
An AOPA article how an iPhone could be a "Heads Up Display" (HUD) in aircraft cockpits.
At this point NO consumer grade electronic device can replace certified instruments, but such a device can be used for improved situational awareness for the pilots.
Aircraft pilots rely on redundancy, with nearly all information display duplicated or triplicated for safety. An iPhone HUD simply adds another layer of redundancy in the cockpit, and the more natural display of the information makes it easier to understand what is being shown.
Note that thrid party devices like Levil Technology or Sagetech make "attitude and heading and reference system (AHRS)" that can provide certified data to devices like iPhone's.
It may not be long iPhone's can be used as certified flight display.s