Flyers, like all vehicles, are limited by the firing arc of the weapons in question (non-vehicles have a 360 degree arc of fire). Turret-, sponson- and pintle-mounted weapons are counted as being able to see (and fire) in an arc equal to the movement they can physically make on the model (counting anything glued in place as if it were free to move), whilst hull-mounted weapons have a fixed 45 degree arc.
A Sunshark, for example, has it's missile pod weapon on a turret at the tail, giving it a 360 degree arc. The Markerlight at the front is also on a turret, allowing it to swivel.
As another example, take a Space Marine Stormtalon - it has a turret-mounted Assault Cannon, which means it has an arc of fire equal to the actual movement of the turret (giving it a full 360), but is also equipped with a hull-mounted weapon system (the optional Heavy Bolters / Typhoon Missiles etc.). As hull-mounted weapons only have a 45 degree arc of fire (22.5 left and right from the mid-point), the optional system can only be fired at targets in it's front arc.
For more information, check out page 74 of the rulebook,
Vehicle Weapons & Line of Sight.
However, future rules questions will get more attention in the
You Make Da Call sub-forum (which is for rules questions and interpretations) rather than Proposed Rules (which is for home-made, houserules).