Originally Posted by
CAFB 04-12
The Dutch roll was caused by a malfunctioning SYD, which is why it occurred at low altitude. An uncommanded yaw (crab) also occurred immediately after takeoff causing some controllability issues for the copilot who was flying the departure.
The AC identified a malfunctioning SYD but didn't turn it off. During a turn to return to base the pilot used rudder to coordinate the turn. This, combined with the oscillating SYD caused the vertical stab to fail. The tail departed the aircraft which caused an unrecoverable pitch down.
The sim does not replicate Dutch roll. Crews are taught that Dutch roll is almost always corrected naturally by the SYD. The only rudder malfunction crews regularly practice is a full-travel hard-over of the rudder, which is quickly diagnosed as such, and the corrective action is boldface.
A couple months prior to the mishap this aircraft experienced an identical malfunction. The crew turned off the SYD and returned to base.
I was surprised at how quickly this mishap developed.
It has been over 20+ years since I flew the R model, but wasn't the SYD always engaged for yaw control? As I recall we pretty much flew the R with our feet flat on the floor or just resting on the rudder pedals. The SYD replaced the rudder axis of the autopilot that was used for yaw control on the A model. You would engage the rudder axis right after takeoff as soon as the water injection ran out (I am really dating myself now). The EFAS on the R model is used for rudder assist during takeoff in the event of an outboard engine failure and worked in conjunction with the SYD to apply up to 10 degrees of rudder below 160 knots. The EFAS gradually decreased rudder authority until all you had was a SYD for normal yaw control. If it was a SYD malfunction, the altitude should have had no bearing on the malfunction. Dutch roll is dutch roll.