Airport slots are the reason six in the morning exists

Uncomfortable departure times are not an airline preference. They are the visible output of a rationing system that decides who may use a busy airport and when.

A slot is permission, not a time

At congested airports, an airline cannot simply choose to operate a flight. It needs an allocated slot, which is permission to use the runway and stands within a short window.

The number of slots is set by the airport's declared capacity, itself a function of runway throughput, stand availability, terminal space and air traffic control staffing.

Because capacity is fixed and demand is not, slots at the most desirable airports become scarce assets that carry real commercial value even though they are permissions rather than property.

Use it or lose it

Slots are retained across seasons on the condition that they are actually operated. An airline that leaves a slot largely unused risks having it returned to the pool for reallocation.

This creates a strong incentive to fly a slot even when a particular sector is only marginally profitable. Holding the position matters more than the individual flight's economics.

Slots can also be traded between airlines, which is why a carrier occasionally acquires a competitor largely for its position at a single airport rather than for its aircraft or routes.

Aircraft have to be somewhere overnight

The other force shaping the timetable is the rotation. An aircraft that will fly four sectors during the day must start the first one early to fit them all in.

Working backwards from a desired evening arrival produces a departure before dawn. The schedule is built as a chain, and the first link is always the least comfortable one.

Crew and aircraft also need to end the day where they can legally start the next one, which further constrains which departure times are usable at all.

Why the cheap fares sit at the edges

Demand is highest for mid-morning and early evening departures, so those flights sell well without discounting. The slots nobody wants have to be filled with price.

The very early and very late services are therefore where the low fares live. The saving is real, and it is paid for in sleep and in ground transport.

Reading a timetable with this in mind

A route with several daily frequencies indicates an airline holding a block of slots, which means more rebooking options when something fails during the day.

A single daily departure, particularly at an odd hour, signals a thin slot position. Disruption on that service usually means waiting a full day rather than a few hours.