Japan’s post-war jet training system was built on American aircraft and on the Fuji T-1, which had flown in 1958. By the late 1970s the T-33s that formed the bulk of the intermediate fleet were worn out, and the supersonic Mitsubishi T-2 was too expensive to use for every stage of training. The answer was an intermediate trainer, the MT-X, sitting between primary propeller training and the advanced and fighter conversion courses.

Three Japanese firms competed. In November 1981 Kawasaki won, but the programme was structured as a national effort: Kawasaki led and assembled the aircraft, while Mitsubishi and Fuji each built major sections. The engine went to IHI, whose F3 turbofan gave Japan a domestically designed powerplant for the type. The original plan called for around 220 aircraft and service entry in 1988, and the programme came close to that target, with four prototypes and 208 production aircraft. A heavier, supersonic development was studied but shelved, leaving the T-4 as a pure subsonic trainer.

Designed for a long working life, the T-4 has now been in service for more than three decades. In 2019 a problem with a baffle inside the F3 engine caused internal damage and led to a grounding of the fleet, disrupting both pilot training and the Blue Impulse display schedule. Japanese commentators have linked the episode to the age of the engines, which were designed in the early 1980s.

Japan began studying replacement options in 2021. At an April 2024 summit, the United States and Japan agreed to explore joint development of a trainer to succeed the T-4, and Tokyo issued a request for information in October 2024. Reported contenders include the Boeing T-7A Red Hawk, the Leonardo M-346 and a Mitsubishi design. The crash of a T-4 near Inuyama on 14 May 2025, which killed both crew members, added urgency. The air force carried out emergency inspections of all 196 remaining aircraft, many of them now well over 30 years old.

The T-4 seats its crew in tandem under a one-piece canopy, with the rear seat raised so the instructor can see forward over the student’s head. Ejection seats are fitted for both crew. The design made extensive use of computer-aided design and manufacture, and composite materials, including carbon fibre, appear in the radome, the vertical tail and the control surfaces to save weight.

The avionics were digital from the start, with ring-laser-gyro navigation equipment from Honeywell, and an on-board oxygen generating system under licence. The Blue Impulse aircraft differ from the training fleet mainly in their paint, a smoke-generating system and a strengthened canopy for protection against bird strikes during low-level display flying. There has been no export version and no major structural variant: the T-4 is essentially the same aircraft today as when it entered service, which is part of the reason Japan is now looking for a modern successor with a glass cockpit closer to the F-35.

• XT-4: four prototypes, the first flying on 29 July 1985.
• T-4: the production intermediate trainer, 208 built.
• Blue Impulse T-4: standard airframes with a smoke system, strengthened canopy and display livery, flown by No. 11 Squadron at Matsushima since 1995.
• Liaison T-4s: aircraft allocated to fighter wings, test units and air command support flights for transport and continuation training.

The JASDF does not publish an hourly cost for the T-4, so any figure is an estimate. Two small turbofans of the 16 kN class burn far less fuel than a fighter engine, but twin engines mean twice the engine maintenance, and an ageing fleet adds inspection and spares costs. Comparable twin-engined trainers in European service are usually quoted at a few thousand US dollars per flight hour, which suggests something in the region of US$3,000 to 6,000 for the T-4. Japanese commentators note that maintenance costs probably began rising in the 2000s, as is typical of aircraft past 15 years of service, and the 2019 engine grounding and 2025 inspections are a reminder that an old trainer becomes steadily more expensive to keep flying.