Dr. Phrixos J. Theodorides, 89, a retired professor of aeronautics at the University of Maryland and a former president of the Pinecrest Citizens Association in Silver Spring, died of pneumonia Sunday at the Sligo Gardens nursing home in Takoma Park.

Dr. Theodorides was born in Serres, Greece. He was educated in Greece and Switzerland, where he took his doctorate in physics at the Federal Institute of Technology in Zurich. He later taught at the University of Smyrna and the University of Greece in Athens. At the latter school, he helped introduce the study of aeronautics in his country.

During World War II, he served with the Greek Air Force. He came to the United States in 1946 and was a visiting lecturer at Harvard University until 1951. He then moved to the Washington area and worked for the Naval Ordnance Laboratory before joining the University of Maryland in 1952.

In addition to being president of the Pinecrest Citizens Association, Dr. Theodorides was a delegate emeritus of the Allied Civic Group Inc., of Montgomery County. He also was a founding member of the Arena Stage and a member of Sts. Constantine and Helen Greek Orthodox Church.

Survivors inlcude his wife, Marcelle, of Silver Spring, a son, Dr. J. Theodorides of Paris, France, and one grandson.