Which condition prohibits inflight selection of wing and/or cowl anti-icing?

In the context of aircraft operation, particularly regarding anti-icing systems, selecting wing and/or cowl anti-icing inflight is restricted under specific conditions to ensure safety and optimal performance. When the aircraft is using the Auxiliary Power Unit (APU) as a bleed source in manual mode, the anti-icing systems may not function properly. This limitation is primarily due to the possibility that the APU does not provide adequate airflow or pressure required for the anti-icing systems to engage effectively, as necessary for inflight operations.

Using the APU in manual mode indicates that pilots are directly controlling the bleed air operation, which could lead to insufficient anti-ice performance during critical flight phases. The design of the systems is such that, under these circumstances, pilots are prohibited from activating anti-icing features while inflight to avoid complications and ensure the aircraft operates within safe parameters.