Legality of GPS/GNSS approach

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Hop Harrigan
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Legality of GPS/GNSS approach

Unread post by Hop Harrigan » Mon Dec 19, 2016 4:26 pm

Ok, so this has been concerning me for quite a while...
My a/c is certified for GNSS (RNAV) and so am I but it isn't fitted with an ADF or DME. So if I want to perform a published approach to either an:
1) NDB, it would make sense to use the Nav receiver in 'OBS' mode to fly a pseudo VOR approach over the NDB GPS position and
2) VOR/localiser approach requiring DME info, it would make sense to use the GPS to provide the necessary distance info
However the plate requires that an ADF or DME is required...
So it's ok to use the GPS to fly a published GNSS approach but is it ok to use it to provide the ADF or DME info?
Or is this just a case of the law lagging ((20 years) yes, that's how long GPS has been around) behind the technology??
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Re: Legality of GPS/GNSS approach

Unread post by SandPiper » Mon Dec 19, 2016 9:00 pm

Nope. Highly illegal.

Substituting the dme with gps only allowed when specifically authorized.

The minimum required equipment is exactly that.
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Re: Legality of GPS/GNSS approach

Unread post by tansg » Tue Dec 20, 2016 7:12 am

SandPiper wrote:Nope. Highly illegal.

Substituting the dme with gps only allowed when specifically authorized.

The minimum required equipment is exactly that.
Quite correct as the protection areas used for the "conventional" nav aids are fundamentally different in nature to GNSS with different errors and tolerances and so it is possible that a "conventional" approach flown on GNSS may actually go outside the protection areas to which that procedure was designed. This has been found to be the case with a number of approaches in Norway and has prompted an ICAO Doc 8168 Vol II update due very shortly which will require all "conventional" approaches which may be flown by GNSS equipped aircraft to have an Aviation Study performed to prove that the aircraft will not exceed the the designed protection areas. Further the chart will be noted to say " May be flown with GNSS"

The use of GPS distance instead of is a lot easier and there is a protocol out in South Africa, AIC 25.4 with phraseology for reporting GNSS distance to ATC however there is nothing yet in place to allow the substitution of GNSS distance for DME on approaches. The fact of the matter is that if you are not equipped with a DME, then you are excluded from flying any approaches requiring a DME unless a protocol to cover that eventuality has been developed. The number of times this is required is considered so little that it was not worth doing the Aviation Study and developing the protocol. However this does not exclude any member of the aviation community performing this Aviation Study and developing this protocol on their own and submitting it to CAA for evaluation and possible publication. SAA has done this on a number of occaisions. However be aware the requirements of an Aviation Study are stringent and will require you to prove the safety of the proposal to an "acceptable level of safety".
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Re: Legality of GPS/GNSS approach

Unread post by Hop Harrigan » Tue Dec 20, 2016 1:16 pm

What about the use of GPS to provide a bearing to an NDB position and then flying the approach using only bearings to that point? I would guess that the GPS bearing is more precise than the ADF needle anyway, and not affected by static.
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Re: Legality of GPS/GNSS approach

Unread post by SandPiper » Tue Dec 20, 2016 2:56 pm

Still no go.

Approved operators fly NPA with GNSS but that is a different certification process altogether.

Raw data must still be monitored during the appproach.

The USA is way ahead re. general aviation shooting gps approaches as the norm.
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Re: Legality of GPS/GNSS approach

Unread post by Jayson v Schalkwyk » Thu Dec 22, 2016 2:40 pm

A GPS approach with WAAS capability is deemed a precision approach that can be flown to a decision altitude all be it, on most occasions to a slightly higher decision altitude.

If you are flying a NDB approach using a GPS (overlay) you are still required to have the NDB tuned and a bearing to the beacon indicated.
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Re: Legality of GPS/GNSS approach

Unread post by Andrew Smit » Tue Dec 27, 2016 9:37 am

Jayson v Schalkwyk wrote:A GPS approach with WAAS capability is deemed a precision approach that can be flown to a decision altitude all be it, on most occasions to a slightly higher decision altitude.

If you are flying a NDB approach using a GPS (overlay) you are still required to have the NDB tuned and a bearing to the beacon indicated.
In SA yes, as the approach was designed using NDB obstacle clearance criteria. The US does have the option of excemption for this this, but this is not the norm.
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Re: Legality of GPS/GNSS approach

Unread post by Andrew Smit » Wed Dec 28, 2016 12:17 pm

Jayson v Schalkwyk wrote:A GPS approach with WAAS capability is deemed a precision approach that can be flown to a decision altitude all be it, on most occasions to a slightly higher decision altitude.

If you are flying a NDB approach using a GPS (overlay) you are still required to have the NDB tuned and a bearing to the beacon indicated.

Here's a better explanation from a procedure design specialist. I've left his name out as this was sent as a PM

A bit of background on why this is not approved in SA. The protection areas for NDB approaches and GNSS based approaches are fundamentally different and it has been found that GNSS protection areas can in some cases be larger than those used for NDBs and as such could introduce a borderline obstacle into the protection area that you would not know about unless you had done a Safety Case/Aeronautical Study of the procedure before flying it with GNSS. This was found when a Norwegian Air 737-800 had an incident when it had GPWS warning while flying a NDB approach. When it was investigated it was found that they had flown it using GPS and had wandered outside of the NDB protection area although they were still way within parameters for the GPS. This is mainly due to the way these procedures are programmed in the database and which path terminators are used. This obviously came as huge shock to everyone on IFPP who had all assumed that since the protection areas for GPS were so much smaller that all would be safe but it turned out that the orientation of the protection areas was more important that size. The FAA decided not to stop this type of operation,
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Re: Legality of GPS/GNSS approach

Unread post by Hop Harrigan » Wed Dec 28, 2016 2:58 pm

Hi Andrew,
Ja, agree with that possibility if you're using the loaded approach from the NavCom. However, if you are merely using the NDB GPS position, which with a WAAS GPS is accurate to a few meters, and flying the approach either as a bearing to an NDB position or using "OBS" mode and flying it as a VOR, it has to be more accurate than the basic ADF/VOR equipment.
My opinion is that the law is just plain 20 years behind the technology.
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Re: Legality of GPS/GNSS approach

Unread post by Andrew Smit » Wed Dec 28, 2016 10:01 pm

That was my view on the issue as well. I was apparently wrong. I'll try and get an expert to comment on the issue.
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