What happens to HCA with lag pursuit?

When engaging in lag pursuit, the horizontal component of the closure rate between the aircraft and the lead is intentionally reduced. This occurs because the pursuing aircraft is positioned behind the lead aircraft and is unable to follow its path directly in a timely manner. As a result, the relative distance between the two aircraft can lead to an increase in the Heading Crossing Angle (HCA).

The HCA measures the angle between the lead aircraft's path and the line connecting the pursuing aircraft to the lead. In lag pursuit, as the pursuing aircraft attempts to catch up from behind and below, this can cause the HCA to increase since the pursuing aircraft must adjust its heading to align with the lead, which may be moving in a different direction.

Thus, during lag pursuit, the nature of the maneuver results in an increase in HCA due to the geometry of the relative positions and motions of the aircraft involved in the maneuver.