The Smiths 'Para Visual Directors', of which there were 6 on each Trident flight deck, rotated to highlight attitude errors. The 'barbers pole' rotated until the error was corrected in a natural sense; the azimuth director was in front of each pilot and 'streamed' left or right until the correct roll input was made, the pitch directors (one on each side of each pilot) streamed forwards and backwards. This was a sort of wide area flight director and was part of the equipment fitted for the world's first operational 'blind landing' airliner.