GB,
Even if it is on your upper arm it will still be visible through a white shirt, most PAX and employers probably wouldn't be happy about someone with visible tatoos. Why not have it on your back or somewhere less visible. Also you can get these "temporary" tatoos nowadays that only go into a couple of layers of skin and last between 2 and 10 years (ideal if you dont want one anymore in later life).
I personally wouldn't have a problem with tatoos and quite fancy getting one myself, although I'll probably get mine on my lower back where it can't be seen easily and it'll be quite small. Remember its hard enough to get a job flying these days without giving any additional objections to a potential employer. Unfortunately despite everything it is still a predjudiced world we live in and as the old saying goes "First Impressions Count".
Thats just my opinion anyway.
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Aargh, that was the taxiway we landed on!