Revenue per available seat kilometre

Revenue divided by available seat kilometres. Set against cost on the same basis, the difference is the margin per unit of capacity.

A carrier earning ₹4.05 against a cost of ₹3.85 makes 20 paise per available seat kilometre.

That spread is why airlines are so fragile. A 4.9% fall in revenue per seat kilometre wipes out the entire margin, and so does a 14% rise in fuel prices on a carrier whose fuel costs ₹1.40 of its ₹3.85, and the industry has no ability to store its product: an unsold seat on a departed flight is gone.

Yield and load factor are the two components, and airlines trade one against the other constantly.