I don’t know if you’ve tried this, but it’s true: Smiling opens doors.
When I want to make a picture with a person or in a place that is not necessarily public, I just put a smile on my face, take a deep breath and ask permission. I usually give a little preface and a reason for them to acquiesce, something like, “I’m practicing my photography and I wonder if you’d let me make a picture of you in the cockpit so I can show my daughter at home?”
Who wants to deny a smiling father a picture for his daughter? That’s right, nobody wants to say no to that. When I use this method I never feel anxious about sneaking a picture, and I often get even more access than I ask for. Give it shot, put a smile on your face and ask permission.