The Honeywell T55 (formerly Lycoming; company designation LTC-4) is a turboshaft engine used on American helicopters and fixed-wing aircraft (in turboprop form) since the 1950s, and in unlimited hydroplanes since the 1980s.

[1] It is produced by Honeywell Aerospace, a division of Honeywell based in Phoenix, Arizona, and was originally designed by the Turbine Engine Division of Lycoming Engines in Stratford, Connecticut, as a scaled-up version of the smaller Lycoming T53.

The T55 serves as the engine on several major applications including the CH-47-Chinook, the Bell 309, and the Piper PA-48 Enforcer.

The T55 also serves as the core of the Lycoming ALF 502 turbofan.

Since the T55 was first developed, progressive increases in airflow, overall pressure ratio, and turbine inlet temperature have more than tripled the power output of the engine.