Cost per available seat kilometre

An airline's operating cost divided by the seat kilometres it flew. The industry's unit cost measure.

A carrier with a total cost of ₹3.85 per available seat kilometre, of which ₹1.40 is fuel, has an underlying cost of ₹2.45 excluding fuel.

Excluding fuel is standard because fuel is a commodity nobody controls, so cost excluding fuel is the measure of actual operating efficiency. Low-cost carriers win on this figure through aircraft utilisation, a single fleet type, and high seat density, and the advantage is structural, so it doesn't fade.

Comparing it against revenue per available seat kilometre gives the spread that decides whether the airline makes money at all.

Usually that gap is a matter of paise, and the whole industry turns on it.