DF approaches
Quote : "...Wikipedia is wrong.
They are both bearings...."
No. The meaning is in fact "heading with no wind".
Those were the days...when an airfield did not have a weather station to give a pilot an altimeter setting but just the "D factor", the difference between the actual airfield elevation andf the pressure altitude as read on the altimeter with a "standard" setting...
The procedure was long and rather complicated as first one had to determinate that the first given sector was correct (one could well have a 180° error) so a heading right or left of the given QDM would after a time cancel that "doubt" (steering left of the QDM would see an increase in the direction).
Then, one had to use a succession of QDMs, using a perpendicular heading to the QDM to compute a distance (with wind), to be correlated to the time it takes to make a vertical-of-station. With the drift that had to be applied to the successive tramsmissions, one could compute the wind velocity and direction.
The rest is about straightforward. One needed to find the outbound leg through a "QDM variation" and fly a wind-correected let-down.
Minima were set at around 6 to 800 ft depending on the airport.
The exercise was very good for materialising one's position relative to the DF station.
The avent of the VOR RMI is , as a matter of fact, the simplification of the VDF let-down, eliminating the ground operator and giving a continuous QDM information to the pilot. The principles are the same.
Quote : "Is my memory correct that in the days when men were men and ATC Controllers knew their stuff, a QGH approach involved the Controller calling for a series of transmissions which, seen on his VDF equipment as QDMs, enabled him to give a series of heading and descent instructions to place one on the glidepath.
"
As seen above, the whole procedure lies on the pilot. He just informed the controller of his position in the approach -with the usual clearances, of course.
This exercise was part of the French airline pilot students training until the late seventies.