I'm sure you're all familiar with the "Cabin crew, x minutes to landing" anouncement, whicht tends to be either 10, 15 or 20 minutes. As cabin crew told me to switch off my electronics (how rude to interrupt my audio book just as it was getting exciting...) I had 20 minutes to ponder the question why it was only 10 audio-free minutes on the outbound and why this number is harder to guess than when the next IROPS will occur.

So is there any reason behind this number? Or does it every captain according to his/her liking?