There are no ads whatsoever coming from the Flight Crew View app (that's why you paid for it). However, these ads can get installed or pushed via Safari from a bad website or app.
Remove Unused Apps
First, remove any apps that you no longer use.
Block Pop-Ups in Safari
Follow Apple's instructions here: How to block pop-ups in Safari
Clear Safari History & Website Data
NOTE: These steps will remove all of your website history and also any saved passwords, so make sure you have them backed up before continuing.
- Turn on airplane mode
- Open Safari
- Tap the overlapped squares in the lower right
- Close every page by tapping the "x" in the left upper corner of the page
- When there are no pages left press HOME (On an iPhone X series swipe up from the bottom edge) and wait a couple of seconds
- Double-press HOME (On an iPhone X series swipe up from the bottom edge and pause)
- Swipe up on the safari image to kill it (On an iPhone X series touch and hold Safari, then swipe up)
- Press HOME again (On an iPhone X series swipe up from the bottom edge)
- Go to the device's Settings app (gear icon). Scroll down and tap ono Safari
- Tap "Clear History and Website data"
- Turn off Airplane mode
If you still get that pop-up, please let us know. Even though it isn't caused by Flight Crew View, we completely understand how annoying it is and are more than willing to help track it down.