Each destroyable part has a value. Each time a hit is registered, whether from ammo or collision, the calculated value of the damage is applied to the destroyable part involved. Once the damage threshold is reached, the part is destroyed. Depending on the part involved, it could result in pilot death, a wing falling off, a gear falling off, etc.
Sounds like your bullets were hitting the landing gear and not the wing; they are two separate "destroyable objects".
