Go to your aircraft schedule:

Find the aircraft in the list on the left side of the schedule. Click on the day you wish to add the maintenance activity on.

Chose either Trip or Non-Trip activity. Either option will open a new window in edit mode that looks almost identical whether it is a “Trip” or a “Non-Trip Activity”, the only difference being that in the Trip Activity you can add a “Scheduled Trip” name.

If you have chosen the Trip activity, you need to start by naming the activity.
Activity type, chose Maintence.
Add the start date for the maintenance and the end time. As the aircraft is not likely to move during this period, put in the same airport in both the "From" field and the "To" field. Add 0 passengers. Click save and you have now blocked your aircraft for a longer period of time. 

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