A proportion of the mines laid were timed to become effective at various dates so as to escape being swept before the operation was due to begin. Operations continued as in phase 2 , with additional fields being laid in the vicinity of Ushant, and on the general line between the Gasquets and Ushant.
Minelaying with special mines, timed to become effective at varying dates, was augmented in the fields to the north of Le Havre and to the north and northwest of Cherbourg. During this phase, aircraft executed an extensive series of minelaying operations in the Kattegat, Baltic Heligoland Bight, Frisian Islands, and in the Bay of Biscay, with the object of retarding the movement of enemy vessels from these more distant waters toward the Channel and assault area.
As these operations involved some risk of compromise to the new types of mines, it was not originally intended that they should be laid from aircraft before 20 May. But to enable full advantage to be taken of especially suitable minelaying conditions, this date was anticipated by five days in the minelaying in the Baltic.
During this phase, operations were primarily directed to the laying of special mines off Ijmuiden, the Hook, the Scheldt, Calais, Le Havre, Cherbourg, St. Malo, Morlaix, Brest, and the Biscay ports. Aircraft also executed further lays off Aalborg and in the approaches to Aarhus.
Operations in Phases , were, of necessity, related to a fixed date. Those in Phase 5, on the other hand, were planned to be carried out concurrently with the assault. In the event, only the lay off Etretat was executed. In view of the shipping congestion in the approaches to the assault.
