Blind take off in fog experiments.
In the Fifties I think a BAC One-Eleven was used for the first of Cat 3 landing experiments at Heathrow in thick fog. About that time trials took place on the practicability of blind take offs using a device called a PVD (Para Visual Direction?) I understand it was a device that showed you if you were tracking correctly during the take off run in zero visibility.
Does anyone have any information on the use of the PVD?