Retired Navy Capt. Theodore C. Lonnquest Jr., 54, an aviator and aeronautical engineer during a career that spanned 30 years, died Wednesday during heart surgery at the Bethesda Naval Hospital.

From 1974 until his retirement for medical reasons in 1975, he commanded the Naval Aviation Intergrated Logistic Support Center at Patuxent River, Md. Since then he had been a senior systems engineer with Systems Consultants, Inc., in Washington.

Capt. Lonnquest was born in Pensacola, Fla. He graduated from the U.S. Naval Academy in Annapolis in 1945. He then served for two years in the Pacific aboard the USS lowa, a battleship.

He was operations officer on the destroyer, the USS Zellars, in the Eastern Atlantic and Mediterranean areas, then trained as a naval aviator. From 1948 until 1959, he had various flight assignments and also studied at the Naval Postgraduate School and Princeton University, where he received a master's degree in aeronautical engineering.

Capt. Lonnquest came to Washington in 1959 to serve at the Bureau of Naval Weapons. Assignments followed in the office of the deputy chief of naval operations for development and as special assistant for air in the office of the assistant secretary of the Navy.

In 1967, he was transferred to Norfolk, Va., as assistant material officer on the staff of the commander Air Force, U.S. Atlantic Fleet.

After duty in command of the Naval Air Rework Facility in Norfolk, Capt. Lonnquest returned to Washington in 1972 as manager of the Ship and Air Systems Project in the office of the chief of Navy material and then as executive director of the research and technology group of the Naval Air Systems Command, an assignment he held until going to Patuxent River.

He is survived by his wife, the former Mary Elizabeth Kunz, and a son, John, both of the home in Chevy Chase; another son, David, of Durham, N.C.; a daughter, Mrs. Scott Moore, of Wintson-Salem, N.C.; his parents, retired Navy Rear Adm. and Mrs. T.C. Lonnquest, of Chevy Chase, and a sister, Mrs. Samuel H. Rickard III, of Williamsburg.

The family suggests that expressions of synpathy be in the form of contributions to the Navy Relief Society. CAPTION: Picture, CAPT. THEODORE C. LONNQUEST