The DH Trident was one of the earliest commercial aircraft to be equipped with PVD (Para-Visual Display).
A device resembling a horizontal striped barber's pole, it uses the ILS localiser signal to assist centre-line guidance during low-viz take-offs. The stripes remain stationary so long as the aircraft is on the runway centre-line. If the aircraft deviates, the stripes appear to move in the direction of the runway centre-line, helping to guide the pilot back to the middle of the runway.