How to send a text instead of iMessage

I will add the background story for flavour to introduce my question.

Last week I flew from the Netherlands to Orlando. I promised my girlfriend to send her a message as soon as I arrived so she'd know I had landed safely. However, I had to wait in line for two hours to get past security and there was no WiFi in the arrival hall. There were signs everywhere that it was forbidden to use a cell phone in the arrival hall so I couldn't call her. And no matter what I tried (turning off iMessage, deleting my girlfriend from my contacts, long pressing the message) I could not get it to send a text. The only feedback I got was that I needed an internet connection to send the iMessage, very frustrating!

TLDR Question: How to send a text instead of an iMessage, for when there is no internet connection available?