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The Vigilant T1 is a Grob 109B adapted by the Royal Air Force for use on Volunteer Gliding Squadrons. The aircraft are maintained by an Engineering Flight at RAF Syerston.

The Vigilant is a low wing cantilever motorglider with folding (and detachable) wings, a 'T' tail and side-by-side seating with dual controls. Entry and exit from the cockpit is via 2 perspex doors which open upwards individually, a modification from the original G109 canopy. The cockpit can be heated, providing that the engine is running. Seat backs can be adjusted and cushions of differing thickness inserted to accommodate a wide range of body sizes. It is mainly constructed from Glass-Reinforced Plastic(GRP) and has a taildragger undercarriage arrangement.

Grob Vigilant on final approach at BAE Salmesbury

It is sometimes referred to as a touring motor glider (TMG), as the propellor cannot be retracted into the fuselage.

The Vigilant's power is provided by a Grob 2500 E1 horizontally opposed, 4 cylinder, air cooled petrol engine. It develops approx 95 BHP at 3400 RPM.

The propellor is a 2 bladed, variable pitch propellor driven directly from the engine. there are 3 pitch settings for the propellor:

Fine, for take-off and general flying. Coarse, for cruising. Feathered, for gliding with the engine off.

Hot air can be supplied to the carburettors when there is a risk of icing. However it is standard procedure to use hot air whenever the engine is not at full power.

The control column operates the elevator and ailerons. Therefore the control column is used to pitch and roll the aircraft.

The rudder pedals operate the rudder and wheel brakes and are also adjustable, forward and backward, to suit individuals of differing leg length. The rudder and brakes are used to control the aircraft on the ground.

The throttle regulates fuel flow to the engine. Engine RPM (power) is controlled by the use of the throttle.

The airbrakes are used to increase the rate of decent and are primarily used during the approach and landing.

All of the above controls are duplicated for both seats.