Successfully complete pilot school with at least all bronze awards to unlock the Rustler. The Rustler is airplane based on the P-51 Mustang. Press L to fire the wing machine guns. Additionally, the Rustler can be found at several locations. If you finish the pilot school at your airstrip, it will spawn in the first hanger. It also spawns at the airport in Los Santos and San Fierro; look in the hangers there. You can get also it by entering the races at the Las Venturas Airport. Select the race with the Rustler, then ignore the race and instead take it back to your airstrip and park it in your own big hangar. You will fail the mission, but can re-do it. Once you get back to your airstrip and look in your hanger, your Rustler will be there. Save the game to keep it. This trick can also be used for other aircraft.