The G109B addressed several issues learnt from operation of the earlier G109A which included improvements to power (95Hp up from 80Hp on the G109A), performance of the wings when wet, crosswind behaviour and limits and cockpit space.

The G109B aircraft is a low-wing cantilever motor glider, with a T-tail, folding (and detachable) wings, and side-by-side seating with dual controls.

The cockpit can be heated, providing that the engine is running, and the seat backs can be adjusted and cushions of differing thickness inserted to accommodate a range of body sizes.

The G 109B is powered by a Grob 2500 E1 horizontally opposed, four-cylinder, air-cooled petrol engine that develops approximately 95 bhp (71 kW) at 2,950 rpm.

The rudder pedals, which also operate the wheel brakes, are adjustable forward and backward to suit individuals of differing leg length, and airbrakes are used to increase the rate of descent during the landing approach as well as limiting airspeed to Vne (velocity never exceed), if required.

The Vigilant was also used by the Royal Air Force Central Gliding School, at RAF Syerston in Nottinghamshire, which trains VGS instructors as well as providing standards checking, syllabus development and engineering support.

The remainder of the fleet were transferred to the Defence Equipment Sales Authority, removed from the military aircraft register and sold to civilian operator Aerobility in 2020.

[7] The G 109A was fitted with a 2,000 cc 80 horsepower (60 kW) Limbach Flugmotoren flat four engine (maximum power delivered at 3,400 rpm), and the wing span was increased to 16.6 metres (54 ft).

The G 109Ar was a modification used on 2 Grob 109A by the Aero Club Valkenburg of two aircraft with a Rotax[8] boxer-type engine for increased performance on short runways and reliability.

Developed out of the Grob G 109A; the wingspan was further increased to 17.4 metres (57 ft), the one-piece canopy of the earlier versions was replaced with opening doors, and the main landing gear was moved rearwards to ease weight on the tail wheel.

Modifications to the Vigilant T1 include a landing light, a higher maximum all up mass (AUM) of 908 kg and a throttle for use in the left hand seat.

The Vigilant T1 with Rotax 912is3 engine by charity Aerobility Data from Jane's all the World's Aircraft 1985–86[11]General characteristics Performance Avionics