Retired Navy Rear Adm. Theodore C. Lonnquest, 87, who was an aviator, aeronautical engineer, and staff officer before retiring from active duty in 1956, died of a heart ailment March 9 at Bethesda Naval Hospital. He lived in Chevy Chase.

Adm. Lonnquest received the Legion of Merit for his work as engineering director of the Navy's Bureau of Aeronautics. After World War II, he held posts dealing with atomic energy and was on the staff of the joint task force commander at nuclear bomb tests in the Pacific.

During the late 1940s, he again served with the Bureau of Aeronautics as its deputy chief for research and development. He then represented the Bureau at Wright Patterson Air Force Base in Ohio before retiring.

He was a past president of the American Society of Naval Engineers, a fellow of the Institute of Aeronautical Sciences, and a member of the Cosmos Club. He belonged to All Saints Episcopal Church in Chevy Chase. He was a 32nd degree Mason.

Adm. Lonnquest was a native of Massachusetts and a 1917 graduate of Dartmouth College, where he was a member of Phi Beta Kappa. He earned a master's degree at the Massachusetts Institute of Technology.

He served in the Navy's flying corps during World War I. Between World Wars, he attended the Naval Postgraduate School at Annapolis and served with air units aboard the carriers Langley and Saratoga.

Survivors include his wife, the former Marie Born, and a daughter, Elaine Richard, both of Chevy Chase, and five grandchildren.