Prince George's County Executive Parris Glendening announced the appointment of a 16-member task force yesterday to consider alternatives to TRIM, the charter amendment that has frozen property tax collections since 1979.

Glendening said he expects the task force to come up with a proposal by late fall that he can send to the County Council for possible placement on the 1984 ballot.

In his campaign last year, Glendening unsuccessfully urged voters to amend the measure to allow for inflationary increases and taxes added by new construction.

He said yesterday he had planned the task force for some time, but acted this week when he learned that the police and fire unions were close to completing their own proposal to modify TRIM.

Glendening said he wants to prevent splintering of efforts to modify the tax proposal and said the goal of the task force was to achieve "a united and cohesive effort, incorporating the concerns of all citizens and special-interest groups."

The panel will be headed by Raymond G. LaPlaca, a partner in Carrollton Enterprises and immediate past president of the county's Chamber of Commerce. It includes business and community leaders, plus representatives of the schools and public safety employes.