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

"... and the airspace boundary are unknown to the system"

Wow. As usual, I've learned something new from you, Alpha. It's counterintuitive, as I've seen airspace boundaries on (TSO'd) GPS displays.

Thanks.

Yes, perfectly, thanks very much :ok:

So if you are given the BK 11 LNAV approach by Sydney Terminal when you are OCTA approaching RAKSO does that allow you to enter and leave the class C airspace on descent to WI?

Current cycle in ARINC 424 format.

one wouldn’t think so - you have ATS approval to conduct the approach but no clearance to enter controlled airspace. For an LNAV only approach it’s pilots choice to get permission to re-enter and leave or dive’n’drive. If it were +VNAV and the profile was going to fly you through CTA then the chart would(should?) be very clear that a clearance was required.

I’m a little confused by this thread - I would never expect a G1000 or other system to keep me clear of controlled airspace - only to fly the approach appropriately (I.e to the manufacturers specs as alfacentauri explained). How can your FMC know what clearances you have or don’t have?

UTR

Only in Australia from my experience would an IFR aircraft on an IFR approach go in and out then in again of controlled airspace.

Once I flew the approach with a US trained pilot coming in from overhead Richmond. It was beyond comprehension to him that he was not cleared for the approach. I explained that we would actually be in uncontrolled airspace when the approach started and he was on his own. No descent clearance would be given.

Agreed, it's very poor form in a supposed first world country and for all the boasting about how much airspace is 'controlled' by this mob.

Given or cleared? ATC can't 'clear' you for an approach that is outside controlled airspace, that is your choice and decision. They shouldn't really be 'giving' it to you either, that implies a clearance.

You can only ever have one IFR aircraft ( that hasnt reported visual) in the BK CTR at one time. My understanding is the APPCH permission from SY CEN is for sequencing into BK even though you are OCTA from SY airspace for the majority apart from the aformentioned segment of the IAF-IF leg, which without a clearance to enter ( different to approach permission), you must avoid.

Lead Balloon,

To clarify, the airspace data and approach data are in separate parts of the database. They are unknown to each other, and so there are no smarts to compare approach path and airspace data…..

Thats kind of the point of the prediction arc in large fms systems. It allows the pilot to know if an altitude is going to be reached by a certain point and adjust if required.

Just out of interest, is this 'procedure' published anywhere? ERSA, DAP's/Jepps?

Not to my knowledge but have tried to find a reference. I am aware of it via local knowledge of flying there. Whenever it's less than VMC and Special VFR and IFR aircraft are operating, they can only ever have one aircraft operating under those rules.

I believe it is to do with ATC are required to seperate IFR from IFR in Class D. Under normal circumstances, most IFR aircraft when VMC prevails operate inbound via the VFR reporting points, and report visual with their inbound call, which enables ATC to employ visual separation to IFR aircraft as they are visual, which obviously cant be done in conditions less than VMC with aircraft on IAP's. Radar/ ADSB is only available for situational awareness to controllers in BK tower to my knowledge.

The only quote that somewhat references it is in the ERSA " IFR Operations in VMC", " Pilots electing to terminate an IFR flight under the VFR should communicate itention to ensure their arrival is processed efficently "

So what it looks like is a 'local procedure.' Unpublished, but IFR pilots are expected to understand and comply. Contributing to confusion and misunderstanding (as in Dick's question).

Great airspace system (sarcasm).

tossbag, it's pretty simple really. You're IFR operating in Class D airspace so subject to a clearance to conduct the approach in D, and as you're required to be separated from other IFR aircraft in D you have exclusive use of the airspace until you've either landed or conducted a missed approach.

Tossbag, Dick's question had nothing to do with your "local procedure". He asked about why his FMS didn't stop him clipping a CTA step in when in full auto. He was given the answer.

I see nothing "local" about being cleared for an approach. The fact that you start out OCTA and, because of the way you operate your aeroplane/FMS, you then clip a CTA step, sound like a personal problem.

There is much nashing of teeth about hypotheses here. To go anywhere with this, we need to know exactly what ATC says. Obviously they won't "give" you the approach (your post #27).

As for
Read note 4. And observe the 20nm range ring from SY.

Perhaps a more-specific caution on the chart re the Sydney step might be in order. AVSEF it.

I'm just wondering if a note is missing from the RNP chart, there is a star next to the @2500 limit at WI, which resembles the same star on the NDB RWY 11C chart which states you have to be 2500 by 17 DME SY. Although this procedure is horribly non aligned with rules concerning CTA as some of them seem to pass in and out of CTA steps. The marked descent line seems to infer all is ok as long as you are 2500 at WI. It would be interesting what a controller familiar with the area would expect an aircraft to do, ie remain clear of the 20 DME step or as long as you are 2500 by 17 DME SY no probs...

That there is some confusion about being 'cleared' for an approach whilst being OCTA and whether it constitutes a clearance to clip CTA or not suggests there is a problem. Which should be cleared up with airspace users. There should be no confusion, especially when you're operating Single Pilot IFR, there is enough to do in the cockpit without confusion.

Captain Bloggs, you'll note in my previous posts I was asking questions to understand what is being said by ATC. You obviously have someone sitting beside you to help clear up any confusion, sit in the cockpit by yourself in IMC and manage the workload of this procedure and see how you go. Personal problem? No, Dick is confused by whether he has a clearance to clip, the last thing you need when SP-IFRing in IMC is to be chipped by ATC for busting CTA.

I think you need to calm down a little, look at the broader implications of what is being discussed.

I say again:

Garmin navigators will provide vertical guidance on almost every approach in the database. On LNAV approaches it will give you LNAV+V where the the +V refers to a pseudo glide path that is a mathematically derived 3 deg slope. This is very handy but care must be taken because it will not account for any mandatory crossing altitudes.

One gotcha with GARMIN systems is that the system switches from terminal mode to approach mode (ie LPV, LNAV/VNAV or LNAV) when the active leg is the leg to the FAF. If there is a fix between the IAF and the FAF it will not sequence to approach mode until after that waypoint which means there will be no glide path until after you pass the waypoint at which point you will be above the glide path.

The work around is to activate vectors to final after you pass the IAF which will cause the FAF to be the active to waypoint.

This is a terrible idea. Never activate vectors unless you are actually “being vectored”. The work around to your dilemma is to use your 3 times tables