Grob Aerospace has revealed more about its plans to develop a stretch version of the SPn once the all-composite light jet is certificated in the final quarter of the year - with a 2.2m (7.2ft) to 3m fuselage plug adding as many as four seats to the six- to eight-seat cabin.

It comes as the Swiss-owned manufacturer - which has its production plant near Munich in Germany - prepares to fly its fourth prototype of the SPn. It will be the third aircraft in flight test - number two crashed killing the pilot in 2006. The first two series examples are also at early stages of production at the Tussenhausen-Mattsies factory. These two, which will be sold and leased back by Grob as customer demonstrators, will also join the later stages of the test programme. The prototypes have completed a total of just under 500h of flight testing, with a target of 1,000h by certification.

 Grob SPn
 © Grob

Managing director Andreas Strohmayer says it was Grob's intention since the launch of the SPn in 2005 to develop a family of light jets in the Part 23 commuter category based on the aircraft's composite design. "We are bound by the 19,000lb [8,636kg] Part 23 maximum take-off weight limit," he says. "Within that, we will do whatever makes sense." The SPn's maximum take-off weight is 6,300kg. "A 2.2m plug in the 2m constant section of the fuselage would give us four more club seats, taking us to 12 seats. With 3m we could do that and have an even bigger galley, for instance," he adds.

Stohmayer says, however, that although studies have been made, no new variant will be launched until after SPn deliveries have begun and the programme is moving towards full capacity production of 45 aircraft a year, expected during 2010.

Grob expects to have notched up 80 orders for the SPn by next month's EBACE business aviation convention in Geneva, with almost half coming from North America, a market the company thought it would not be able to penetrate when the aircraft was launched.

Grob is also deciding on the future of two other prototypes, which have flown, but have been on ice since the SPn's launch. The four-seat G140 is a single turboprop-powered aerobatic trainer, which can also be used in a general aviation role, and is a development of Grob's G120 piston single trainer. The G160 Ranger is a six-seat variant, aimed at the business aviation market.

In January, Grob clinched a deal to be a development partner on the new Bombardier Learjet 85, and will build the first three prototypes in Germany, using composite primary and secondary structures.

Flight International will be publishing a cutaway drawing and technical description of the SPn in our 20-26 May issue.