Eyrix — Blog

Tail assignment planning: building aircraft rotations that actually close

Assigning a specific aircraft — a tail — to a flight looks like a lookup: find a tail of the right type that isn't already flying, and drop it on the leg. At a handful of aircraft that works. As soon as the fleet grows and legs chain into multi-day rotations, tail assignment stops being a lookup and becomes a routing problem: each tail has to be in the right place, at the right time, legal to fly, for every leg you give it.

A rotation is a chain, not a list

The mistake most manual planners make is treating a day as a list of flights to fill rather than a set of rotations that have to close. A rotation closes when every leg hands off cleanly to the next: the aircraft physically ends leg n at the station where leg n+1 departs, with enough ground time in between. Break that chain once — a tail that "flies" a leg departing from a station it never arrived at — and the whole rotation is fiction, even though every row looked full on the sheet.

The four things a valid tail assignment has to satisfy

1. Fit. The tail's seats, range and payload have to cover the leg. A regional turboprop can't take a leg that needs jet range; a smaller tail can't take a leg sold at higher capacity.
2. Continuity. The tail has to be at the departure station — because a previous leg put it there, or because you've planned a positioning leg to get it there.
3. Turnaround. The ground time between two legs has to clear the minimum turnaround for that station and aircraft — refuel, offload, board, pushback.
4. Availability. The tail isn't in a maintenance check window or AOG across the leg's time.

Miss any one and the assignment is infeasible in practice, however clean it looks on a Gantt row.

Why utilisation and tail assignment are the same problem

Good tail assignment isn't only about feasibility — it's the main lever on aircraft utilisation. Put the wrong tail on a leg and you tie up an expensive asset a cheaper one could have flown, or you force an empty positioning leg that a better rotation would have avoided. The rotation that closes with the fewest deadhead hours is usually also the one that keeps the most tails productive.

What an engine does that a grid doesn't

A scheduling engine treats tail assignment as a set of hard constraints — fit, continuity, turnaround, availability — and refuses to place a tail on a leg unless the rotation still closes around it. Then, among the assignments that are feasible, it optimises for utilisation rather than grabbing the first available tail. Crew sit on top of the same rotation: the right tail is only half an assignment until a qualified, legal crew can close the same chain. That is the difference between a grid that records where tails went and an engine that works out where they should go.