Abstract: A window assembly in which two window units are slidably mounted and in which one of the window units cannot be removed until the other has been removed. The removal of a first of the window units releases the sash locking member for lateral movement within the frame to permit overlapping lips of the other window unit and its associated frame to move out of overlapping engagement with one another.

Abstract: A ground-based flight simulator having a television type terrain visual display with simulated weather conditions. The visibility of the terrain image is varied over the picture area to simulate ground fog and cloud conditions, either one alone or both together. Either weather condition provides a zone of transitional visibility, i.e., spatially changing visibility between spaced zones of uniform visibility, e.g., between one of uniformly clear visibility and another zone of uniformly zero visibility. The transitional zones may be varied in width according to aircraft pitch and the entire weather pattern rotated, together with simulated terrain, according to aircraft roll.