If someone wants an existing but offline only checklists app you can try mine: Fast Lists [0]. It includes options for nesting and hiding the already ticked items so it can be used with large lists.
I haven't added synchronisation yet as I'm not sure the return would be worth the time and I'm also concerned about conflict resolution and data loss.
The OP has a picture of a cockpit which would imply a safety critical environment. I would think long and hard before selling my app in that way especially for an online service.
When it comes to your valid concern about the safety of pilots using a always-online application I agree with you, that would not be very smart, but nowadays there is technical soulutions around this problem [0]. Alsow it is a hint to everyone who have read the book "The checklist manifesto" by Atul Gawande, I recomend it!
I haven't added synchronisation yet as I'm not sure the return would be worth the time and I'm also concerned about conflict resolution and data loss.
The OP has a picture of a cockpit which would imply a safety critical environment. I would think long and hard before selling my app in that way especially for an online service.
[0] https://itunes.apple.com/app/fast-lists-checklists-for/id481...