Definition
A weight-and-balance value used in large aircraft loading calculations, equal to the basic operating weight of the aircraft multiplied by the horizontal distance from the reference datum to the aircraft's center of gravity, then divided by a reduction constant to keep the resulting number manageable. The index represents the moment of the aircraft in its basic operating condition (empty aircraft plus crew, crew baggage, and required operating fluids and equipment, but without payload or usable fuel) expressed as a single working number rather than a large raw moment value.
Plain English
A single number that captures the balance condition of the aircraft when it is ready to fly but has no fuel or payload loaded. Loaders add or subtract index numbers as they add fuel, passengers, and cargo, which is faster than recalculating moments from scratch each time.
Context Anchor
Seen in weight-and-balance records, loading schedules, and load planning for aircraft that use index numbers instead of full moment numbers.
Derivation
The word index comes from the Latin index, meaning 'pointer' or 'indicator.' Here it points to a balance condition by standing in for the full moment calculation. 'Basic operating' refers to the aircraft in its standard ready-to-operate state, before fuel and payload are added.
Why Pilots Care
Keeps the center of gravity within certified limits after fuel, passengers, and cargo are added, preventing loss of control.
Intuition Check
Basic does not mean approximate here. It means the aircraft’s standard starting condition before the flight’s added load is included. Index does not mean a page reference here. It means a shortened number used to represent the aircraft’s balance calculation.
Example Sentence 1
The dispatcher started the load sheet with the Basic Operating Index from the aircraft's weight-and-balance record before adding fuel and passenger loads.
Example Sentence 2
After installing new avionics, the mechanic recalculated the Basic Operating Index to keep future weight-and-balance records accurate.