Gaia GPS App. Works great on my iphone 6. Been using it for 3 years. Never failed me. It has a ton of different maps you can download for free for the area you are going to be in(Google satelite, google terrain, public/private land, road maps, etc etc). It does not work in airplane mode. Airplane mode, at least on iphones turns all cell, wifi, and gps functions off. But in the new iphone operating system its takes literally about .25 of a second to turn airplane mode on or off. I don't use a GPS any more, just my phone and this app.
After you learn how to download maps/areas, it is super easy and fast.