Air Transat flight 961 was at 35,000 feet when the composite material rudder broke off from its hinge, the aircraft soon encountered major stability problem. The crew responded by descending the aircraft and that soften the issue, they landed safely.
Modern jet airliners with swept wings suffer from a particular form of yaw stability issue called “Dutch Roll”, because the opposite side swept-back wing actually face the air head on if the plane yaws into the other direction, producing more lift and more drag, causing the plane to roll into the yaw, then the yaw reverses due to stabilization effect of the vertical fin and the drag difference produced by the wings and producing another opposite roll.
The cycle repeats several times over and depending on airspeed, altitude, CofG, it may go on for quite a while, this is at least very nauseating for occupants, you don’t want half of the plane throw up their breakfast five minutes into the flight, so modern airliners are equipped with “yaw dampers” that uses gyroscopes and accelerometers to detect any yaw disturbances, then drive the rudder the other way to counteract the Dutch Roll, this makes the turn always coordinated and flight controls much easier for the pilots, they don’t have to coordinate the rudder input with aileron inputs, which is already difficult for 200-foot long huge airplanes, feet always on the floor and the plane always fly nicely.
On some airplanes, the yaw dynamic stability can actually becomes negative at higher altitudes and speeds, meaning if the yaw damper fails, the plane would start rocking by itself, the amplitude increases until it breaks apart.
This is a huge safety problem, such aircraft are required to have multiple yaw damper computers, data source and actuators, if one fails, the pilots must immediately fly the plane out of the dangerous speed/altitude in case failures continue to occur and leaves them without any yaw stability.
Back to the 961. Rudder breaking means they lost any meaningful aerodynamic control effect from the rudder, the total surface area of the vertical fin is greatly reduced, thus they immediately started to swing. Luckily the stability is still good enough to not swing the airplane apart, and they were able to descend in time to escape the lowest stability region of the flight envelope.
Author – Tom Chai