Oldie question for legacy model 12 with the original sight glass window. Does anyone know where the total stated tank capacity would come up to in the little round windows?? I have installed the new gauge on top but as the window was not causing any issues I left it in place. Full on the gauge seems less than the tank will actually hold.
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How Full is Full?
- Thread starter Jamesey
- Start date
Because it is, by a small amount. If you look closely, you can see that the aft part of the fuel tank is slightly higher in elevation that the middle, where the mechanical gauge is located. That means the mechanical gauge will indicate full before the area above it at the back of the tank is filled with fuel.
Neither the sight window or the mechanical gauge are meant to provide an absolute indication of the fuel level in the tank. It is meant to provide a visual reference just like you have in a traditional aircraft fuel tank when you can remove the cap and visually look at the fuel level to confirm that it appears to match what is indicated by the fuel gauge on the instrument panel.
The biggest benefit of the mechanical gauge is while fueling because it is visible through the rear window. When the gauge needle stops moving while filling the tank, you know that you better slow down the flow because the tank is nearly full.
Piper J3
Well Known Member
The Moeller mechanical float and the float for the EFIS are both located near the front of the fuel tank. Both floats will rise and touch the top of the tank at ~ 16 gallons. If you top-off the tank by adding an additional 4 gallons, neither of the mechanical floats will budge because they are already pinned against the top of the tank. This is why both the mechanical gauge and the EFIS indicate maximum 16 gallons even if the tank is filled to 20 gallon capacity.
When you start flying with a full fuel tank it will register only 16 gallons and both gauges will show 16 gallons until the first 4 gallons are consumed at which point the gauges become active and are surprisingly accurate all the way to near zero. No problem… best to have correct indication when fuel is at critical levels. If you could monitor the side sight-gauge you would be able to see gallons 16-20 consumed during the first hour of flight.
The legacy 12 fuel tank is a fully-usable design with the bottom of tank tapered toward the exit port. I have flown my plane down to ~ 1 gallon remaining on two different occasions when preparing to remove the fuel tank for maintenance. If the Moeller float is “bobbing” around you still have usable fuel. Try that in a Cessna or Piper….
The above scenario works for level flight and when airplane is pitched slightly nose-down during landing phase. It is critical to have minimum 4 gallons of fuel for takeoff so fuel tank doesn’t un-port as fuel moves to rear of tank when the nose of plane is in climb attitude.
When you start flying with a full fuel tank it will register only 16 gallons and both gauges will show 16 gallons until the first 4 gallons are consumed at which point the gauges become active and are surprisingly accurate all the way to near zero. No problem… best to have correct indication when fuel is at critical levels. If you could monitor the side sight-gauge you would be able to see gallons 16-20 consumed during the first hour of flight.
The legacy 12 fuel tank is a fully-usable design with the bottom of tank tapered toward the exit port. I have flown my plane down to ~ 1 gallon remaining on two different occasions when preparing to remove the fuel tank for maintenance. If the Moeller float is “bobbing” around you still have usable fuel. Try that in a Cessna or Piper….
The above scenario works for level flight and when airplane is pitched slightly nose-down during landing phase. It is critical to have minimum 4 gallons of fuel for takeoff so fuel tank doesn’t un-port as fuel moves to rear of tank when the nose of plane is in climb attitude.
