I live in a flat under a flight path with a plane going over every 15 mins.
The planes go directly in front of the flat, do you think I would be ok to use
my lasers shining it parallel to the ground in the back or would that cause problems ??
Definitely not when they're flying by, you'll still bring attention to your residence. In the US laser sightings are almost always reported as a 'laser incident' by aircraft, even when it's not aimed anywhere near an airplane or if it's from an outdoor concert. The FAA evaluates all of this data reported by pilots for flight plans, routes, ect over time. Your best bet is to play it as safe as possible outside when the skies are quiet :beer: