Last year cwright developed a LUA script for flyable aircraft in Far Cry. I've now produced a Crysis version and it is working pretty well. I've made a Spitfire model in XSI Mod Tool and used it as the test bed.

Control Options:
Key 8: enter VTOL
Key 9: enter 1. Spitfire
Key 0: enter 2. Spitfire
Key ?: leave
Key E: turn on the engine
Key C: ascent flight and steering the machine with arrow keys
Key H: Fire rockets
Key space: switching between outer and inner sight

Features:
* Repacked as mod
* Spitfire planes

Installation guide:
1. Go to your Crysis game folder -> Electronic Arts -> Crytek -> Crysis -> Mods. (if not existing please create this folder inside your Crysis root folder)
2. Copy the mod folder from the downloaded file into the current game folder (Crysis/Mods).
3. Start Editor per batch file (is included in the Spitfire mod folder.
4. Load level Island