Whether a flight leaves on time is usually decided on the ground rather than in the air. The turnaround between arrival and departure is where the schedule is won or lost.
An aircraft only earns while it is flying
Ownership, financing and crew costs continue regardless of whether the aircraft moves, so time parked is time spent without revenue against it.
Airlines therefore design schedules to keep aircraft in the air for as many hours as the day allows, which means minimising every ground period between flights.
Short-haul operations feel this most acutely, because an aircraft may fly four or more sectors and every turnaround multiplies across the rotation.
The turnaround is a sequence with a critical path
Cleaning, catering, refuelling, baggage and boarding overlap partially, but some tasks cannot begin until others finish, and those form the constraining chain.
Boarding is usually the last item and the hardest to compress, since it depends on passengers arriving at the gate and stowing bags at their own pace.
Anything that delays an earlier step pushes boarding later without any possibility of recovery. The lost minutes cannot be won back at the runway.
Delay propagates down the day's rotation
A late arrival hands its lateness to the next departure, which arrives late in turn and passes the problem on again.
Because the schedule contains little slack, the delay accumulates rather than dissipating. A short morning problem can become a substantial evening one.
This is why the same aircraft appears repeatedly in a day's disruption reports. The passengers change but the underlying rotation does not.
Buffers are deliberately kept small
Airlines could build generous ground time into every turnaround, and punctuality would improve immediately. The cost would be fewer flights from the same fleet.
Schedule planners choose a level of buffer that trades expected delay against aircraft use, which means some delay is an accepted outcome rather than a failure.
Where reliability matters most, buffers are placed selectively rather than evenly, typically before flights with tight onward connections.
The last flight absorbs everything
Evening departures inherit the accumulated slippage of the whole day, which is why late flights are cancelled or delayed disproportionately often.
Crew duty limits compound this. A crew approaching its permitted hours cannot simply keep working, and finding a replacement late at night is difficult.
Booking earlier in the day is therefore less about airports being quieter and more about sitting nearer the start of the aircraft's chain of commitments.