Airport Tools contain tools to construct an air transport network. Tools may build or remove: taxiways & runways for aircraft, Airstops (used to pick-up & drop-off, cargo or passengers), aircraft depots (to purchase and manage air transport vehicles) and and various airport buildings. If playing with timeline, then as time passes in Simutrans more tool-options may appear.

Click airplane-icon at top of game-view to open toolbar.
Hover mouse-cursor over tool-option (after opening or clicking on toolbar) to see name & where appropriate: construction cost, maintenance cost in brackets, and maximum speed limit.

{Tips: One way to build a simple airport is:
i) construct and connect Taxiway(s) and Runway(s);
ii) place airstop(s) on end(s) of taxiway(s);
iii) add airport buildings as required.}

Tools may include, from left to right:

Taxiway: tool builds ground movement areas for aircraft to go from an Airstop to a Runway. Taxiways are built on level ground in game-view.
IMPORTANT: If Taxiways are connected to the end of Runways aircraft may not take-off and land.
To build a Taxiway: click on tool to select (changes cursor to taxiway), then click on terrain for start-point (shows a bulldozer in game-view and displays map co-ordinates at bottom right of display), & finally click on terrain or runway for end-point.
{Tips: Use Destroy/Remove to remove individual pieces of Taxiway. Undo [z] does not refund constructions costs.}

Runway: tools build an airstrip, used by aircraft to take-off and land. Runways are built on level ground in game-view and may cross each other.
To build a Runway: click on tool to select (changes cursor to way), then click on terrain for start-point (shows a bulldozer in game-view), & finally click on terrain for end-point.
{Tips: Leave ends of Runways unconnected otherwise aircraft may not take-off or land. Use Destroy/Remove to remove individual pieces of Runway. Undo [z] does not refund constructions costs.}

Remove airstrips: tool removes a segment of Taxiway or Runway, when no aircraft are present, between two points in game-view. Use of tool incurs a construction cost.
To remove an airstrip: click on tool (changes cursor to a red-cross); then click on strip that is to be deleted (selects deletion-point shown by a red-cross in game-view); & finally click at another point on strip, to remove portion up to first deletion-point. {Tips: Leave end of Runways unconnected otherwise planes may not take-off or land. Use Destroy/Remove to remove individual pieces of airstrips.}

Air Depot: tools build hangars for purchasing & managing aircraft. Hangars have a maintenance cost & are built on the end of a piece taxiway.
To build a Hangar: click on tool (changes cursor to a depot), then click on an end of a taxiway.
{Tips: Remove depots with Destroy/Remove.}

Airstops&boarding bridge: tools build Stops in game-view, for aircraft to pick-up and drop-off passengers or cargo.
An Airstop when not built adjacent to an existing Stop will create a new Stop.
Airstops are built on the end of a piece taxiway; and have a maintenance cost and catchment area for goods, passengers & mail. An aircraft will only land at an airport if there is an free Airstop available.
To build an Airstop: click on tool to select and then click on an end of a taxiway.
{Tips: Remove Airstops with Destroy/Remove. Press [v] to show/hide catchment area for goods & passengers in game-view.}

Airport buildings: tools construct extensions for Stops which may increase maintenance costs and the capacity & catchment area for goods & passengers. In the corner of some tool-options an icon (used in Station List & Stop Information) shows which items the extension enables the Stop to handle.
To build an extension: click on tool to select extension (changes cursor to extension), then click required position, next to an existing Stop in game-view. The new extension is now considered to be part of the Stop.
{Tips: Remove extensions with the Destroy/Remove. Press [v] to show/hide catchment area for goods & passengers in game-view.}