If set up like the 747, there are 2 possible ice detection possibilities. A Primary system is associated with Auto Anti-Ice on nacelles, wings, or both. An Advisory system does not have the automatic feature, so you must turn on the anti-ice yourself.
Icing Detection (Primary)
The icing detection system detects airplane icing conditions in flight. Two probes
on the forward fuselage and system logic:
• control nacelle and wing anti-icing valves,
• display anti-icing system operating indications on primary EICAS, and
• display EICAS alert messages
Icing Detection (Advisory)
The icing detection system detects airplane icing conditions in flight. A probe on
the left side of the forward fuselage and system logic:
• display anti-icing system operating indications on primary EICAS, and
• display EICAS alert messages