docbert

Google Maps has a full world map available at all times, even offline.

It will also cache areas that you've been, even where offline maps are not available/downloaded. If you put your phone in airplane made and then pan around in maps you'll see what level of detail is available when offline.

For example, I was in Tokyo last week. Offline maps are not available for Japan, but I can still zoom in to street level for the area around Tokyo even 5 days after I left. If I look elsewhere in Japan, I see only very high-level detail (no roads, only very major city names, etc).

Panning over the area that your flight is going to be going in advance does seem to cache enough information to show good detail when flying.