Background
A Piper Arrow, registration N8604N, and a Piper Mirage, registration N146PM, collided at the intersection of runways 12R and 7 during landing and takeoff operations, respectively.
Sequence of Events
The pilot of the Piper Arrow contacted the local controller and reported inbound for landing when the airplane was approximately 6 miles north-northeast of the airport, tracking a course of about 200 degrees. The airplane remained on that course, and the pilot was cleared to land when the airplane was about 2 1/4 miles north-northwest of the airport. About 20 seconds later, the airplane began a left turn towards the airport, flying a standard base leg to final approach.
When the local controller cleared the Piper Mirage for takeoff, the Piper Arrow was about 1 1/2 miles northeast of the airport on final to runway 12R, with the data tag on the D-Brite radar showing a ground speed of 110 knots. Assuming that ground speed remained at or below 110 knots, the airplane was about 1 minute away from touchdown. This meant that the Piper Mirage had less than 1 minute to power up, taxi onto runway 7, begin its departure roll, and travel about 725 feet to pass through the intersection before the Piper Arrow crossed the landing threshold for runway 12R.
Controller Actions
The Piper Arrow was cleared to land on runway 12R 54 seconds before the Piper Mirage was given a takeoff clearance on the intersecting runway. During this time, the local controller and local assist controller were both viewing the D-Brite radar and looking out the tower cab's rear windows for a helicopter that had entered the Class D airspace without authorization. With both controllers' attention diverted to the helicopter, they failed to effectively monitor the runway operation and did not ensure proper separation was maintained between the two airplanes operating on the intersecting runways.
Conclusion
The collision occurred at the intersection of runways 12R and 7. The minimal time available for the Piper Mirage to depart before the Piper Arrow arrived warranted close monitoring by the controllers to ensure that separation was maintained at the intersection.