NDB Approach Following GPS Arrival

[QSK?]

A question for those of you who are either PANS-OPS designers or IFR Instructors, familiar with Oz procedures.

1. A pilot is conducting a GPS Arrival procedure into an Australian airport, using the nominated NDB as the reference aid. The published MDA for this GPS Arrival is 910 ft.

2. Whilst conducting the GPS Arrival, the pilot notices that his inbound track is also exactly aligned to the final approach (inbound) leg of the runway aligned NDB approach which has a published S-I MDA of 640 ft. The prevailing wind is favouring the use of this runway.

Under these circumstances is a pilot permitted to transition from the GPS Arrival procedure once he has reached the GPS Arrival MDA of 910 ft, and make a direct entry onto the final approach leg of the NDB approach, to take advantage of the lower runway aligned MDA of 640 ft? The pilot has been using the nominated NDB as the reference aid throughout and is also able to positively fix his position reference the NDB using the GPS distance readout.

Comments, supported by references, would be appreciated.

Thanks very much.

[Chimbu chuckles]

NDB approaches are just that...they start overhead the aid and are designed to be flown, tracking wise, as published. The GPS doesn't give you a distance from the terrestrial aid it gives you a distance from co-ordinates on a theoretical sphere that are in the database and labelled xyz ndb.

The only way you could fly as you are asking is if the approach was an NDB/DME where a Initial Approach Fix was defined by DME/GPS distance on that inbound track...common with VOR NPAs.

The bottom line with instrument approaches is you chose the approach with the lowest minima (MDA/DH) and fly that approach. System degradation after passing the FAF may dictate a higher minima but to suggest flying to a lower minima by doing the first half of one approach and the last bit of another is begging for CFIT.

[Capt Fathom]

In your example, the answer is NO.

However, if it was an NDB/DME approach with a straight-in leg published, eg Albany 14 NBD -Y approach (http://www.airservices.gov.au/publications/current/dap/ABANB01-101.PDF), you could join the final leg at 10nm (but not below the MSA).

Have a read of the AIP links below. That should help give you a good idea. Others can probably fine tune it for you.

AIP ENR 1.20 Descent, AIP ENR 2 Approach Procedures (http://www.airservices.gov.au/publications/current/aip/enr/15739.pdf)

AIP GEN 8.5.6, Use of GPS in Lieu of DME (http://www.airservices.gov.au/publications/current/aip/gen/15122.pdf)

[swh]

QSK?,

I have done this before to get under LSALT for a leg.

The NDB procedure had IAF for the straight in approach. Use that technique I could keep a constant descent for the whole procedure.

This is similar to a RADAR 1 NDB type procedure. Need to obey normal speed and configuration limitations at the IAF and FAF.

I dont see this being valid for a tear drop NDB approach, always need to start an approach at the IAF.

:ok:

[Inka Dinka]

Fundamentally, this is not what the designers of these approaches permit.

Under the IFR, there are specific limits to where you can join an approach and apart from a few variations (see ENR 1.5 para 1.4), you must join via an Initial Approach Fix (IAF). Prior to the IAF you can’t descend below LSALT/MSA for the route segment being flown. ENR 1.5 para 1.5 further states that ‘Having arrived over the IAF…further descent must be in accordance with the entry and holding procedures’ … and ‘the approved instrument approach procedure’.

So the way I see it, unless there is an IAF on track (as is the case on the 280 track to Casino, and in this case you need a DME), you must join overhead the NDB at the IAF not below LSALT/MSA.
All (?) ‘NPA without distance measuring equipment’ (ENR 1.5 para 2.3) start overhead the aid as this is the only place you can get a ‘positive fix’.

[QSK?]

Thanks to all who responded. You have certainly contributed to improving my understanding of the procedures.

However, the obvious question that comes to my mind from all your responses is:Why don't procedure designers simply prescribe an IAF on the final (inbound) leg of NDB teardrop patterns based on GPS distance from the aid reference?Surely, facilitating a direct entry onto the final leg of the NDB approach, following a GPS Arrival, must offer significant safety and efficiency benefits to the industry (particularly for SPIFR ops); compared to forcing a pilot to conduct a MAP off the GPS Arrival and then re-positioning and flying another separate approach.

Or, have I missed something significant here, as well?

[Capt Fathom]

It is meant to be conservative. Otherwise some ninkinpoop will try to fly it straight-in without a DME or GPS.
I have seen people fly VOR/DME approaches without a DME! (No GPS back then)

[Icarus2001]

Why don't procedure designers simply prescribe an IAF on the final (inbound) leg of NDB teardrop patterns based on GPS distance from the aid reference? Well had happens when your inbound track is 90 degrees or more from the inbound of the base turn? You would then need a procedure to transition and turn on to the inbound leg.

An IAF on the final leg would be defined by what? A DME or GPS distance? The system we have gives an acceptable solution for most situations and allows adequate buffers for ham fisted operators using poor techniques.

[QSK?]

Icarus2001:

Thanks mate, but I'm not quite sure where you are coming from. All my comments are only related to the operational scenario that I put forward when I started the thread (?)

[OzExpat]

Sorry to be so late coming into this discussion but I've been away and am only now getting back up to speed in here. From the procedure design viewpoint, yes, it is possible to add an IAF at some point further out, aligned with the NDB final approach track. It is done in my part of the world wherever possible, but is currently only available to aircraft equipped with DME.

We allow use of GPS distance in lieu of DME for Arrivals, but have not yet extended this approval to approaches. This is because the size of the fix tolerance area exceeds the final approach protection area. We have information on studies that have been undertaken by CASA and other authorities, which suggests that the fix tolerance area for GPS is even smaller than for DME and we were going to act on that.

However, GNSS has overtaken those plans and we don't have the resources to do both. So we won't be making any changes to the existing procedures because the exclusive use of GNSS will solve the whole problem.

I would suggest, however, that if you were to lodge a request for such a fix with AirServices, they would evaluate it and, if appropriate, provide it.

[QSK?]

OzExpat:

Thanks for the feedback. I will take your advice and correspond with ASA or CASA as appropriate.

[OzExpat]

The folks in the Procedure Design Section of AirServices Australia in Canberra are the ones to talk to. They have all the ncessary delegations from CASA for providing and modifying approach procedures, so you should take this matter up with them. CASA will only refer you to them anyway.

[DirectAnywhere]

OzExpat
We allow use of GPS distance in lieu of DME for Arrivals, but have not yet extended this approval to approaches

Perhaps specific to company tailored charts but referring to several current Jepp charts, my company allows the use of a GPS distance in lieu of DME referenced to the applicable point.

[OzExpat]

I think you might be talking about the practice in Australia DA. I heard, a while ago, that CASA was going to approve it but I didn't follow up to find out if they did. From what you say, it looks like they have in fact approved it already, so thanks for that.

As I said in the post under reference, we (in PNG) would've gone down that track too, except that our resources wouldn't cover the work on that AND the GNSS Implementation Program.