When is it not permissible to issue clearances as filed?

The correct choice highlights that it is not permissible to issue clearances as filed when alternate routing instructions are necessary. In aviation operations, a filed clearance refers to the original flight plan that has been submitted by a pilot. However, situations may arise that require alterations to this plan due to changes in air traffic conditions, restrictions, or other operational considerations.

When alternate routing instructions are issued, it indicates that the initial routing as filed is not suitable for safe or efficient operation. This may be due to traffic congestion, temporary flight restrictions, or environmental factors. Consequently, it becomes essential for controllers to provide revised clearances to ensure safety and compliance with current operational requirements.

The other scenarios provided may involve factors that do not inherently negate the ability to issue clearances as filed. For instance, during an emergency declaration, controllers may actually expedite or alter clearances to assist a distressed aircraft. Similarly, if an aircraft is approaching the departure airport or if weather conditions are below minimums, there might still be valid reasons to issue the originally filed clearance—with appropriate precautions and procedures in place—unless practical operational adjustments are warranted. Thus, the necessity for alternate routing stands out as the key reason why clearances as filed cannot be issued in that situation.