Story ID: 030
MU-300: Japan’s Sparkling Gem
By: David Donald
Caption: The Diamond evolved into the Beechjet
Word Count: 1515
Mitsubishi was Japan’s best-known builder of aircraft before and during World War II. In the immediate aftermath of the conflict, the country’s vast industrial empires were broken up and their historical names were forbidden from use. The vast Mitsubishi enterprise was separated into three entities.
With the official ending of the Occupation in April 1952, the companies resumed using their historic names, and the element that had included the former aircraft production became Mitsubishi Heavy Industries (MHI). From 1964, the three elements merged back together under the MHI banner.
With its long history of aircraft production, MHI was naturally at the forefront of the rebirth of the Japanese aviation industry in the post-Occupation period. It was at the heart of a consortium that began development of the YS-11 twin-turboprop airliner in the mid-1950s. In 1956, work started on the MU-2 high-wing, small-cabin twin that went on to become quite a success, with over 700 having been built by the time production ended in 1987.
At the same time, as the Cold War heated up, MHI began building U.S.-designed fighters to swell the ranks of the Air Force. F-86 Sabres were built in considerable numbers in the immediate post-Korean War period, to be followed by F-104 Starfighters, F-4 Phantoms, and F-15 Eagles. Today, MHI remains one of the powerhouses of Japanese industry and one which remains committed to aviation.
Mitsubishi’s Business Jet
Success with the MU-2—especially in the U.S., where Mooney Aircraft sold and supported the type from 1963, and assembled it at San Angelo, Texas, from 1965—encouraged Mitsubishi to explore the general aviation/executive market further. In 1977, a business jet program was born, which crystallized as the MU-300 Diamond.
Initial development was accomplished rapidly. The Diamond was an entirely conventional jet with all-swept surfaces and a T-tail, constructed almost entirely from aluminum alloy. The wings had a quarter-chord sweepback of 20 degrees and incorporated overwing spoilers for roll control and lift-dumping. Powerful flaps—double-slotted inboard, and single-slotted outboard—bestowed good field performance on the type.
Power was provided by a pair of aft-mounted Pratt & Whitney Canada JT15D-4 turbofans, each rated at 2,500 pounds thrust. The circular-section cabin accommodated two pilots and seven or eight passengers, with a toilet in the aft and storage for baggage.
Mitsubishi built two prototypes, the first of which flew on Aug. 29, 1978. After the manufacturer’s trials in Japan, the pair was shipped to the U.S. for FAA Part 25 certification. They were based at the former Mooney plant at San Angelo, which Mitsubishi had acquired in 1969.
Certification was not obtained until Nov. 6, 1981, the delay being caused primarily by the need to incorporate new requirements introduced following the crash of an American Airlines DC-10 in Chicago in May 1979. Answering the new requirements required significant changes that added 600 pounds to the Diamond’s weight.
Apart from the prototypes, all of the Diamond Is were assembled at San Angelo from Japanese-built kits. Customer deliveries got underway in July 1982, and 61 production aircraft were completed. Impressions were favorable, although the aircraft initially showed a propensity to “snake” at high speeds.
In the year after deliveries began, Mitsubishi announced a development, the Diamond 1A. This featured JT15D-4D engines, an MTOW increase from 14,630 to 16,230 pounds, and an electronic flight instrumentation system. More apparent was the addition of an additional port-side cabin window. Deliveries from San Angelo of the Diamond 1A began in January 1984. Twenty-seven were built.
Following swiftly behind was the Diamond II, which was announced in October 1984. This had a reduced MTOW of 15,780 pounds but had increased fuel capacity. The main change, however, was the installation of the uprated 2,900-pound thrust JT15D-5 engine, which also sported a thrust-reverser. It is unclear how many Diamond IIs were delivered by Mitsubishi—quoted figures range between one and 11—because a big change was afoot.
Changing the Name and Joining the Military
In December 1985, the design rights for the Diamond II were sold to Beechcraft, which itself had been owned by Raytheon since 1980. The sale also included 64 Mitsubishi-built kits, which were assembled at Beech’s Wichita, Kansas plant. The Diamond II was renamed the Beechjet 400 and was certificated as such in May 1986. Once the initial 64 kits had been completed, Beech assumed full manufacture of the type.
In 1989, Beech introduced the Beechjet 400A, with Collins Pro Line 4 avionics installed, redesigned cabin interior, and some changes to optimize cruise performance. Under its various names, this model became the most successful, with nearly 700 being built.
Nearly 200 of these aircraft were trainers for the military. In 1990, Raytheon/Beech landed a major order from the U.S. Air Force for a “Tanker/Transport Training System.” In the event, 180 Beech 400Ts—designated T-1A Jayhawk in service—were delivered to the U.S. Air Force for service as multi-engine trainers.
To equip them for military service, there were numerous small changes. An additional fuselage fuel tank was incorporated and better air conditioning was installed. The wing leading edges and windshield were reinforced for increased birdstrike resistance, and the main avionics bay was moved from the nose to the rear fuselage to ease maintenance. The fuel system was adapted for single-point pressure refueling.
Deliveries began in 1991, and the Jayhawk has served uncomplainingly since the following year. In the late 1990s, the fleet was given GPS navigation, and from 2018 has undergone a major update with Collins Pro Line 21 avionics and ADS-B flight surveillance in order to maintain currency with air traffic mandates and newer front-line equipment.
Somewhat ironically, the Japan Air Self-Defense Force also turned to the Beechjet when it sought a new multi-engine trainer. Thirteen 400Ts were acquired, designated T-400 in service. They differ from their USAF counterparts in being fitted with thrust-reversers.
Another Name, Another Life
In 1993, Raytheon added the British Aerospace business jet line to its portfolio, bringing with it the historic name of Hawker. The Beechjet 400A became the Hawker 400 to bring it in line with the larger Hawker 800 (formerly BAe 125-800) and continued to sell well. In 2003, the 400XP was introduced, with several small improvements. The most important of these was a 200-pound MTOW increase that gave the aircraft extra payload, hence the "XP" designation.
Raytheon sold off the division in 2006, after which the aircraft were branded as Hawker Beechcraft products. One of the new company’s first acts was to announce the Hawker 450XP, a major overhaul of the 400 design. It was to feature Pratt & Whitney PW535D engines with Fadec control. Performance improvements were promised across the board, including the all-important hot-and-high take-off and climb-out figures. FAA certification was planned for the second quarter of 2010, but the 450XP was canceled in 2009 due to an economic slump.
That down-turn presaged the eventual failure of Hawker Beechcraft itself in 2013. While Beechcraft reemerged a year later under Textron ownership, it did so without its Hawker business jet line, which would have competed with Cessna, its more successful Textron stablemate. Therefore, production of the relatively simple aircraft that had begun life across the Pacific as the MU-300 came to an end. Production reached over 1,000, including the military trainers.
While no more 400s would be built, Textron continued to support the type, which remained popular with corporate, air taxi, and private owners on account of its adequate performance and low operating costs. Large numbers continued in service, making the type attractive for update/remanufacturing.
Nextant Aerospace identified this potential market before Hawker Beechcraft’s demise and launched the Nextant 400XT remanufacturing program. This took a Beechjet 400 airframe and effectively “zero-lifed” it while adding Williams FJ44-3AP FADEC engines for greatly improved performance and efficiency, and Collins Pro Line 21 avionics systems for a state-of-the-art flight deck. The result was a jet with a purchase price of approximately half that of a comparable new-build aircraft with similar performance and features. Range compared to the original 400A was improved by 50%.
The first remanufactured aircraft took to the air in March 2010 and was certified by the FAA in October 2011, with deliveries beginning the following month. Further upgrades were introduced by the 400XTi in 2014. In 2018, the 4500XTe was announced, a baseline model with a three-screen cockpit and no VIP interior, aimed primarily at air taxi and charter operators.
Textron’s answer to the Nextant 400XT was the 400XPR upgrade, which is also centered on the Williams FJ44 engine. As with the 400XT, the performance is dramatically enhanced, including the ability to reach FL450 in just 19 minutes with four passengers. The XPR features fuel-saving winglets and is offered with a choice of either the Pro Line 21 suite or Garmin’s G5000 avionics. It first flew in 2012 and received certification in 2016.
Both programs have ensured that the “little Japanese jet” continues to be a common sight around the executive airfields, especially in the U.S., and will remain so for many years to come.