In Leon it is possible to add Hold Item List to the aircraft, in section $$Maintenance > Hold Item List$$ (more details about HIL section can be found here ).
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If you have added an open HIL to one of your aircraft within a certain period of time, in section $$PLanned Flights$$ su ch aircraft appears in orange (see screenshot on the right).
If you try to add a new flight from $$Planned Flights$$ section, Leon will issue a warning:
Aircraft A-BCDE has a hold item list open in section “Maintenance > Hold Item List”
The same applies if you add a new flight from $$New Flight$$ section.