A rudder lock occurs when the force on a deflected rudder, developed during a steady sideslip, suddenly reverses because the vertical tail stalls. The rudder may then stick at full deflection and the pilot cannot re-center it. A mechanical linkage between the pedals and the rudder is required for this phenomenon, which is usually known as ‘rudder lock’.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a rudder lock in aviation?
A rudder lock is a condition that prevents the rudder from returning to neutral and leaves it stuck at full deflection, making the pilot unable to re-center it. This catch-up situation accentuates the floating tendency of the rudder, especially when aerodynamic balance allows the floating angle to catch up with or overshoot the required rudder deflection, demanding opposite pedal forces to operate the rudder. Rudder lock occurs when the force on a deflected rudder in a steady sideslip suddenly reverses as the vertical stabilizer stalls, which could violently swing the aircraft into the sideslip and result in a higher trimmed sideslip angle.
To guard against wind-induced damage while the aircraft is inactive, a Rudder Wind Gust Lock is used. It consists of a modified rudder push rod end bearing and locking mechanism that reacts rudder gust loads on the aft pressure bulkhead. The device, designed by Lockheed Martin, is simple, cost effective, and easily activated from within the aircraft during mooring. It incorporates a mechanical fuse which the pilot can override if the lock is inadvertently left engaged or if extreme gusts exceed design limits.
What causes rudder lock in an airplane?
Rudder lock occurs when the force on a deflected rudder in steady sideslip suddenly reverses as the vertical stabilizer stalls, leaving the rudder stuck at full deflection. This aerodynamic reversal takes place during an excessive sideslip, and it is particularly likely when the vertical tail stalls at a high sideslip angle.
The phenomenon is aggravated if rudder cables are slack or break. The surface slams back and forth against internal stops, tearing the rudder skins at the hinge opening and bending the vertical stabilizer hinge points. Wind gusts, jet blast, or propeller wash supply the same damaging impulse, and once the hinge points are distorted, the locking mechanism jams under the force of impact, freezing the rudder in place.
An example is the loss of the complete fin and rudder assembly on American Airlines Flight 587 after the pilot used full rudder deflections while following in the wake of a very large jet. The same sequence of over-travel, torn skins, and bent hinges has been repeated in smaller aircraft when gust locks were left equipped or re-engaged after lift-off.
Although rudder lock is an aerodynamic event, not a mechanical failure, it precipitates a crash: if the rudder remains at full deflection, the airplane enters an uncontrollable roll or unusual attitude from which recovery is impossible, as seen in USAir Flight 427.
What is an external rudder lock on a plane?
An external rudder lock is a gust lock which is an external device attached directly to the aircraft's control surfaces, and it locks control surfaces while preventing rudder pedal movement.
An external rudder lock is a clip or pin that is physically placed on the rudder itself while the aircraft is parked. It aligns the rudder with the adjacent structure, holding the surface in a neutral position and preventing random movement caused by wind, jet blast, or propeller wash. Because the lock is fully outside the cabin, it must be attached and removed by hand on the ramp.
External rudder locks are significantly more dangerous than internal locks because they are set and forgotten. Their remote location offers no cockpit reminder, and they lack a safety feature that locks out the throttle or engine-start controls. Consequently, a forgotten lock stays in place during taxi and take-off, creating a severe hazard. For this reason, external rudder locks must be tagged with a large red remove-before-flight streamer.
