorry, I don't want to be rude. But I think there is nothing more to explain.) I don't need to think about it in detail, as I'm very happy that the system I use for my own aircraft is perfectly fine. My aircraft's nominal fuel capacity is 140L, of which 4L is unusable/undrainable. The empty mass and balance of the aircraft defined in my SkyDemon profile includes the 4L, and the capacity of the tanks is set up as 136L. I'm still struggling to see how this is a problem. Avgas fuel density can vary by up to about 6.5% according to temperature, but I've never met any GA pilot who worries about that in their W&B calcs. Ok, so it is exactly as I said. You don't use the unusable fuel feature. I can't find such a feature in standard SkyDemon. You could effectively create one by specifying a load point name 'Unusable Fuel' with the arm/moment the same as the fuel tanks and a default mass equal to the amount of unusable fuel, but the risk in doing that is that the official weight schedule may already include it. Know your aircraft and its documentation and set up the SkyDemon profile accordingly. If you are using a profile that has been handed to you by a Flying School etc, and you think it's wrong, you should get them to change it. There's nothing wrong with SkyDemon in my opinion, you can configure it to do what you want. Sorry, but I really don't understand the whole discussion. You haven't even found the feature but you tell me that it is correct and you let me explain its problems over and over... Apologies, I've found it. The 'Unusable Fuel' value is actually 'unusable' unless the Aircraft weighing schedule doesn't include unusable. A tick box should be added to define how it should be taken into account. Apologies again.
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Erik	E Erik posted 6/27/2026 9:49:38 PM Post Details Share Post
E Group: Forum Members Posts: 3, Visits: 5	+x miaelch - 4/11/2026 4:40:19 AM There are two ways for the user to deal with this. FWIW, I have kept this post as a reference text and obviously support the suggestion of the checkmark to determine which situation applies. Many thanks for elaborating. Setting up W&B for an Aquila 210, I am using method 1. The A210 empty weight includes unusable fuel. The mentioned disadvantage still exists (usable fuel entered as 'capacity' and zero entered for unusable) but, on the other hand, the volume I enter while planning corresponds to what the dip stick and gauges indicate. Both indicators are calibrated to show empty when the tanks hold just the amount of unusable fuel. The dip stick resembles a T bar hanging from the upper wing surface, as opposed to resting on the tank bottom. The fuel gauges are calibrated as mentioned per maintenance manual. The only remaining complication is that 54.8 liters per tank require more brain capacity for quick calculations than 60 would. ;-)
	Reply Like 2
GalaxyzA11+	G GalaxyzA11+ posted Yesterday @ 10:17 PM Post Details Share Post
G Group: Forum Members Posts: 37, Visits: 2	Hi, I'm trying hard to understand all of this. In my mind I can use unusable fuel feature if the empty weight does not incluse it, And wont use it if it is still included in empty weight and CoG data. Btw the key is "what does the manual book say " ? Include or not ? If included, the gauge or the stick should start at included = differential zero If not included, the gauge or stick should start at zero = absolut zero and you use unusable fuel feature. Of course if you say 10L included in empty weight but , but gauge indicate 10L at unusable, and the CoG does not include it and so on, then you cannot solve the problem because your data are uncorrelated. What I mean is that your data must be correct/coherent with the first point -unusable fuel is included or not in data book- and then the other data depends on that first point. So far I dont see something wrong in SkyDemon. (No need of a tick box ?) HTH. Edited Yesterday @ 11:07 PM by GalaxyzA11+
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