A330 - Longitudinal Stability
Trim Drag is the drag produced as a by-product of the extra lift necessary to compensate for the aerodynamic download on the tailplane (nothing to do with trim tabs protruding into the airflow.) In the A330 Trim Drag is reduced in the cruise by pumping fuel aft into the tailplane tank, thus reducing the required downward lift on the tailplane. The aft transfer takes place automatically as the aircraft climbs through 25,000 feet and, if any fuel remains in the tailplane tank during descent, it is pumped back into the mainplane tanks as the aircraft descends through 25,000 feet. The resulting decrease in Total Drag should give up to 5% reduction of fuel burn in the cruise.
However, you don't get it for nothing. The result is a reduction in longitudinal stability, which is why it is restored prior to landing.
Hand flying an A330 at altitude in Normal or Alternate Law in benign conditions demands a high degree of skill and concentration, considerably more so in Direct Law. In moderate or greater turbulence it becomes progressively more difficult. Should consideration be given to inhibiting or reversing the aft fuel transfer when severe turbulence is forecast for the planned route and so give the pilots more positive longitudinal stability to help them handle the steed?
Perhaps a Test Pilot could comment on the resultant difference in handling qualities.
Neppie