Depending on the sort of flying you do, trades can also be useful second lines of work. Sure, if you work for a 900-hour-per-year LCC, you will be lucky if you have enough free time to sleep and see your family and friends. Corporate doesn't involve much flying per se, but requires a lot of standby time and availability on a short notice, making side jobs hard to schedule. But, if you work in the leisure and charter sector, for a good half of the year you will be doing hardly any flying. Between November and April, 5 flights in a month is considered an awful lot in many airlines of that kind. And the basic salary without flying is often not too impressive. So, if this is where you are likely to end up, you'd better build some strategy how to spend your winters wisely. Being an on-call home improvement professional can be a good option, especially once you have done it for a while and have a "database" of happy clients who will call you when needed and possibly refer you to their friends. The trouble is, you can't pick up the necessary skills too quickly and you also need experience in order to be trusted with working on people's homes. So, this is ideally worth thinking about BEFORE you start flying.
...and here goes a piece of advice which will never go out of style - get your second qualification before you commit yourself to flying.