Primarily for general aviation discussion, but other aviation topics are also welcome.
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By Norman
#58325
A question for all you clever IMC and IR types.

Reading Thom, he says that RNAV will only give you lateral deviation from a course between 2 waypoints (which can be real or pseudo).

But I seem to recall someone telling me that his RNAV system will effectively allow a pseudo VOR/DME to replicate a real VOR/DME, with an angular deviation from a specified pseudo radial to the pseudo VOR/DME.

There is no reason why it shouldn't happen like that because the computer would know where you are, and it would only be a matter of arithmetic to give angular deviation from a specified radial. But does it?

On a related point, can the pseudo VOR/DME also give you a QDM, like an ADF?

And finally, if T BLiar finds a pseudo WMD in Iraq, will that let him off the real hook?
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By Timothy
#58362
An ADF will only give you a QDM on a windless day.


No. It always shows a QDM. Just like any other QDM you have to allow for drift when flying it. Only on a windless day, or when the wind matches the QDM or its reciprocal will the drift be zero. Very different.

Timothy
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By Norman
#58388
Looks like Thom is right and my recollection of what I was told is wrong.

No-one has suggested that you can create a pseudoVOR, then specify a radial from it, and locate and track that radial using the VOR dial in the normal way. I am sure that is what this chap told me his RNAV can do. But then he was trying to sell me a share. :scratch:



An ADF will only give you a QDM on a windless day.


We do get a lot of wind in the North West. It is all that cabbage water we eat with our dry bread. :drunken:
By Guest
#58389
No-one has suggested that you can create a pseudoVOR, then specify a radial from it, and locate and track that radial using the VOR dial in the normal way. I am sure that is what this chap told me his RNAV can do. But then he was trying to sell me a share.


This is exactly what an RNAV can do, the chap is not fibbing even remotely. However, when tracking towards the pseudo-VOR the side-side displacement of the VOR needle indicates a linear displacement from the required track, rather than an angular displacement. If you fly with the needle centred, this distinction is totally academic.
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By Norman
#58391
Thanks 2Ds. I am still confused about one point. I thought that the RNAV system would only enable you to follow a track from one waypoint to another, both of which can be pseudo.

What I was wondering about is whether you can use the OBS in the same way as you can with a real VOR.
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By David Williams
#58393
Norman wrote:I am still confused about one point. I thought that the RNAV system would only enable you to follow a track from one waypoint to another, both of which can be pseudo.

Perhaps what you're thinking of is the fact that with a KNS80 you can pre-program the offsets for a number of "pseudo-VORs", thus defining a little sequence of waypoints. Hence, when you reach one of your pseudo-VORs you just push a button, change VOR frequency and start tracking to the next pseudo-VOR without the need to fiddle around entering the offset. I think a KNS80 can give you four waypoints like this if my memory is correct - it's been a while since I've used one.

The first time you use RNAV, you'll find it is indeed the dog's danglies (assuming you don't have GPS). Just don't forget the box is in RNAV mode when you next fly the aircraft, otherwise you could find yourself tracking towards a random point some distance away from the "real" VOR you think you're tracking. ;) When I was last flying a shared aircraft with a KNS80 I added an extra item to my shut-down checklist to swicth the KNS80 back to "normal" mode so it doesn't confuse the next pilot or lead them into trouble.
By Guest
#58395
What I was wondering about is whether you can use the OBS in the same way as you can with a real VOR.


The answer is "yes" you can. Twiddle the dial, and the needle will deflect just as it does for a real VOR. The difference, as noted before, is that whereas ordinarily, the dots of deflection indicate degrees away from the correct radial, the dots indicate linear distance when in RNAV mode.
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By David Williams
#58400
2Donkeys wrote:The difference, as noted before, is that whereas ordinarily, the dots of deflection indicate degrees away from the correct radial, the dots indicate linear distance when in RNAV mode.

... the implication of this of course is that the needle doesn't become hyper-sensitive as you get close to your waypoint, so even a numpty like me can keep it centred!
By Guest
#58403
That is the implication. However, most RNAV units have two modes for enroute and approach navigation.

In the case of the KNS 80/81, pressing the APR button whilst in RNAV mode doubles the needle sensitivity. KNS users are advised in the manual to select APR for RNAV in order to ensure acceptable accuracy,
By bookworm
#58436
David Williams wrote:... the implication of this of course is that the needle doesn't become hyper-sensitive as you get close to your waypoint, so even a numpty like me can keep it centred!


The converse is that if you set the pseudo-beacon to a point a long way away, the needle is hyper-sensitive to the OBS setting, and not very sensitive to your position.

It's worth having an appreciation of errors when playing with a VOR shifter:

1) Measurement errors

I watched a beginner mark precise positions on a map, and then measure bearing from a VOR 30 miles away to the nearest 10 degrees. That will give you a waypoint position to the nearest 5 miles (which may be fine, or may give very unexpected results).

2) Instrument errors

Note that the instrument error depends on your distance from the VOR/DME that you're shifting. If you are 100 miles from your waypoint and you use a VOR/DME close to your present position to put a pseudo-beacon on the waypoint, the error will be negligible, provided you measured accurately. If you use a VOR/DME close to the waypoint, you'll get your lateral position plus or minus about 10 miles. Don't be too surprised if a little wobble in the VOR signal causes the deviation indicaator to bang off the stops.