Load factor and CASK determine survival.
An airline flies a fixed number of seats whether they sell or not, so an empty seat is revenue gone forever the moment the door closes. The business is a knife-edge between how full the planes are and the cost of flying each seat one kilometre. Fuel is a third to a half of costs and moves with crude, which the airline cannot control, so the only durable edge is a structurally lower cost per seat. That is the entire IndiGo story. Debt and unhedged fuel are what bankrupt the rest.
An airline sells something stranger than most products: a seat that exists for only a few hours and then vanishes forever. Understanding that one fact explains almost every decision an airline makes.
Think of an airline seat like a hotel room at midnight: the moment the plane pushes back from the gate, any seat that did not sell is gone for good, there is no next customer to sell it to later. The plane, the crew and the fuel cost roughly the same whether the flight is empty or full, so every seat filled at any reasonable price is almost pure extra profit, and every seat that flies empty is a loss that can never be recovered. This is why airlines discount tickets right up to departure rather than let a seat fly empty.
Since an airline cannot really raise or lower how many seats a plane has, its whole economic game comes down to two numbers: how full the planes fly on average, and how cheaply it costs to fly each seat. An airline that fills 85% of its seats while spending less per seat than its rivals will make healthy profits; one that fills only 70% while spending more will bleed cash, even flying the exact same routes with the exact same planes.
Load factor is simply the percentage of available seats that were actually sold and flown, seats filled divided by seats available. Because the fixed costs of a flight barely change whether it is half full or completely full, small moves in load factor swing profit disproportionately, a few points either way can be the difference between a profitable quarter and a loss-making one.
RASK is revenue earned per available seat kilometre flown, essentially how much pricing power and demand the airline commands. CASK is the cost of flying that same seat one kilometre, covering fuel, crew, airport fees and aircraft leasing. The airline is profitable exactly when RASK is above CASK, and the size of that gap, not the size of the airline, is what determines whether it makes money.
Jet fuel typically eats 35-45% of an airline's revenue, and its price moves with global crude oil, something no airline controls. An airline that does not hedge its fuel costs, locking in prices in advance, is essentially betting its entire profit margin on the direction of oil prices every single quarter. Combine that with debt taken on to buy or lease aircraft, and a spike in oil prices at the wrong time can turn a healthy airline loss-making within a single quarter.
Because airlines routinely operate on single-digit or even negative net margins, the sector has a long history of bankruptcies, and the market tends to value airlines on their ability to survive a downturn (cash on hand, debt levels, cost structure) as much as on growth. A low-cost structure is the only durable moat in this business, since there is little brand loyalty when a cheaper fare from a competitor is one click away.
Every sector reduces to a demand metric, a pricing metric, an efficiency metric, a capital metric, and a risk metric. For airlines, these are the ones that matter.
What each number tells you, and how to read it.
| Metric | Why it matters |
|---|---|
| Load Factor | Seats filled over seats available. Above 85% is efficient; below 80% means yield dilution. |
| RASK (Revenue/ASK) | Revenue per available seat kilometre, the pricing-power metric. |
| CASK (Cost/ASK) | Cost per seat kilometre. Lower CASK is a structural cost advantage, IndiGo's moat. |
| Fuel Cost % | Typically 35-45% of revenue. When crude moves, margins move. Unhedged means high volatility. |
| Fleet Utilisation | Hours flown per aircraft per day. Higher means more revenue from the same fixed asset base. |
| Yield | Revenue per passenger kilometre, a pricing-power indicator. The RASK-yield gap shows ancillary revenue quality. |