H.M. (Jack) Horner, 79, chairman emeritus of the United Technologies Corp. who had long been active in the aviation industry, died May 9 in a hospital here. The cause of death was not reported.

Mr. Horner, who lived in Hartford, was a native of New Haven, Conn., and earned a bachelor's degree in mechanical engineering at Yale University in 1926. Earlier that year, he had joined Pratt & Whitney aircraft company as an outside stock chaser. By May 1940, he was general manager of Pratt & Whitney. Two years later, he was named vice president of United Aircraft, Pratt & Whitney's parent company, and a predecessor of United Technologies. In 1943, he was elected United's president.

During World War II, Pratt & Whitney supplied seven out of 10 military propellers. Its Chance Vought division was a major builder of fighter planes, including the famed F4U-4 "Corsair," for the Navy and Marine Corps.

Mr. Horner served as chairman of United Aircraft from 1956 to 1968. He remained on the board of directors until 1970. During his years with the company, it grew from less than 100 to more than 75,000 employes. The concern is now involved not only in flight propulsion, rocketry and space technology, but is a leading producer of air conditioning machinery, elevators and escalators.

Survivors include his wife, the former Lela Thomas Shumate of Hartford; a son, Leonard Mansfield Horner of Fort Worth, Tex.; a daughter, Lela Burwell Horner of Haddonfield, N.J.; three grandchildren, and four great-grandchildren.