VOR approaches in NDA

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Raptor256
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VOR approaches in NDA

Post by Raptor256 »

Shooting VOR approaches in NDA with HSI running in Slaved mode.

What should you set as the inbound track on the course indicator? I was under the impression that even though you are running in Slaved mode, you should still set the inbound course as you see it on the approach plate and not convert it to magnetic, because of the way the VOR receiver works.

And I know we should be running in free mode, but we don't.

Any thoughts?
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sakism
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Re: VOR approaches in NDA

Post by sakism »

Set the course as it appears on the plate. Your HSI doesn't know whether it is in slaved mode or not. It will properly indicate your position relative to the radial that you have your needle pointed at.

Since the VOR itself is aligned to true, and the plate is in true - if you want to fly the track on the plate you must point it where the plate says, regardless of the slave/free mode of the HSI.
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ahramin
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Re: VOR approaches in NDA

Post by ahramin »

You must set the CDI needle to the IB radial as specified in the approach plate or you will not be on the correct inbound track. This is basic IFR navigation. If the inbound track is 090, and you set 110, what do you think is going to happen?

Secondly, if you know you should be running in free mode, why don't you?
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just curious
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Re: VOR approaches in NDA

Post by just curious »

Running a slaved compass in the polar regions will only aid you when you happen to be aligned with both true north and magnetic north. Unless you are running off the sea ice north of Barrow, then you are really risking having the compass want to drive itself off course constantly.

I suppose you don't have a compass compensating system for transport wander either...

You may wish to ask a few more questions about flying up in a region where alternates are sparse, and the environment is unforgiving.
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PanEuropean
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Re: VOR approaches in NDA

Post by PanEuropean »

Raptor:

I think it might help you figure it all out of you think of things this way:

The scenario you describe (a VOR approach) involves using two totally different and totally independent "systems" (a stabilized directional gyro system, and a VOR). However, the information from both systems is displayed together in a single combined instrument - the HSI.

Each of the two systems needs to be set to use a common 'reference system' for heading in order to be congruent with each other. Typically, in the south, we use magnetic north as that reference system. So, as you know, if you were flying a heading of North (000) as displayed on the gyro system, and at the same time you were tracking outbound on the 000 (or 360) radial from a VOR, the indications on the two different systems (the stabilized directional gyro system and the VOR receiver) would be congruent.

Up north, the VOR is referenced to true north, rather than to magnetic north. Thus, if you want the directional gyro system to "make sense" when you look at it with the overlay of the VOR receiver displayed on top of it - which is what the HSI presents to you - then you need to ensure that the stabilized directional gyro system is also using the same reference system.

It's not particularly important whether the reference system used is magnetic North or True North, but you have to use the same reference system (at the same time) on both devices. As pilots, we don't get much choice in the matter, because someone else has already decided what reference system the VOR on the ground will use, and we can't change that.

Heck, everything would work just fine if you used the location of the local Pilot's Bar and Grill as your reference system, although you would first have to successfully convince the operator of the VOR to re-align it so that the 000/360 radial exactly overlaid the location of the local Pilot's Bar and Grill. However, if you were successful in getting the VOR re-referenced, and you then adjusted your stabilized directional gyro system so that it showed North (000/360) when you were pointed directly at the local Pilot's Bar and Grill, hey, you would then have the world's first VOR approach that was based on your own unique reference system (rather than true north or magnetic north), and you could fly it without any problems, as long as you set your stabilized directional gyro system up so that it was using the same reference system as the VOR at the appropriate time (i.e. according to the instructions published for the approach).

I hope this helps you understand the concept OK.

Michael
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CID
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Re: VOR approaches in NDA

Post by CID »

It's extremely dangerous and irresponsible to operate in NDA with a slaved compass. In NDA it's not just a matter of acknowledging the magnetic variation, it's also a matter of understanding what that compass indications may be erratic. The entire reason for the special classification of that airspace is for the lack of horizontal magnetic flux density due to the proximity to the poles.

Your compass system just may not have enough of a magnetic field to even display the magnetic heading.

If your compass card is not capable of displaying true heading or true track, you have no business flying in NDA.
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