Any aircraft rudder is subject to considerable forces that determine its position via a force or torque balance equation. In extreme cases these forces can lead to loss of rudder control or even destruction of the rudder, as on American Airlines Flight 587 (the same principles also apply to water vessels, of course, but it is more important for aircraft because they have lower engineering margins). Maximum rudder deflection is controlled by a rudder travel limiter. The largest achievable angle of a rudder in flight is called its blowdown limit; it is achieved when the force from the air or blowdown equals the maximum available hydraulic pressure.