Abstract: This specification discloses an ice accretion detector for aircraft comprising a sensing hole or port formed in part of the aircraft structure where icing is known to occur and connected to the key control port of a OR/NOR fluidic key and an air supply supplying air under pressure to the supply port of the key while a pneumatic indicator is connected to the output port of the key. When the sensing hole is closed by the formation of ice the fluidic key is switched into its other state and the pneumatic indicator senses this change and indicates the formation of ice.