Press ''T'' then type the cheat.

Unlimited ammunition: you got what i need
Reload weapons: food goes here
Refuel: big gulp
Invincibility: youre here forever
Disable crashes: damn that corner
Repair damage: chiliburger
Plane cannot be hit: spindive
Auto level and fly upside down: upside down
Paper airplane: paperairplane
Cannot crash on water: water boy
Display throttle: show throttle
Display speed in mach units: show mach
Display air speed: show velocity
Display ground speed: show ground speed
Display fuel usage: show fuel
Display fuel usage as %: show fuel usage
Display altitude in feet: show altitude
Display angle of attack: show aoa
Toggle blackouts: blackout
Toggle flameouts: flameout
Toggle flatspins: flatspin

As it is said in official guide: "If you are undertaking a Campaign mission, the number of weapons available is the total number you can have for the entire campaign, unless friendly transport aircraft or convoys bring in more. If the transports in a particular campaign get shot down, you may face ammunition shortages in that Campaign."

If at some point in campaign you find yourseld in situation where you can't loadout your plane, use alt+tab, go into main game folder and delete FSLOT01.cmp file. This file is created when your .sav game file is created.