Hello FolksA techno boff is needed.I notice in my Bendix radio stack the DME bit ,there is displayed a miles to or from a Vor (please excuse my lack of knowledge here )Next to the miles to/from there is a constantly changing figure in what appears to be kts. This figure in kts doesn't seem to have any connection to my current airspeed, I can be chugging along happily at 140kts in my dc3 but that figure in the DME(kts) display (not miles to /from that is obvious in what its showing) constantly changes.Any explanation of this much appreciated please.Andy

It is probably your G/S. It would only be accurate when tracking toward or away from the VOR/DME and because it is working over a slant distance, would otherwise be very inaccurate. When passing abeam the station, it would be of no use as in indication of G/S. Try tracking toward or away and see if it roughly agrees with your TAS plus or minus the wind component. If you're in a DC3 at low level, your TAS will only be a little above your IAS, so you can easily check your G/S. Colin B

Hi Andy, it's supposed to simulate a DME speed readout and they fluctuate a bit. IRL they work by measuring the time it takes to send a signal to the aircraft and receive a reply back, or maybe the airplane sends the signal and gets a reply back from the DME station, I dunno, but I suppose if the signal gets briefly interrupted by a cloud, mountain, or an A-380, the reading gets sporadic. Also like Colin said, you have to be tracking either directly to or away from the station or the reading has no value at all. I think you have to look for an average, it won't agree with your airspeed indicator unless you're cruising at sea level with an OAT of 59° F, Altimiter: 29.92, and no wind ("standard day", they used to call it). It should roughly agree with your GPS groundspeed though whenever you're on a course directly to or from the VOR station. Jim

Thanks again guys.Your a clever lot.Sometimes when I see instrument displays where I am not quite sure off what is ACTUALLY being displayed ,over the course of a long flight I can sometimes have a Eureka moment and see AH! now that's what's going on ,but in this case no light-bulb was illuminated.As an aside I keep telling myself I may go back to light aircraft and try and get to grips with all the navigation stuff (DME included) as its too easy to fly FS and not REALLY understand what the instruments are actually doing and telling you. I am sure I have heard it said that real trainee pilots don't always find learning the actual flying of the plane the hard bit(guess I will never know being rather lacking in the bank balance dept ) ,but learning all the map reading ,navigation stuff (technical term)Anyways enough of my ramblingscheers again Andy