UPDATE September 29, 2017: Compatibility testing between ForeFlight and iOS 11.0.1 is complete and we are issuing the “all-clear” to ForeFlight customers, with an important caveat for turboprop and jet pilots.
We have found that devices on iOS 11 using Bluetooth or plug-in GPS receivers for position data in ForeFlight can experience loss of GPS signal when carried on aircraft exceeding 300 knots. The issue does not appear on slower aircraft or with devices that provide more than just GPS, such as Stratus or installed Garmin avionics.
We expect that this issue will be resolved in a future iOS update, so we recommend that anyone who relies on a Bluetooth or plug-in GPS receiver for position data in ForeFlight, and who regularly exceeds 300 knots in flight, to hold on updating their iOS version until the issue is resolved.
The following support articles from Bad Elf provide details about the GPS issue:
ORIGINAL POST September 19, 2017: We are performing compatibility testing between ForeFlight and iOS 11. Given the scope of this iOS release, we recommend that you do not yet update devices that you regularly use for flight planning or flying. We will update this post with an “all clear” as soon as testing is completed. Please also stay tuned to our Facebook page or Twitter feed for updates.