To be able to use trigonometry to work out an aeroplane's location over Earth would involve more than just knowing that there is a 90° angle involved. A second angle would need to be calculated to use this method.
Also, no measurements for the sides of the triangle are known - the distance from the viewer to the aeroplane, the distance from the aeroplane to the ground and the distance from that place on the ground to the viewer. Again, more information is needed.
