Using external conditioned ground air simultaneously with airplane air conditioning packs is:

Using external conditioned ground air simultaneously with the airplane's air conditioning packs is prohibited because it can lead to potential issues with the aircraft's environmental control system. When ground air is introduced at the same time as the aircraft's packs, it can cause conflicts in the temperature and pressure regulations set by the environmental control system, which relies on specific operational parameters to function correctly.

Additionally, the use of external air while the packs are operational could create inefficiencies in the system, leading to inadequate cooling or heating of the cabin, which could be unsafe for passenger comfort and aircraft operation.

By strictly prohibiting this practice, safety measures ensure that any environmental control issues can be avoided and that the aircraft operates within its designed parameters, particularly during critical phases of operation like pre-flight checks and boarding.