Since the B-24's ultimate purpose was to bomb targets, the bombardier had the most important job. The bombardier and the pilot had to be familiar with each other's job and cooperate during the short bombing run during which the bombardier took over control of the aircraft either through the auto-pilot or Pilot Directional Indicator (PDI). The auto-pilot connected directly to the bombsight; the PD1 transmitted desired course changes to the pilot via a needle instrument in the cockpit. Bombing accuracy depended on calculating altitude, true airspeed, bomb ballistics for each type of bomb (intended trajectory from bomber to target), drift caused by lateral winds, and air density.