Bankstown RNP RWY 11 approach on Garmin G1000
 

When carrying out the Bankstown RNP RWY 11C approach from the RAKSO IAF with a Garmin G1000 equipped aircraft while coupled to the autopilot, strange things happen. Fly over RAKSO at 3,700 feet and the Garmin unit doesn’t start descending to the 2,500’ SBKWI IF until there is a 3˚ approach angle. This would put the aircraft into the 2,500’ step.

In fact, if you look at the approach plate, it shows the descent should start at RAKSO from 3,700’, then go at a constant angle to WI at 2,500’. Once again, that means that the aircraft will clip the 2,500’ 20 DME Sydney Class C step. In effect, it means that the expensive and safer autopilot coupled system can not be used in this case.

Does anyone have an answer to this as to why the Garmin approach doesn't actually follow what is required in practice?

Dick,

The approach coding for that procedure does not have any coded vertical guidance prior to SBKWI. Any vertical advisory provided before this point is Garmin system derived, and not from coded data. Therefore the only vertical advisory the system can generate is based on a known final altitude......the only coded 'known' final altitude is the 2500ft at SBKWI and so it generates a path to this point. In this particular instance, because there is a break in the descent, the vertical advisory provided will not ensure you reach 2500ft by 6.5nm to SBKWI.

Its a case of the navigation systems 'smarts' being smarter than the approach. It happens.

Alpha

I am sure that the Collins Pro Line in my CJ3 flew the approach correctly.

So why the problem with Garmin?

Sounds like that particular approach may be coded wrong for that particular units database, it happens.

If it's any consolation Dick, a 'modern' 747-8 would have the same problem with its new 'Integrated Approach Navigation' mode. We won't arm this mode for RNP/RNP-AR descent until all altitude constraints are complied with and we're approaching the FAF.

That's why you have a pilot as the "master monitor" of the aircraft. It's why you're there mate.

Have not flown G1000 in 9 years and unfamiliar with the approach, I’m just looking at the plate:

https://www.airservicesaustralia.com..._24MAR2022.pdf

I’m not understanding why a 3 degree from Rakso to SBKWI would encroach the 2500’ shelf. That’s 20.3 miles and it should start descending approximately 3.7 miles out.

My FMS would do the same. For obvious reasons, it tries to stay as high as it can. Since there is no limiting step on the chart at 6.5WI, the FMS will stay up until in needs to descend to reach 2500ft at WI. In fact, my FMS would probably go through RAKSO around 5000ft (and WB at around 3000ft, for that matter). Airspace steps are not in my database and the box knows nothing of them.

If this particular airspace config doesn't change, then an "at or below" step at 6.5WI would be in order. Whether that is an allowed design feature for an IAF>IF leg is another issue. And don't be naughty and edit the approach to put in a step. :oh:

Or, "request a clearance to leave and re-enter"... (note note 4).

I do not agree. A line on the profile chart doesn't mean you (or the box) have to fly exactly along it. It is merely depicting that a descent is necessary at some point between RAKSO and WI. You could actually pass RAKSO then whip it into VS, zip down to 2500ft and then wait for WI.

The box would only have to commence descent from 3700ft at 3.7nm from WI (assuming it will try to descend at around 3°), but the step is passed at 6.5WI. You'd be crossing 6.5WI still at 3700ft.

Just another thought, the approach plate has a SYD CTR frequency, they’re the ones clearing you for the approach.
The hard altitude is 2500’ that one should be respected by the G1000.
Should be 4 ways to do this :

• VNaV 3 degrees
• VNaV release the altitude, descend early to intercept the path
• Descend in Vertical Speed
• Descend in FLCH

The chart is a LNAV chart, i.e. 2D, there is no descent angle coded in database. What annunciation did you have on the HSI, LNAV or LNAV+V ?

The initial segment from RAKSO to WI is actually from the Westmead Hospital RNP 052 chart, that charts has crossing WI at 2700 (it’s steeper at 3.07 degree). To fly the BWU approach at near 3 degree all the way, cross RAKSO at 6700 ft (13.3 nmx318 ft/nm +2500), cross 6.5 WI at 4600 (6.5x318+2500), that is the 2500 ft SY step, cross WI at 2500.

There is actually SWH....see attached extract of procedure coding (my red circle is the vertical path angle for the final segment)..........this only goes back to the FAF. There is no coded vertical path prior to the FAF. (Note this is source coding....not final coding for a specific navigation system)
https://cimg9.ibsrv.net/gimg/pprune....4dcf77a7ed.png
So back to Dicks issue. Each navigation manufacturer can tweak the procedure coding to optimise performance for that particular navigation system. The final version of coding for each system will differ and its why the Collins Proline and Garmin system each fly the procedure differently.

You will also notice that the 2500ft altitude at SBKWI is an AT altitude, so this altitude will be honoured. How the aircraft gets there is up to each individual manufacturers navigation logic.

Alpha

Alpha, why would each equipment manufacturer elect to code the descent differently?

Both systems would put the aircraft into the 2500' class C controlled airspace step if the pilot relies on the automation.

What do you mean by that? What are you expecting the GARMIN to do?

Which is why the plate Note 4 says approach approval required from SYD ATS.

I dont see 3 degrees in the Jepp database, just crossing altitudes, and no temperature limitations on the chart.

Tossbag, different manufacturers modify the coding in order for their specific navigation system to perform in a certain way for certain aircraft. For example its widely known that Boeing and Airbus navigation systems perform vertical path generation differently depending on the aircraft being flown. All whilst utilising the original coded data from Jeppesen. Even Garmin post process the Jeppesen data for different navigation systems.

SWH, that screenshot is from the Jeppesen database, granted its an older cycle so unless it have been taken off, which I am not aware it has been, then it is accurate. How are you viewing the coded data? The procedure doesn't have to be a VNAV procedure to have the vertical path angle in the coding, hence the lack of temperature limitations.

Alpha

Thankyou Alpha.

Boeing & Airbus "in order for their specific navigation system to perform in a certain way for certain aircraft" as in, due to the different aerodynamic properties of individual airframes?

Tossbag, yes pretty much.

From my own experience it seems that Boeing aircraft systems seem to favour a 2.5-3 degree profile, whereas the Airbus logic is not quite as straight forward and in some cases goes towards a more dive and drive profile. This can vary based on weight, wind, and temperature. Its just the different logic used in the navigation systems.

Garmin systems being more GA focused seem to also favour a 3 degree profile. The key thing to remember here is that the system is only as smart as the information it hold. In the Bankstown case for the RAKSO-SBKWI leg there are only 2 known points that being RAKSO or SBKWI (the 6.5nm fix and the airspace boundary are unknown to the system). So starting at 3700ft at RAKSO and wanting to be 2500ft at SBKWI presents some options..
a) I can give you a 3 degree descent from 3700ft @ RAKSO to 2500ft, or
b) I can give you a 3 degree descent to 2500ft @ SBKWI from 3700ft, or
c) I can plot a constant descent from 3700ft @ RAKSO to 2500ft SBKWI which would be 1.5deg (approx)

These are pretty much the only coding options available......each nav system manufacturer may choose a different option....and each of them are valid for a specific system/aircraft. For example option a) may be suited Cessna 400 series piston aircraft allowing for deceleration. Option b) may be more suited to turboprop aircraft and option c) may be suited to jet aircraft. In all cases the start, end and minimum altitudes are honoured.
I hope this helps explain it.
Alpha