H145-D2: RNP Approach to LNAV minima

[peeush]

Hi,
I am uncertain about Helionix/AFCS response during RNP horizontal approach. While on RNP approach to LNAV minima, APP mode, which must automatically get armed at IF (Intermediate Fix) with NAV mode preceding from IAF (Initial Approach Fix), did not yield expected response. The NAV mode continued to remain engaged till MAPt. Referring to Flt manual & FOBN (Flight operations briefing notes by Airbus Helicopters), I seem to have checked all boxes in checklist. (Since it was a practice RNP approach under VFR, it did not have other implications)

Any inputs of what may have played against to prevent APP armament and (following) engagement ?

Cheers

[FloaterNorthWest]

Hi,
I am uncertain about Helionix/AFCS response during RNP horizontal approach. While on RNP approach to LNAV minima, APP mode, which must automatically get armed at IF (Intermediate Fix) with NAV mode preceding from IAF (Initial Approach Fix), did not yield expected response. The NAV mode continued to remain engaged till MAPt. Referring to Flt manual & FOBN (Flight operations briefing notes by Airbus Helicopters), I seem to have checked all boxes in checklist. (Since it was a practice RNP approach under VFR, it did not have other implications)

Any inputs of what may have played against to prevent APP armament and (following) engagement ?

Cheers

On the GTN, was the transition “vectors to finals” or via an IAF/IF?

Did APP appear below NAV?

FNW

[peeush]

On the GTN, was the transition “vectors to finals” or via an IAF/IF?

Did APP appear below NAV?

FNW
Hi. Transition through IAF; No, the APP did not appear below NAV as expected. Thanks for quick response

[FloaterNorthWest]

Hi. Transition through IAF; No, the APP did not appear below NAV as expected. Thanks for quick response

Hi,

Did you activate the approach at any point or just load and use direct to?

What airfield was it to?

FNW

[peeush]

The approach was loaded and activated from arrival route. No, the 'Direct to' or VTF function wasn't used. Airport - VAID

​​​​​​Thanks

[FloaterNorthWest]

The approach was loaded and activated from arrival route. No, the 'Direct to' or VTF function wasn't used. Airport - VAID

​​​​​​Thanks

I will see if I can fly it on HATS and let you know.

FNW

[peeush]

FNW
​​​Appreciate your interest. Thanks

[212man]

Don’t know the system at all but could it be ANP greater than RNP?

[peeush]

Hi.
We can put that away as this is one of '2nm short of FAF' checks. ANP did taper down to 0.3 in our case. Nevertheless, thanks for your interest.

[PlasticCabDriver]

Interesting. My first thought was the same as 212man’s, ANP > RNP, but you say it went to the 0.3 as normal, which is APP mode. Wonder if it is just a wigglyamp anomaly. Have you had the chance to fly the approach again?

[peeush]

Hi. I am looking forward to go for it again. Will share more of it then

[gipsymagpie]

From my notes, I think it's an issue of SBAS. App mode needs SBAS to function and the aircraft uses NAV instead if reliant upon RAIM instead. You should be under the GAGAN SBAS at that airfield. You should check its enabled on your GTN750 and if so, check you have 3D DIFF NAV in the GPS Status menu.

It's still perfectly valid to fly the approach using NAV, being reliant on RAIM for integrity. The scaling of the RNP is Garmin thing all approaches it generates are angular- of course strictly for an LNAV it should be a fixed lateral width all the way in. See the excellent PBN Manual produced by the PPL/IR group for more.

[gipsymagpie]

So I have spoken to someone in the know and an alternative solution has presented itself. Rhey had the same issue a while ago and it was due to some settings being different in a replacement GTN.

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/768x1024/img_20211011_wa0000_9c9473ef71a013ef51a62079467fedb89c5c2e6c .jpg
Correct settings on left/top. Replacement GTN on right/bottom.


Access settings by holding HOME whilst switching on...well your engineer should do that bit.

[peeush]

Thanks for the input.

We had SBAS based navigation on arrival route as well. Shouldn't a message/caution be flagged if drop of SBAS signal caused APP mode deviation.

Just an argument, since I ain't sure of Helionix logic.

[peeush]

So I have spoken to someone in the know and an alternative solution has presented itself. Rhey had the same issue a while ago and it was due to some settings being different in a replacement GTN.

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/768x1024/img_20211011_wa0000_9c9473ef71a013ef51a62079467fedb89c5c2e6c .jpg
Correct settings on left/top. Replacement GTN on right/bottom.


Access settings by holding HOME whilst switching on...well your engineer should do that bit.


Hi. Thanks big time. Will revert if it plays up.

Cheers

[peeush]

So I have spoken to someone in the know and an alternative solution has presented itself. Rhey had the same issue a while ago and it was due to some settings being different in a replacement GTN.

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/768x1024/img_20211011_wa0000_9c9473ef71a013ef51a62079467fedb89c5c2e6c .jpg
Correct settings on left/top. Replacement GTN on right/bottom.


Access settings by holding HOME whilst switching on...well your engineer should do that bit.

Attaching settings in my aircraft. Apparently, the search for answer continues.

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/1600x1200/img_20211011_wa0013_bf6e6668503ad8ab1d1492d568bb0092294dd36f .jpg
We had same settings on GTN 1 & 2

[Flying Bull]

Hi peeush,
in the GTN-Trainer-App, which is free for iPad - there is no RNP for VAID - probably older database? So no way to check it there in the dry.
After selecting the approach, did you check the Waypoint-list in the FlightPlan on the GTN and on the NAV-Display?
SBAS was indicated?
Flight Manual:
At the Final Approach Fix (FAF):
Autopilot mode indication (FND or NAVD) − Check APP indicated
Autopilot ALT.A, FPA or V/S mode − Activate, as required

[gipsymagpie]

NAVBLUE has the procedures as per original message (one for each runway with LNAV and LNAV/VNAV minima). They definitely exist.

From the FOBN:

For APP mode to engage (assuming 2D approach)

a. A flight plan including an RNP approach with horizontal guidance only (LP, LNAV, or LNAV+V) must be active in the FMS
b. The FMS must receive valid SBAS signals
c. The NAV mode must be previously engaged

I think b. is the problem. Have you got GAGAN enabled in System-GPS Status-SBAS?

[peeush]

NAVBLUE has the procedures as per original message (one for each runway with LNAV and LNAV/VNAV minima). They definitely exist.

From the FOBN:

For APP mode to engage (assuming 2D approach)

a. A flight plan including an RNP approach with horizontal guidance only (LP, LNAV, or LNAV+V) must be active in the FMS
b. The FMS must receive valid SBAS signals
c. The NAV mode must be previously engaged

I think b. is the problem. Have you got GAGAN enabled in System-GPS Status-SBAS?
Hi.
Yes. In fact we were on SBAS (GAGAN) navigation on arrival route using NAV mode. Approach was loaded and activated via IAF. There wasn't a caution or message flagged during the entire procedure.

Thanks anyway
​​​​​

[FloaterNorthWest]

NAVBLUE has the procedures as per original message (one for each runway with LNAV and LNAV/VNAV minima). They definitely exist.

From the FOBN:

For APP mode to engage (assuming 2D approach)

a. A flight plan including an RNP approach with horizontal guidance only (LP, LNAV, or LNAV+V) must be active in the FMS
b. The FMS must receive valid SBAS signals
c. The NAV mode must be previously engaged

I think b. is the problem. Have you got GAGAN enabled in System-GPS Status-SBAS?

Point b. is incorrect in the FOBN. You do not need SBAS for APP only mode.

The RFM states that for APP mode you need the “required Level of Service” to be available.

You do not need SBAS for RNP approaches to LNAV minima. (FLM 2.5.7.1).

You can see this in the aircraft if you do a LPV or LNAV/VNAV and you switch both SBAS off on the GTN.

I have seen what has been described above I.e no change to APP mode from NAV and on the second approach it worked. Without knowing every button push and set up its hard to know if it’s user error, database error or Helionix related.

I would suggest raising a WebTek with us and attach a download of the flight file for analysis.

I will see if I can get some information from our Flight Test department or Tech Reps.

FNW

[peeush]

Point b. is incorrect in the FOBN. You do not need SBAS for APP only mode.

The RFM states that for APP mode you need the “required Level of Service” to be available.

You do not need SBAS for RNP approaches to LNAV minima. (FLM 2.5.7.1).

You can see this in the aircraft if you do a LPV or LNAV/VNAV and you switch both SBAS off on the GTN.

I have seen what has been described above I.e no change to APP mode from NAV and on the second approach it worked. Without knowing every button push and set up its hard to know if it’s user error, database error or Helionix related.

I would suggest raising a WebTek with us and attach a download of the flight file for analysis.

I will see if I can get some information from our Flight Test department or Tech Reps.

FNW
​​​​​​ Thanks FNW. I'll see what it gives me during another approach.

​​​I'll do the needful.