A diversion is a fuel decision made in the air

A diversion feels like an improvised response to bad weather. It is closer to the execution of a plan that was written before the aircraft left its origin.

Every flight carries a named alternate

Flight planning does not simply calculate the fuel needed to reach the destination. It adds fuel to fly on to a nominated alternate airport, plus a reserve to hold above it.

That alternate is chosen for weather, runway length and the ability to handle the aircraft type. It is written into the plan whether or not conditions suggest it will be needed.

The consequence is that a diversion airport is rarely the nearest one. It is the one the paperwork already accounted for, which may be well past the destination.

Holding burns the margin

When arrivals back up, aircraft are stacked in a holding pattern and released in sequence. Every minute in that pattern consumes fuel that was set aside for the alternate.

Each crew calculates a point at which continuing to hold would eat into the reserve. That figure is not negotiable, and reaching it converts a delay into a diversion.

Aircraft therefore leave the hold at different times, which is why one flight lands at the destination and another from the same stack ends up two hundred kilometres away.

The ground is the harder problem

An unplanned arrival needs a stand, steps or a jet bridge, ground handlers, and often a fuel truck. A small airport at night may have none of these on duty.

Immigration is a further constraint. Passengers cannot always disembark, because the airport may not be staffed to process an international arrival that was never scheduled.

This is why aircraft sometimes sit on a remote stand for hours with passengers on board. The delay is a staffing and permissions problem rather than a flying one.

Crew hours end the flight before the aircraft does

Flight and duty time limits cap how long a crew may work. A long hold followed by a diversion can push a crew to that limit while the aircraft is still airworthy.

Once the limit is reached the sector cannot continue, regardless of how close the destination is or how willing the crew might be. A replacement crew has to be positioned.

Where the passenger ends up

Onward travel from a diversion is arranged by the airline, and the usual solution is a coach rather than a second flight, because refuelling and re-slotting an aircraft takes longer.

Bags stay with the aircraft unless it is going nowhere, so a coach transfer often separates passengers from luggage for a day. Carrying essentials in the cabin is the practical hedge.