Using GPS as an ADF replacement

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Hop Harrigan
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Using GPS as an ADF replacement

Unread post by Hop Harrigan » Thu May 19, 2016 5:40 pm

ADF IF approaches are awkward things to fly and not precise at all. Some GPS systems (G430/530 for example) have an 'OBS' mode whereby it is possible to use a GPS point on the ground as a VOR and fly the approach using the CDI exactly as one would if it has been a physical VOR. (I think this is termed an 'overlay approach?)
This method is more accurate than a physical ADF approach. It is also easier to stay current on only one type of non-precision approach than on both. However, as I understand it, this isn't legal even though the aircraft and pilot may be GNSS certified.
This is surely an antiquated piece of legislation which if updated, could improve safety on these type of approaches of which there are a number in SA?
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Re: Using GPS as an ADF replacement

Unread post by Flugmaour » Fri May 20, 2016 7:41 am

NDB approaches are slowly being withdrawn worldwide including SA. They are all replaced by GNSS approaches, so I recon in the next 10 years you won't find any of them around anymore.
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Re: Using GPS as an ADF replacement

Unread post by Iceberg » Fri May 20, 2016 8:45 am

There is an AIC somewhere stating that all NDB's are being phased out in SA by 2019.

Wonderboom has had a VOR/DME (fully functional) for 2 years now but one still has only the NDB approach if you need to get in in IMC.

From a practical point of view, I use the GPS for the NDB approach, using the ADF needle as backup.
At Wonderboom, I also set the inbound course (288) on the VOR for extra backup. Works like a charm.
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Re: Using GPS as an ADF replacement

Unread post by Hop Harrigan » Fri May 20, 2016 12:29 pm

Hi Karl,
As far as I know nobody even sells a single engine GA aircraft with an ADF as standard equipment anymore. However, as far as I understand it one may not legally execute an NDB approach using GPS (even if GNSS certified) without an ADF in the panel.
This sort of antiquated legislation is a hindrance to reducing the GA accident rate. I would really like to CAA apply their minds to updating the rules to make GA flying safer.
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Re: Using GPS as an ADF replacement

Unread post by Andrew Smit » Fri May 20, 2016 9:07 pm

Yes you can. But not in SA. The FAA issue a C300 authorization which allows suitably equipped machines to fly NDB/VOR approaches without the actual beacons been available.
An A320/330/340 for example don't actually fly a VOR radial during a VOR approach. They rather use FMS coded track and waypoint data (RNAV really) to compute the approach path.
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Re: Using GPS as an ADF replacement

Unread post by Kwagga » Sat May 21, 2016 7:35 pm

I have heard but is also not sure, that by tuning to the NDB/VOR frequency and receiving the morse signal it indicate that the approach is available and runway serviceable. If there is a problem at an uncontrolled airfield the beacon will be shut down and you can not continue with the approach, you will not know this flying the overlay approach. I wonder if this will really be done. ATC can also warn you well in advance when flying to a uncontrolled airfield if there is a problem, but this is the argument that i once heard.

The distance that the 430/530 display is it horizontal distance or 3D distance like DME. I know the difference will only be noteworthy at higher levels when it is less critical but theoretically you can end up outside the protected area because 5 nm horizontal is then "further" than 5 DME.

There is just no way that I will fly IFR without GPS, I am just not current enough.
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