A gas turbine engine variable guide vane has a fixed portion on an upstream side, a movable flap on a downstream side and a transfer slot between a fixed portion trailing surface and a movable flap leading surface. The movable flap has opposite pressure and suction sides along a chord line between leading and trailing edges, and is rotatable about an axis along a movable flap span over a range of angular positions between open and closed. The trailing surface has a substantially U-shaped profile with first and second branches respectively partially around the pressure and suction sides. The transfer slot has inlet and exhaust ports respectively on the pressure and suction sides. In the closed position the suction side contacts the second branch closing the exhaust port, and in the open position the second branch directs a first air flow through the transfer slot tangentially over the suction surface.