There are two sides on the use of an iPhone in flight.
First is the easier one, the legal part. Quite simple in almost all countries the use of active mobiles is forbidden in cockpit, or to say other - there is no certification of an iPhone for use in the cockpit and whoever uses it, does impose additional risk. This risk might be lower now, compared to what we knew a couple of years, but it definitely is a risk. So, legal wise - you are obliged to put the iPhone in Flight Mode if it is on during flight. If you don't, you usually will hear disturbances in COM and intercom and maybe a misfunctional compass.
Second is the more complex, technical part. The design of the iPhone is somehow flawed as it was never designed to operate in a cockpit environment. So, in order to reduce costs they chose a chip design, where the mobile part and the GPS part are on one chip set. So, if you disable mobile net, you disable GPS - which is quite bad. BTW: this is one of the most important differences to an iPad, where you are able to switch off mobile data and the device is quiet network wise. So, if you have a choice - go take the iPad instead of an iPhone. Another option would be to switch on Flight Mode on the iPhone, then switch on Bluetooth and use an external BT GPS - which could be more wise anyways.