I was wondering how much of that was actually usable fuel. One user commented that all of that is usable fuel as only usable fuel is indicated on the FQI, not the total fuel quantity including unusable fuel.
I'm not familiar with the A320, maybe someone can confirm, that indeed only usable fuel is indicated and when the boost pumps run dry, the indication actually reads zero.
I'm familiar with the MD-11 though and that aircraft shows the total fuel quantity, including unusable fuel, on the cockpit indications. I have defueled and drained an aircraft for weighing just recently, so there is no doubt about that. The MD-11 has about 500-600kg of unusable fuel after defueling and still about 150-200kg of undrainable fuel after draining.
What's puzzling me is that the weight of ununsable fuel is already included in the ZFW. Now when you add the total fuel (including unusable fuel) indicated on your instruments, to the ZFW, the unusable fuel would be accounted for twice.
Additionally there is usable fuel in the manifolds that is not indicated on the cockpit instruments. I'm not talking about the engine feed lines, but in case of the MD-11, the fill/crossfeed manifold that contains about 270kg of fuel which can be drained into tank #2 if the quantity of that tank drops below 900kg.
So now I have about 500kg of unusable fuel that are counted twice and 270kg of usable fuel unaccounted for for TOW calculation. What do the regulations actually say about that?
All I could find regarding unusable fuel was this:
Sec. 25.959 — Unusable fuel supply.
The unusable fuel quantity for each fuel tank and its fuel system components must be established at not less than the quantity at which the first evidence of engine malfunction occurs under the most adverse fuel feed condition for all intended operations and flight maneuvers involving fuel feeding from that tank. Fuel system component failures need not be considered.
Also how is the amount of unusable fuel determined? According AMM, when weighing, all (unusable) fuel is removed, only the undrainable amount of fuel may remain in the tanks. As it is included in the basic empty weight of the aircraft it has to be added to the result of the weighing process somehow.
Empty weight. The weight of the airframe, engines,
all permanently installed equipment, and unusable fuel.
Depending upon the part of the federal regulations under
which the aircraft was certificated, either the undrainable oil
or full reservoir of oil is included.