An air vehicle that is launched from inside a launcher, includes a release mechanism for releasing fins of the vehicle from a stowed condition to a deployed condition. The release mechanism includes a pin that is located within a cavity in the fuselage of the air vehicle. Pressurized gasses initially fill the cavity in the fuselage. The launcher includes a reduced-pressure portion such as from a muzzle brake. When the air vehicle passes into the reduced-pressure portion of the launcher, the gas pressure behind the air vehicle is reduced. This causes the pressurized gas within the cavity to drive the release mechanism backwards out of the cavity. The length of the pin may be used to control the timing of the fin deployment, the delay between the initial movement of the release mechanism out of the cavity, and when the fins are released.