https://www.skybrary.aero/index.php/...cts_on_Engines
https://www.boeing.com/commercial/ae...icle_03_2.htmlIce building up on the inlet, fan, or spinner would likely shed outward into the fan bypass duct without causing a power loss. Therefore, in these power-loss events, it is reasonable to conclude that ice must have been building up in the engine core.
It is now believed that ice crystal icing can occur deep in the engine where surfaces are warmer than freezing (see fig. 2). Both older generation jet engines and the new generation of jet engines (high bypass ratio engines with electronic engine controls) can be affected by ice crystal icing.