Use of Anti-ice systems
Some aircraft use anti-ice systems as de-icers, so it is not possible to give a 'one size fits all' answer. In the case of the B747Classic Engine Anti-ice should be ON during ground or inflight operations whenever icing conditions exist or are anticipated. On the other hand Wing Anti-ice should be turned on following an icing encounter. Also, if icing exists it is important to use wing anti-ice before extending any flap so that the LE devices are cleared before extension. (due no anti-icing action for LE devices available after extension). Some aircraft have an auto detection/activation of anti-ice system and manual de-ice operation. (Prop & Engine anti-ice Auto - Wing Leading edge de-icer manual)