Doesn't surprise me. I got on a flight that had to go back to gate due to an "iPad failure" about 6 years ago. The captain was apologising as we disembarked. I asked him what happened. He said software issue across both their flight plan iPads. Hardware was fine. Not sure if it grounded other flights but we were back on a different plane about 2 hours later.
To be clear, we're actually talking about two separate things, they both just use an iPad.
You're talking about what's known as an electronic flight bag, which is using a tablet as a replacement for paper maps and charts. This is legal and has been for some time.
What I and Animats are talking about is running an app on a ipad as a "replacement" for equipment or training that is required for safe, legal, operation. NOT legal.
Maybe because the iPad has a more user-friendly interface than most avionics.
Here's the cockpit of the Gripen, Saab's current fighter aircraft.[1] One big screen across the whole panel. No sign of a classic altimeter or compass. There's a HUD as well, so there's a second device for the basic flight instruments.
