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Thread: K factor and fuel remaining error

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    Senior Member Andy's Avatar
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    Default K factor and fuel remaining error

    Almost every time I refuel my FX-3 the indicated fuel remaining plus fuel added exceeds 39 gallons usable. This means the fuel computer is underestimating fuel flow.

    I have loaded data for every refuel in a spreadsheet in an attempt to come up with a better K factor. However, the data show there is a large variation in the fuel used error when K factor is unchanged. With factory set K factor of 68000 the error has varied between 0% and 9.8%. I do not think the data are stable enough to compute a new K factor but I have run the last 4 refuels with K factor 64600.

    I'm suspecting that the fuel flow sensor calibration is very dependent of flow and that, if K factor is calibrated for high power settings, it will be very inaccurate at low power settings. (The only zero error refuel was during early phase 1 flyoff when high power was used).

    If you log your FX-3/EX-3 fuel usage what variation are you seeing in the error of indicated fuel remaining?
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    Senior Member turbopilot's Avatar
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    Default Re: K factor and fuel remaining error

    Quote Originally Posted by Andy View Post
    With factory set K factor of 68000 the error has varied between 0% and 9.8%.
    It is possible that using the assumption the "tanks are full" based on fuel up to the neck of the fuel port may be the problem. Given the problematic nature of the 4 tank 44 gallon fuel system in the FX and optional on the SS, I think it is possible air may be getting trapped in one or more tanks. This may also play into the inconsistent reports of actual unusable fuel we covered in another thread.

    Have you every filled both sides then vigorously rocked the wings, then try to put more in?

    I owned a couple of Bonanza B36TC's. This aircraft had 2 tanks a side with an interconnect and a single fuel port on the outboard tank. I could often get it to take more fuel by rocking the wings.
    Bob Anderson, CC11-00435, N94RA (for sale CC11-00316, N382RA)

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    Senior Member Andy's Avatar
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    Default Re: K factor and fuel remaining error

    Quote Originally Posted by turbopilot View Post
    It is possible that using the assumption the "tanks are full" based on fuel up to the neck of the fuel port may be the problem.
    I think there is bound to be some error in the "it's full" assumption but I try to be consistent about how I fuel it. Now I always select one tank or the other to prevent cross feed while fueling. I did not for the first few times I fueled it. The gurgle gives a pretty good indication of transfer between the inner and outer tanks in one wing. That gurgle is so useful I really miss hearing it if someone keeps an engine running when next in line for the pump.

    I guess that my "full" error is no more than a gallon and that would contribute 2.6% error.

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    Senior Member turbopilot's Avatar
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    Default Re: K factor and fuel remaining error

    Quote Originally Posted by Andy View Post
    The gurgle gives a pretty good indication of transfer between the inner and outer tanks in one wing. That gurgle is so useful I really miss hearing it if someone keeps an engine running when next in line for the pump.
    Not sure the gurgle will tell whole story. Based on the parts manual, there are three tubes connecting the two tanks. Two 1.13" ID tubes, one forward and one aft low between the tanks and a .44 ID vent tube high between the tanks at about the same place as the top tube for the site gauge on the inboard side of the tank.

    It would appear depending on the size of the tires, the height of the tail wheel or the grade the aircraft is parked on fuel can only rise to a point where the inter tank vent tube is covered by fuel. That would leave an air pocket in the forward end of the inner tank which would be depended on the aircraft nose up attitude during fueling.

    So when filling the outboard tank via the fueling port (which is forward of the vent port) you should get more fuel higher in that tank than is possible for the inboard tank. The question is how big is the air pocket and how much can it change based aircraft attitude. I would guess the same principle might apply if the aircraft was parked on a slope with one or the other wing down. Once fuel covers one of the vent ports between tanks that inner tank will stop filling.

    Last edited by turbopilot; 11-17-2020 at 03:51 PM.
    Bob Anderson, CC11-00435, N94RA (for sale CC11-00316, N382RA)

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    Senior Member Andy's Avatar
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    Default Re: K factor and fuel remaining error

    The gurgle is not a signal to stop fueling. It's a signal to reduce fill rate and carefully watch the rising fuel level (thanks to Brad for that tip). In any event all my refuels were done on level ground, with the same tires, and with similar weights on the tail. This results in a pitch attitude of 12-12.5 degrees on 8.50 tires.

    26 inch tires went on yesterday so some recalibration of the pilot may be required. I doubt it's going to make a difference to fuel flow calibration.

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    Senior Member turbopilot's Avatar
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    Default Re: K factor and fuel remaining error

    Quote Originally Posted by Andy View Post
    The gurgle is not a signal to stop fueling. It's a signal to reduce fill rate and carefully watch the rising fuel level (thanks to Brad for that tip). In any event all my refuels were done on level ground, with the same tires, and with similar weights on the tail. This results in a pitch attitude of 12-12.5 degrees on 8.50 tires.

    26 inch tires went on yesterday so some recalibration of the pilot may be required. I doubt it's going to make a difference to fuel flow calibration.
    I am going back to 8.50 tires. Just blew out a 26" tire over the weekend. Looks like it hit a sharp object. Just a hole in the tire. Made it back to hangar before it went flat. $1,200 for a puncture is pretty pricey. I don't see how it could be repaired.
    Bob Anderson, CC11-00435, N94RA (for sale CC11-00316, N382RA)

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    Senior Member Andy's Avatar
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    Default Re: K factor and fuel remaining error

    I now suspect that the difference between indicated fuel remaining and actual fuel remaining may be caused, at least in part, by fuel overflowing from the tank cap vent tubes.

    My preliminary calculations show that, with completely full tanks, fuel will flow out of the tank vent tubes at 3 degrees roll when fuel selected to both and at 9 degrees roll when selected to either left or right.

    I have not been able to find any scaling specification for the GDU 465 slip/skip indicator so I don't yet know how far the slip ball would have to be displaced for fuel to spill. Fuel spillage seems likely in an intentional slip.

    I can't be the first owner to have thought about this so what are the ways to prevent fuel spillage? (other than don't fill the tanks and never slip or skid)

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    Senior Member turbopilot's Avatar
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    Default Re: K factor and fuel remaining error

    Quote Originally Posted by Andy View Post
    I now suspect that the difference between indicated fuel remaining and actual fuel remaining may be caused, at least in part, by fuel overflowing from the tank cap vent tubes.
    Sounds plausible. Have you ever seen a fuel plume coming off the wing? I once had a fuel cap come off on rotation. Looks like a smoke generator on the wing. Can't miss it.
    Bob Anderson, CC11-00435, N94RA (for sale CC11-00316, N382RA)

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