A pilot direction indicator, or PDI, is an aircraft instrument used by bombardiers

Bombardier (air force)

A bombardier , in the United States Army Air Forces and United States Air Force, or a bomb aimer, in the Royal Air Force and other Commonwealth air forces, was the crewman of a bomber responsible for assisting the navigator in guiding the plane to a bombing target and releasing the aircraft's bomb...

 to indicate heading changes to the pilot in order to direct them to the proper location to drop bomb

Bomb

A bomb is any of a range of explosive weapons that only rely on the exothermic reaction of an explosive material to provide an extremely sudden and violent release of energy...

s. The PDI is used in aircraft where the pilot and bombardier are physically separated and cannot easily see each other.

PDI's typically consist of a dial that is installed in the pilot's instrument set on the main console, with an arrow pointer than can be moved to indicate how far and in what direction to correct the heading. The bombardier typically has a switch to move the pointer to the right or left, and a repeater dial so he can see the setting.

The Norden bombsight

Norden bombsight

The Norden bombsight was a tachometric bombsight used by the United States Army Air Forces and the United States Navy during World War II, and the United States Air Force in the Korean and the Vietnam Wars to aid the crew of bomber aircraft in dropping bombs accurately...

was originally designed with the idea of automatically directing a PDI and thereby simplifying the bombardier's task. In later versions, the PDI was supplanted by a