The Aircraft Tree is a hierarchical technology tree featured in Ace Combat Infinity and Ace Combat 7: Skies Unknown. When the player first begins, they're given a basic aircraft and a single tuning part at the earliest tier. The player can then advance on the tree by playing the game to unlock more powerful aircraft and parts.
The term tree refers to the player's ability to choose which sequence, or branch, to advance. Completing all branches is not mandatory; it is entirely the player's choice which branches to advance depending on what aircraft or parts the player wishes to unlock.
Each vertical column of the tree is considered to be a higher tier than the last. The right-most tier of the tree houses the most powerful aircraft and the left-most tier houses the least powerful aircraft.
At the start of the game, the player is provided with one aircraft for free. The starting aircraft branches out to connect to a few other aircraft and the player must purchase one of them to proceed on the tree. The player is also provided with one tuning part located below the starting aircraft, but this tuning part is not connected to any branches on the tree.
Exclusive to Ace Combat Infinity, the player could not purchase an aircraft or part without first filling up its research gauge. A technology's research gauge would only be available upon unlocking it from any prerequisite branch. For example, all aircraft branching out from the starting aircraft, the F-4E Phantom II, began researching at the start of the game since the F-4E was already provided. Every online sortie filled up the research gauges depending on certain factors:
- The player's performance
- The team's performance
- Whether the player used Forced Sortie
- Whether the player used an Additional Plant Investment Contract
- Whether a special research event was occurring