Pitch control operates a C*U control law. C* describes a blending of pitch rate and load factor; at low airspeeds, pitch rate is the main element at high airspeeds, the load factor.
The U term refers to change in airspeed from "referenced trim speed"; if the aircraft is trimmed to a particular airspeed, deviation from it will cause a pitch load, forcing return to that airspeed, thus giving some speed stability.
The pitch-trim switches on the control wheels do not directly control the horizontal stabiliser in the usual fashion. In flight, they request a new referenced trim speed; the PFC output pitches the aircraft nose-up or down, to achieve that new airspeed; the stabiliser then automatically trims to off-load the elevator. As on conventional aircraft, trimming reduces column forces held by the pilot.
On the ground, the pitch trim switches move the horizontal stabiliser directly; the alternate trim levers always move it directly, but first signal a change in the referenced trim speed to the PFCs.
Pitch effects caused by configuration changes and thrust alteration are fully compensated in normal mode.
The pitch-control law incorporates landing-flare compensation, for landing behaviour consistent with a conventional aircraft, which would have been otherwise affected by the C*U control law.
Source: Flight International
