We can’t provide a blanket answer about the legality of the iPad as an EFB due to the many variables in flight operations and the associated requirements. iPad’s are in use legally in all forms of flight operations from Part 91 to 121 and 135. It is incumbent upon the pilot and flight operation to determine the legality of the iPad as an EFB for their particular needs.
In general, for most Part 91 operations with a maximum gross takeoff weight (MTOW) of less than 12,500 pounds, iPad’s are legal to use as an EFB. Part 91 operations with aircraft having a MTOW of over 12,500 pounds don’t need FAA approval, but will need to pay attention to AC 120-76C - in particular the EFB testing and documentation requirements. Part 121 and 135 operations will have to get approval from the FAA on a case by case basis.
AC 91-78 discusses the use of Class 1 (portable and not attached to the airplane) and Class 2 (portable and non-certified devices that are mounted to the airplane) EFB’s. This is the AC that states it is legal for Part 91 piston aircraft pilots to use the iPad as an EFB as long as, amongst other requirements, the data is current and valid. A backup data source, either digital or printed is encouraged but not required.
Part 91F operations will need to adhere to the EFB testing and documentation requirements as listed in AC 120-76C.
Part 121 and 135 operations will need to pay close attention to AC 120-76C as they seek approval to use the iPad as an EFB.