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Interesting that you mention the 707 since the UK ARB determined it to be wholly unsafe until Boeing was forced to install hydraulic boost for the rudder. Seve

Interesting that you mention the 707 since the UK ARB determined it to be wholly unsafe until Boeing was forced to install hydraulic boost for the rudder. Several of the type had already been lost in training accidents due to lack of pilot command on the rudder. The US regulator showed little interest. > Altering the 737, with known and proven characteristics, is low risk But that's the problem; altering aerodynamics, adding three tonnes of empty weight, new FBW wing spoilers and a higher thrust line means that the characteristics are no longer proven and you should be testing from scratch again.

WalterBright 8y ago
The 707 issue you mention is not relevant to this. The yaw damper is, and was put in to correct dutch roll instability. I believe all jet airliners have such a device installed. Airplane design is constantly changed throughout production. This is normal, and Boeing knows how to do these things. Re-engining, altering wing shapes, doing stretch versions, all are routine. All airplanes have instability issues, and there is no indication that the crash was caused by an unknown aerodynamic instability.