AAL A-300 accident

[Flight Safety]

My apologies in advance for begin so focused on the design issue...

Managing the side forces on the vertical fin is THE central issue here, in regards to maintaining the structural integrity of the fin, as loss of the fin usually means loss of the aircraft.

We all know that the rudder's deflection angle has to be progressively limited as the airspeed increases, to keep the side forces generated by the rudder, within the design load limits of the vertical fin.

The design issue for me is how the rudder limiting system interacts with the pilot, to help or hinder the pilot's ability to correctly and accurately modulate those side loads.

As airspeed increases in the 767 design, the rudder limiting system does not affect the length of rudder pedal travel or the pedal forces required, but instead progressively limits the rudder's deflection angle at the fin. This system essentially maintains a constant relationship between the amount of rudder pedal travel and pedal force applied, to the amount of aerodynamic control force and side force being generated at the fin, regardless of the airspeed.

As airspeed increase in the A300-600 design, the rudder limiting system progressively and directly limits the amount of pedal travel and pedal force, thus resulting in the progressively limited deflection angle of the rudder at the fin. The problem with this system, is that NO constant relationship between pedal force and pedal travel to side forces generated at the fin, is being maintained here.

You can think of the human interface design problem presented here in terms of "modulation gain", where "gain" (normal use of the word, expressed as a ratio of controlling power to the amount of power being controlled) refers to the ratio of the amount of controlling power and movement of the pilot's legs, to the amount of side force begin generated by the pilot's control inputs.

In the 767 system, the control "gain" is basically constant, meaning the amount of force and travel appiled to the rudder pedals always produces the same amount of aerodynamic force and side force on the fin, regardless of the airspeed. So 50 percent pedal travel means 50 percent side force, at all airspeeds. This is a very intuitive system for the pilot.

In the A300-600 system, the control "gains" are screwed because the relationship between the amount of pedal force and travel to side force generated at the fin, is constantly changing with the airspeed. So if the pilot wanted to apply about 50 percent side force, he would have to know the airspeed, know the rudder pedal travel limit for that speed, then move the pedal 50 percent of that amount. As you can see, this system makes it much more difficult for the pilot to accurately modulate (or control) the amount of side force being applied at the vertical fin.

So the design question for me is, does the rudder limiting system help the pilot's ability to accurately and correctly manage the amount of side forces being applied to the vertical fin, while recovering from an attitude emergency? In the 767 system I think it does, because the control gains are constant. In the A300-600 system, I strongly believe it does not.

Up to now I've ignored the regulation issue. Many of use know the weakness of the regulations that specify the design load limit of the vertical fin, specifically how the regulatory design load limit is inadequate for a full rudder reversal at speed. If the regulation is ever modified to include greater strength for the vertical fin, I think it should also include a provision for rudder limiting systems that present constant travel and force gains to the pilot, so aerodynamic side forces and side loads on the vertical fin can be accurately controlled by the pilot.