Unusable fuel
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From: Isla Grande
Thread Starter
Joined: Feb 2017
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From: Isla Grande
Joined: Jan 2013
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From: Seattle Area
14 CFR 25.1337(b)(1). I believe EASA CS reads the same, but did not check.
Code of Federal Regulations
Sec. 25.1337
Powerplant instruments.
...
(b) Fuel quantity indicator. There must be means to indicate to the flight crewmembers, the quantity, in gallons or equivalent units, of usable fuel in each tank during flight. In addition--
(1) Each fuel quantity indicator must be calibrated to read "zero" during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply determined under Sec. 25.959;
(2) Tanks with interconnected outlets and airspaces may be treated as one tank and need not have separate indicators; and
(3) Each exposed sight gauge, used as a fuel quantity indicator, must be protected against damage.
Code of Federal Regulations
Sec. 25.1337Part 25 AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES
Subpart F--EquipmentInstruments: Installation
Sec. 25.1337Powerplant instruments.
...
(b) Fuel quantity indicator. There must be means to indicate to the flight crewmembers, the quantity, in gallons or equivalent units, of usable fuel in each tank during flight. In addition--
(1) Each fuel quantity indicator must be calibrated to read "zero" during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply determined under Sec. 25.959;
(2) Tanks with interconnected outlets and airspaces may be treated as one tank and need not have separate indicators; and
(3) Each exposed sight gauge, used as a fuel quantity indicator, must be protected against damage.
Joined: Jan 2013
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From: Seattle Area
Just for clarification, you have to read 25.1337(b)(1) and 25.959 to understand the intent correctly.
The unusable quantity of fuel in the tank is determined at the most critical attitude during which the tank is expected to be used (typically cruise for auxiliary tanks and low fuel go around attitude for main tanks), then the FQIS is set to read zero in level flight at the fuel level corresponding to that unusable fuel volume in level flight. This will typically mean that, if you are in level flight with no longitudinal acceleration, there is some additional fuel in the main tanks below the zero indicated level that is accessible by the pumps. Operators are not permitted to take credit for fuel below the zero indicated level toward their required mission or reserve fuel.
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 failure need not be considered.]"
"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 failure need not be considered.]"
The unusable quantity of fuel in the tank is determined at the most critical attitude during which the tank is expected to be used (typically cruise for auxiliary tanks and low fuel go around attitude for main tanks), then the FQIS is set to read zero in level flight at the fuel level corresponding to that unusable fuel volume in level flight. This will typically mean that, if you are in level flight with no longitudinal acceleration, there is some additional fuel in the main tanks below the zero indicated level that is accessible by the pumps. Operators are not permitted to take credit for fuel below the zero indicated level toward their required mission or reserve fuel.
Joined: Jul 2006
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From: Somewhere over the rainbow
Few days ago, Vistara UK944, Airbus A320 touched down with 260kg of fuel in the tank and was on chock with 200kg with APU still running. So without a doubt the FOB displayed to the pilots is the usable fuel.
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Joined: Feb 2017
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From: Isla Grande
