Yakovlev Yak-40 STR-40DT

The STR-40DT did not start life as a new aircraft but as the second stage of work that the S. A. Chaplygin Siberian Aeronautical Research Institute (SibNIA) in Novosibirsk had begun on its own initiative in 2012: re-engining the Yak-40 trijet. The institute reckoned that more than a hundred Yak-40s still had at least ten years of airframe life left, and it wanted to repeat what it had already done with the An-2 biplane in its TVS-2MS form: first replace the engines of a fleet with usable airframes, then use that experience for a technology demonstrator. It took airframe 40-04, built at the Saratov plant in 1975, and turned it into a flying laboratory registered 87216. With one TFE731-3 and two AI-25s it flew on 8 December 2012; with two TFE731-3s and the central AI-25 on 20 May 2013; and on 29 November 2016 it took off as a twin, with TFE731-5BR engines of 2154 kgf each and the third engine replaced by an auxiliary power unit. That configuration, the Yak-40MS, is the direct basis of the STR-40DT. The Yakovlev design bureau stayed out of it: according to SibNIA, it made clear it had no interest in working on the aircraft.

Meanwhile the Ministry of Industry and Trade had commissioned the institute, under the federal civil aviation programme, to study aircraft for local and regional routes: one contract worth 889 million roubles for 2013-2014 and another of 440 million in 2015. Out of this came the preliminary design of the LMS-19 "light multi-purpose aircraft", seating 19 to 32, with an all-composite airframe but derived from the Yak-40. From 2017 the ministry funded a new research and development project, worth more than 675 million roubles, to integrate technologies for a future platform of light aircraft for local airlines. Within that framework the twin received its new designation: STR-40DT, "turbojet aircraft 40, technology demonstrator".

The stated aim was to carry over to a jet, flying at 600-700 km/h, the composite construction techniques SibNIA had developed for the TVS-2DTS biplane, and to show, in the institute's words, that they could raise the productivity of a new aircraft two- to threefold and halve its production cost. In 2018 the Yak-40's metal wing was replaced outright by an all-composite one, lengthened, of greater span and smaller area and tipped with winglets: the opposite of what a short-field aeroplane wants, and exactly what a fast, economical cruiser needs. The fuselage remained the original one. The engines were two Garrett TFE731-5BRs of more than 2100 kgf each, obtained after overhaul; in the centre engine bay sat a Garrett 3092 auxiliary power unit with a 9 kVA generator, which powered the systems on the ground and allowed electric rather than pneumatic engine starting.

The schedule slipped. The first flight had been announced for the end of 2017; in April that year SibNIA's scientific director, Aleksei Seryoznov, also announced plans to build a composite fuselage leading to an all-composite prototype, and on 7 June 2018 he repeated that the demonstrator would be shown before the year was out. He pitched it at the Russian North, with long range and maintenance costs of no more than 250 dollars per flight hour.

On 4 December 2018 the STR-40DT took off for the first time from Yeltsovka, the airfield of the Chkalov aircraft plant in Novosibirsk. It was flown by a crew led by Igor Moseikin, head of SibNIA's aviation certification centre, and photographs of the flight published on RussianPlanes.net were the first public evidence. The new test phase sought additional data, statistics and the refinement of composite techniques in the wing of a light jet.

In July 2019 SibNIA declared the work complete. According to the institute, two engines instead of three and the composite wing raised cruising speed to 700 km/h, against 500 km/h for the Yak-40, and cut fuel burn to 560-590 kg per hour, against 1150; the flying prototype met the requirements of the ministry programme. Only one example was built.

The STR-40DT was meant to precede a 19-seat aircraft, expandable to 32, built entirely of composites (fuselage, wing, tail surfaces, control surfaces, doors and airstair), with a glass cockpit and only the landing gear inherited from the Yak-40. On paper it would carry 3200 kg, against 2720 kg for the Yak-40 with the same fuselage; weigh 8500 kg empty and 13000 kg at normal take-off; cruise at 650-700 km/h with a range of more than 4000 km, a ceiling of 9500-10000 m, a take-off run of 800 m and a landing run of 600 m; and SibNIA credited it with 15-20 % lower fuel burn than the Dassault Falcon 900, a trijet with similar engines. The projected price was 6-6.5 million dollars, and as early as 2016 director Vladimir Barsuk spoke of a version with two AI-222-25s for 30-40 seats and 5000 km. Development was suspended in 2019, probably because the ministry chose to build the Czech L-410 turboprop in Russia instead.