RTJ You can ask
Seriously, maths is about symbolic reasoning and problem solving - it opens a great many doors. You could potentially move from that into engineering via an MSc, or into many other fields - although you are probably looking at some degree of additional education or training for virtually any other career after a maths degree. But, with the training a maths degree gives you, you can probably fast-track through that training better than most.
I must admit that, starting where I am now if I wanted to do another degree for fun, I'd do either maths or Physics - probably the latter 'cos I'm personally fascinated by quantum physics and relativity, and would love to understand them properly (if that's possible?)
TanveerAgain, this is purely my personal opinion, but the concept of a degree as a backup always troubles me. The majority of degrees do not qualify you to do anything but carry on learning. But, something like a pure science or engineering + (even failed!) pilot training will equip you well to move into one of the technical professions that make aviation happen - whether it's aircraft design, aerodynamics research or aviation psychology. If you want a pure backup, personally I'd suggest getting a vocational qualification - something like accountancy, physiotherapy, sports coaching - whatever you'd enjoy and be good at.
G