Definition
The small amount of fuel and oil that remains in an aircraft's tanks, lines, and components after the usable fuel and oil have been drained. This residual quantity is included in the aircraft's empty weight because it cannot be practically removed during normal operations.
Plain English
The fuel and oil that stay trapped inside the aircraft's tanks, hoses, and engine even after you drain everything you can. Because you can't get this last bit out, its weight is counted as part of the airplane's basic weight.
Context Anchor
Seen in aircraft weighing, maintenance records, and empty-weight calculations.
Derivation
Residual comes from the Latin residuum, meaning 'that which is left behind.' It refers to what remains after the rest has been removed — in this case, the fuel and oil that cling to internal surfaces and low points in the system.
Why Pilots Care
These quantities affect empty weight, center of gravity, and accurate fuel-consumption planning; ignoring them can lead to incorrect performance data.
Intuition Check
Residual does not mean reserve fuel saved for later use. It means fuel and oil left behind because the system cannot be completely drained by normal methods.
Example Sentence 1
The aircraft's empty weight in the equipment list already includes residual fuel and oil, so the mechanic only added the weight of the usable fuel for the loading calculation.
Example Sentence 2
During the post-maintenance inspection, the mechanic recorded the residual fuel and oil quantities to update the aircraft's weight-and-balance record.