Before posting this question, I honestly tried to find the answer to my question on this website, but failed. Therefore, I've decided to create a new 'conversation'. Will you consider the following sentence, please:
"I phoned her yesterday afternoon to find out if we could meet in the evening, but she said she was supposed to/had to/was to fly to Brussels and didn't know when she would return." [=she said she was flying to Brussels]
I've read in various sources that "was supposed to" tends to be used for arrangements or something expected that didn't in fact take place (e.g. He was supposed to fly to Berlin but his flight was cancelled). As for the expression "had to", judging by all the examples I've come across so far, it looks like it's always used only for completed actions (e.g. We had to return home because of heavy rain).
Would you mind telling me if it's correct to use "was supposed to/had to/was to" in my example (in which the action/arrangement is not completed)? Besides, I was wondering if it was correct to use Past Simple instead of Past Continuous: ".... she said she flew to Brussels ...".
I'd be very grateful to you for your help.