If you are using the touch or vive controllers (VR Hands) you can re-center your view by pressing the menu button on the controller SEE HERE
You can also bind the 'reposition view in VR' to any key or button of not using the motion controllers to fly.
Dalfrak,
yes, this is possible, but its not officially documented as you can break the simulator with it if you do anything wrong here:
//
// this function allows you to override the VR head position and orientation
// be very careful, this function is called right in our render loop, ensure you
// are doing just a quick calculation here and no time consuming stuff!
// YOU HAVE BEEN WARNED!
//
__declspec( dllexport ) bool Aerofly_FS_2_External_DLL_VRHeadDataUpdate(
const tm_double time_absolute,
tm_vector3d &vr_head_position,
tm_quaterniond &vr_head_orientation_quaternion,
const tm_uint32 vr_num_controllers,
const tm_vector3d &vr_controller0_position,
const tm_quaterniond &vr_controller0_orientation,
const tm_vector3d &vr_controller1_position,
const tm_quaterniond &vr_controller1_orientation )
{
//
// vr_head_position and vr_head_orientation are the position and orientation as returned by the VR
// the coordinate system is:
// x - forward
// y - left
// z - up
//
...
//
// these code lines are just for sample purposes, so one can output the head data in the extra window
//
{
std::lock_guard<std::mutex> lock_guard{ VR_DataMutex };
VR_Head_Position = vr_head_position;
VR_Head_Orientation = vr_head_orientation;
VR_NumControllers = vr_num_controllers;
VR_Controller0_Position = vr_controller0_position;
VR_Controller0_Orientation = vr_controller0_orientation;
VR_Controller1_Position = vr_controller1_position;
VR_Controller1_Orientation = vr_controller0_orientation;
}
return true;
}Thank you !! That's exactly what I was searching for ! 
Now I have one more question about the vr_head_orientation_quaternion variable.
vr_head_orientation_quaternion is a quaternion, so it has a r, a x, a y and a z attribute. Could you (or someone else) explain me what is the r attribute ?
I guess the x, y and z are attributes of a tm_vector3d, but I don't know why there is a tm_vector3d here.
I mean, maybe the r attribute is the rotation around the vector, but why only one vector ?
Have a nice day,
Dalfrak
Hi Dalfrak, I think Wikipedia might be one of the places where you can read up on quaternions.
Hi, and thank you for your reply. I already checked Wikipedia, don't worry. But I asked for it in order to, maybe, have a simpler explanation. Because Wikipedia is great, I agree, but it's sometimes really difficult to understand when you're searching for specific terms (like quaternion). At least I tried.
Have a nice day,
Dalfrak
Hi, and thank you for your reply. I already checked Wikipedia, don't worry. But I asked for it in order to, maybe, have a simpler explanation. Because Wikipedia is great, I agree, but it's sometimes really difficult to understand when you're searching for specific terms (like quaternion). At least I tried.
Have a nice day,
Dalfrak
The quaternion's x,y,z is the endpoint of a vector that points out from the origo of the 3D coordinate system. This will give you the orientation of the 3D object. Now, the r is the angle that the object is rotated around the vector.
Like if you were cooking a marshmallow on a broach. you are the origo, the end of the broach is pointed at x,y,z and r is how you turn the broach to cook the marshmallow evenly at all sides.
Thank you Kisvakond !
It's way more easy to understand now, and your metaphor helps a lot 
Have a nice day,
Dalfrak
Let's give credit where credit is due: I've heard this example in the 3D engine programming videos of 'thebennybox' on youtube. 
