Your news story, "A300 fin damaged in 1997 incident" (Flight International, 26 March-1 April) did not touch on control breakout force, which could be increasingly relevant as that investigation progresses.
This is the amount of force that must be applied before the control moves. On the Airbus A300-600 at 250kt (460km/h), as was the case on American Airlines flight AA587 in 1997, the breakout force is 10kg (22lb) with 4.5kg incremental force for full authority. This is a fixed ratio system, in that as the speed increases there is less rudder pedal travel available. Airbus calls it rudder travel limiter.
This is not unique to Airbus. Douglas used fixed ratio systems on all its jets, limiting motion with variable stops or blowdown. Blowdown refers to airloads exceeding activator force which keeps the rudder in tune with fin loads, preventing full deflection at high speeds. Boeing used fixed ratio with blowdown limiting on the 727 and 737 and switched to a ratio changer system for the 747 and subsequent aircraft.
With this system, full rudder pedal travel is always available, but results in less and less rudder motion as speed increases. Obviously, there is also a relatively small breakout force in such a system. On the 747-400 the rudder motion runs from 30° to 4°.
Looking at both systems objectively, it should be obvious that in the ratio changer system overcontrol is difficult, but with the fixed ratio/rudder travel limiter, system overcontrol at high speed is more likely.
David Connolly
Aviation consultant, Brussels, Belgium
Source: Flight International
