Why do airlines sometimes lose money even when their planes are full?

Even when a flight is physically full, airlines can still lose money due to a combination of high fixed costs and the nature of the aviation business model. Here are the primary reasons:

• High Operational Costs: Airlines face massive overhead, including fuel, crew salaries, airport fees, maintenance, and aircraft financing [2]. As noted in one analysis, a single flight can incur tens of thousands of dollars in costs before it even leaves the gate [2]. If the tickets sold for that flight are priced too low—perhaps due to heavy discounting or aggressive competition—the total revenue might not cover those substantial operating expenses.
• The "Perishable" Nature of Seats: Airline seats are a perishable commodity [1]. Once a flight departs, any empty seat loses its value forever; it cannot be stored or sold later [1]. To ensure as many seats are filled as possible, airlines often use complex dynamic pricing. While this fills the plane, it can sometimes result in selling seats at prices that are too low to contribute meaningfully to the airline's overall profitability [1].
• External Vulnerabilities: The industry is highly susceptible to "exogenous events"—factors outside an airline's control, such as sudden shifts in fuel prices, political instability, terrorism, or natural disasters [3]. These events can drastically inflate costs or collapse passenger demand unexpectedly, making it difficult for even a full flight to remain profitable [3].

In short, a full plane indicates high demand, but it does not guarantee that the revenue collected from those passengers exceeds the high cost of operating that specific flight [1, 2].