Would it be possible to popup a reminder to close a flightplan? Could be triggered when plane is on ground and standing still or when closing Navigation. Normaly I fly without flightplan but when crossing borders I file a flightplan. If I land on an uncontrolled airfield I could forget to close the flightplan(just did). They send the police to check if the plane had landed an next day I got a lot of angry people in my face.

This is one of the more absurd threads I've seen.
The thought of a reminder taking away personal responsibility isn't even coherent. Does your meeting reminder take away your responsibility for being at the meeting?
Not sure about your "moods" Shifu and frankly glad I don't have to deal with them beyond an idea forum.

If you don't want to lose your sense of responsibility, turn off all automation in your life. No autopilot, no GNSS (because you're not manually calculating a fix from the received signals) don't use your phone as a clock, listen to the BBC and set your wind-up watch by it...

Incredulous.

All airplanes will be unmanned soon enough. Where is your responsibility then?

The idea is a sound one and if not for the pride of an old pilot, then for the sake of the rescue personnel who are set on alert for nothing, over and over. Must be pretty wasteful and despiriting for those involved having to take care of a simple oversight.

If such an option helped to reduce the number of false call outs for unclosed flight plans, then I think the aviation community would be benefited tremendously.

Perhaps you weren't thinking about the greater community when you derided the idea, but you also didn't ask the question either.

At some point the flight plan will be closed by the app anyway, just as it is filed now. Using a cell phone to call and close the plan is no different, even if it is nostalgic for some.
Accept progress and recognize that this idea is a step in the direction things will go anyway. Or be a barnacle on the boat. Whatever you like.

Final helping point: the reason that avionics form a greater and greater share of an airplane's value is safety. They alleviate the cockpit management tasks which make flying the plane easier and safer.
This idea reduces mental clutter for those who don't get to fly so often, and might miss a detail or two. One less thing to worry about means more focus on flying the plane.

Thanks geohawk - for putting the right words to my suggestion.

Just had a sunday meeting in our flightclub where I mentioned the idea. First reaction was that one of the other pilot missed to close a flightplan last week - and he is flying every week IFR(uses Skydemon). Everyone supported the idea. The suggestion of using a rubber band, moving the clock from one arm to the other or using Iphone messages is fine but with the possibility to implement a solution in Skydemon it would not be nessesary. Another help to meet the pilots responsibility.