If you using in airplane mode both cellular and wifi are off as is travel pass. Travel pass is active when you use cellular for texting or calling. If you have an iPhone, using iMessage to other iPhones is via wifi therefore not charged to travel pass. If you text to non iPhone it goes via cellular SMS and goes with $10/day charge.
If on $10 / day Pass, does reading a text trigger the $10 charge? If phone on airplane mode, does this prevent Travel Pass?
You must have “Cellular Data” on to trigger the travel pass. You can keep that off and still receive texts and calls. That is what I do, then respond when I’m on Wi-Fi. Or answer and incur the costs. If you keep your phone on airplane mode only, it will only work when on Wi-Fi.