This is a tracking category.

It is used to build and maintain a list or lists of pages—primarily for the sake of the lists themselves and their use in article and category maintenance. It is not part of the encyclopedia's categorization scheme.

More information:

This category is hidden on its member pages—unless the corresponding user preference (Appearance → Show hidden categories) is set.
These categories are used to track, build and organize lists of pages needing "attention en masse" (for example, pages using deprecated syntax), or that may need to be edited at someone's earliest convenience.
These categories also serve to aggregate members of several lists or subcategories into a larger, more efficient list (discriminated by classifications).
Typically, tracking categories are automatically populated by templates invoked in pages or by the MediaWiki software (see Special:TrackingCategories for an overview of the latter).

Administrators: Please do not delete this category as empty!
This category may be empty occasionally or even most of the time.

This error tracking category contains categories with at least 1 missing parameter, produced by one of the following WP:AIRCRAFT category navigation templates:

((Aircraft by decade))
((Aircraft by nationality))
((Airnd))
((Airnt))
((Airntd))
((Airtd))
((Civairnd))
((Milairnd))
((Civairnt)) (#R)
((nowrap|((Civairntd)) (#R)}
((Civairtd)) (#R)
((Civil aircraft by nationality)) (#R)
((Milairnt)) (#R)
((Milairntd)) (#R)
((Milairtd)) (#R)
((Military aircraft by nationality)) (#R)

Categories are sorted by missing parameter:

1: are missing (({1))}
N: are missing |nation=
T: are missing |type=