A bill entitled the Posse Comitatus Improvement Act of 1989.

Expresses certain congressional findings concerning drug interdiction assistance activities by members of the armed forces of the United States. Authorizes members who are assigned to duty on vessels of the Navy to which a member of the Coast Guard has been assigned, who have been provided with appropriate training in drug law enforcement functions of the Coast Guard, and who have been designated by an authorized member of the Coast Guard to perform drug law enforcement functions of the Coast Guard, to assist or perform, outside the United States, any of the drug law enforcement functions of the Coast Guard. Protects such members from any personal liability arising from any actions taken with regard to such drug law enforcement assistance.

Directs the President to deploy additional radar surveillance AWACS and other aircraft for drug interdiction assistance, especially along the southwest and southern U.S. borders. Requires the Department of Defense (DOD) to make available to the Coast Guard and Customs Service certain aircraft for the pursuit of aircraft or vessels involved in illegal drug operations.

Directs the President to request funding for such drug interdiction assistance activities for FY 1989 through 1991. Directs the Secretary of Defense to create for the Military Drug Interdiction Program a major force program category for the five-year defense plan of DOD. Directs the President to submit in the DOD budget for FY 1991 a request for funds for such Program in the form of the major force program category.

Directs the Secretary to report to specified congressional committees on the use of deadly force by members of the armed forces while engaging in drug interdiction assistance activities.

Requires the Chairman of the Joint Chiefs of Staff to direct the preparation of a plan to provide additional flight hours of specified aircraft for operations off the coast of Colombia in order to implement a certain defense-in-depth concept for air interdiction. Directs the Secretary to report to specified congressional committees on the specific details of the plan.