STACK — Air Traffic Puzzle

What is STACK — Air Traffic Puzzle

STACK is a turn-based air traffic puzzle played on a square radar screen. Every aircraft holding in the sector has a heading, shown by its nose, and a flight level from 1 to 3, shown by its size, its colour and the badge beside it: green is level 1, amber level 2, cyan level 3. Nothing moves until you say so. Clicking an aircraft clears it for takeoff and it flies straight ahead, at its own level, until it crosses the sector boundary. With a mouse, hovering previews the flight; on a touch screen the first tap previews and a second tap confirms.

The interesting part is the wake. Every aircraft the departure overflies at a different level drops one level, and anything pushed below level 1 lands, leaves the board and drags its four orthogonal neighbours down with it, which can land in turn. One takeoff can clear half a sector, and it has to: the budget is a clearing line found by a solver plus a small slack, four spare at first and only two from sector nine. Sectors grow from five squares across to six at sector three and seven at sector seven, and then beacons and weather cells arrive as well.

How to play

1. Click an aircraft to clear it for takeoff, or tap once to preview and again to confirm; it flies straight ahead at its own flight level until it leaves the sector.
2. Every aircraft it overflies at a different level drops one level, while meeting one at the same level is a loss of separation that deletes both and voids the takeoff.
3. An aircraft pushed below level 1 lands and disappears, and its landing wake pushes the four aircraft orthogonally around it down a level, which can start a chain.
4. Empty the sector before the budget runs out; beacons appear at sector four and re-vector whoever crosses them, weather cells at sector six, blocking aircraft at or below their tops.

Strategies that work

• Green aircraft are one push from the ground, and landings chain only through the four orthogonal neighbours, never diagonally. Find greens touching side to side, overfly the one at the edge, and the whole block comes down in a single takeoff. Inside one takeoff the first landing pays 50, the second 100, the third 150.
• When in doubt, launch a cyan aircraft. At level 3 it can only lose separation against another level 3, and weather tops never exceed 2, so nothing stops it while it sinks every green and amber it passes. The catch: a cyan you never launch needs three pushes of its own to land.
• A loss of separation costs far more than two aircraft. The whole takeoff scores zero and the combo resets, so a four-landing cascade can evaporate on the last square of the path. The exception is a sector's final takeoff, where a deliberate conflict removes two aircraft and only the clear bonus is still in play.
• The combo multiplier climbs half a step for each consecutive takeoff that lands an aircraft, up to four, and a takeoff that lands nothing drops it back to one. Do the setup early: a takeoff that only sinks traffic into position is free at the start of a sector and ruinous later.
• Slack is a resource, not a cushion. A spare takeoff is worth 200 when the sector clears and matching the solver line adds 300, so finishing two takeoffs early can beat one more chain. If you have spent the number printed under the board and half the traffic is still holding, undo with Z.

A little history

Air traffic control has been a computer-game subject for as long as screens could draw a radar sweep, and almost every version is a test of nerve: aircraft arrive on a clock, fuel drains, and you lose for being slow. STACK keeps the vocabulary and throws away the clock. Nothing moves until you clear a takeoff, so a sector is a finite position rather than a reaction test, and a solver checks that each one has a clean line. The single borrowed piece of real aviation is wake turbulence, and that one rule builds the whole puzzle.

Frequently asked questions