You are right VDF class is not airspace related but accuracy levels (one can pass PPL exams without understanding these legacy things 
)
I guess from ATC perspective it’s lot of responsibility to give QDM if they don’t have you height or position and you are in IMC…
Ibra wrote:
I guess from ATC perspective it’s lot of responsibility to give QDM if they don’t have you height or position and you are in IMC…
I don’t think so. The VDF has a readout that gives the ATCO the QDM directly. (At least all VDFs that I’ve seen at tower visits.)
I was wondering if giving QDM readouts, means the headings (& altitude) are terrain safe (obviously it’s under line of sight but not necessarly obstacles clearances and terrain/obstacles is always PIC responsability)
Even on emergency, I doubt ATC can take VDF readouts and make something similar to vectoring or let-down on primary radar?
Ibra wrote:
was wondering if giving QDM readouts, means the headings (& altitude) are terrain safe
I expect it does not. It just tells you your QDM to their antenna, which is on airport grounds, or close.
Ibra wrote:
Even on emergency, I doubt ATC can take VDF readouts and make something similar to vectoring or let-down on primary radar?
It gives them (or you) a single QDM, so there is a whole (half-)line that you can be on. One would need another intersecting fix, or a distance. So I expect not. But if you fly high enough, it can get you flying DCT to above the airport.
VDF will get you to the airfield whereas for the approach you would need PAR. IIRC Le Mans Arnage LFRM used to have VDF, I don’t know if it still does, and from there you would pick up the NDB IAP. I don’t remember being given any altitude information. It was a long time ago that I last went there.
The simplest and cheapest back up to GNSS approaches might be old RAF methods. For instance aircraft returning to Northern Ireland during the Berlin Airlift used to follow white painted rocks in the sea and onto final approach.
Some inland airfields also dabbed paint in the middle of fields. Occasionally there was the luxury of lights in fields, but only occasionally as heavy batteries were always being nicked for other uses.
If you couldn’t see the white rocks you went elsewhere or another arrangement was made.
My father worked on Sunderland flying boats during the airlift and was based in Berlin, Denmark and Northern Ireland and used to tell many stories of the ingenious methods used to get the aircraft back on the ground/water, safely.
No luxuries like GPS then.
QDM was compulsory when I got my PPL in 1992. Never used it since
Ibra wrote:
I was wondering if giving QDM readouts, means the headings (& altitude) are terrain safe (obviously it’s under line of sight but not necessarly obstacles clearances and terrain/obstacles is always PIC responsability)Even on emergency, I doubt ATC can take VDF readouts and make something similar to vectoring or let-down on primary radar?
VDF QDMs are not vectors. In principle the information you get is the same as you would get from a VOR station located at the airport – it is only presented in a different way and updated only on request. The pilot is still navigating the aircraft — particularly the pilot has to decide on wind correction.
Got it, still better than flying single NDB, no DME bellow MEA and no radio, maybe 2 or 3 VDF stations would be neat 
I guess one imaginative solution for GPS backup is getting transponder position sent back to you on ADS-B via TIS-B and feeds into your navigator rather than your traffic display but I doubt that works well when one gets low for approach and TXP/ADSB radar is not at your landing airfield?
Maybe worth asking FAA how they handle GPS outage and lack of WAAS in Alaska for RNP approaches?
gallois wrote:
The simplest and cheapest back up to GNSS approaches might be old RAF methods. For instance aircraft returning to Northern Ireland during the Berlin Airlift used to follow white painted rocks in the sea and onto final approach.
That would be back to the old good days !
Right, so ILS + VOR. Nice when the state picks up the bill…
