innage

Innage is the volume or weight of fuel physically remaining in an aircraft's tanks after flight operations end. It is measured in gallons, liters, or pounds depending on the aircraft type and regional practice. This is distinct from outage, which measures the empty space above the fuel surface, and from usable fuel, which accounts for fuel that cannot be drawn by the engines due to tank geometry or fuel system design.

Measurement of innage is performed using fuel quantity indicating systems (FQIS), which may employ capacitance probes, mechanical floats, or more advanced electronic sensors. On some aircraft, technicians verify readings by direct physical inspection of tank sight gauges or by gravity sampling. The accuracy of innage measurement is critical because it directly affects weight and balance calculations, flight planning, and compliance with minimum fuel reserves required by operating regulations.

Innage diminishes throughout flight as fuel is consumed by the engines. Pilots monitor innage continuously and must declare a minimum fuel state to air traffic control when reserves drop to specified thresholds, typically 45 minutes of flight time remaining. Maintenance personnel track innage after each flight to detect abnormal fuel consumption, fuel leaks, or FQIS calibration drift. Significant discrepancies between calculated fuel burn and measured innage may indicate a fuel system malfunction requiring investigation.

Different aircraft designs result in different proportions of usable versus unusable innage. High-wing designs and fuel management systems with multiple tanks and crossfeed valves allow pilots to manage weight distribution and burn reserves more precisely. Low-wing aircraft and single-tank systems may leave larger quantities of fuel that cannot be accessed. This affects operational planning and the real range of the aircraft under various load conditions.

The term itself derives from the verb to gage, an older spelling of gauge. Innage is literally in-age, the amount that is in the tank, measured by age or level. It appears in aircraft maintenance logs, flight planning documents, and weight and balance records as part of the standard accounting of aircraft fuel status.