I have some knowledge here - I've written for a variety of flying magazines since 1996 and after a couple of years break, last year I picked this activity up again and currently write a monthly column in a non-UK flying magazine.
Firstly trip reports - all flying magazine editors are inundated with trip reports, some quite well written, most not, the vast majority far too long to use anyway. A trip report needs to be a quite exceptional piece of writing to get published and very very few will ever see print.
Next flight tests - these are fairly specialist, and people who can get a really good judgment, then write well about it, in a small period of flight time, are pretty rare. The "staff writers" (actually usually just known good regulars rather than people on a salary) therefore tend to pick these up because they can be trusted to get them right. It is also important when writing one of these to appreciate that one of the major objectives of a flight test or equipment test article, in the mind of the editor, is to sell advertising! Thus is why, particularly in UK flying magazines, you virtually never read a critical report.
High quality discussion editorial needs incredibly knowledgeable, and usually regular contributors. Pick up any of the flying magazines and you'll see who these are. Personally I really enjoy writing this, but it's bloody difficult to keep saying something new and interesting, for a deadline, every month without fail. Writers block is no excuse- your editor demands copy, on time, of the required length, of the required quality. Not much of this tends to be one-offs.
All magazines have a house style that needs to be used. Most magazines nowadays will have some guidance on their website somewhere, along with length requirements, whether the editor wants to agree a structure in advance, and so-on. And try, over time, to build a relationship of mutual trust with your editor.
Photographs, as has already been said, are essential for trip reports and flight tests, and helpful to other editorial. Whilst they need to be good, quality is less important than you might think - flying magazines mostly sell on the quality of the writing and relevance of the subject matter - but nonetheless articles need illustrating and if an editor is choosing between two of equivalent quality, then the one with the better illustrations will sell.
Money varies massively and is to some extent negotiable if you are good enough and regular, to very little extent negotiable if you are new. Flight tests usually pay better than anything else. Some pay by the word, some by the page, some a standard rate per article.
Your name may be a good well known one (Dennis Kenyon, Bob Davy, Helen Krasner, Dave Unwin...) in which case you want to use that. However, particularly if you are selling to multiple magazines then agreeing a pseudonym may be practical because magazines prefer to maintain the illusion that any writer is apparently exclusive to them.
Finally the apprenticeship. Learning to write good publishable prose is a learned skill that requires development and time. Writing webitorial that is not subject to critical review, word limits, commercial requirements is unlikely to be a harsh enough environment to get you where you want to be. Writing for club or society newsletters is widely recognised as a good start in - it won't pay, but it will help you refine your skills. Expect a few rejection slips along the way!
Also the level of perfectionism is part of that learning processes. Most people in their first 5 years of writing probably need to expect at-least 4 revisions & re-writes per article, and probably 6 in the first year. After about a decade you can probably get down to one write and 1-2 reviews, per article, before submitting.
But it is gratifying, it can go a long way to paying for your recreational flying (it has done for me anyhow), and it does make a lot of your flying tax deductible! Also the magazines need copy, you don't need a commercial licence to make money from your flying this way, and the market will always be there.
G