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A 320 L DEV for RNP AR approach

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A 320 L DEV for RNP AR approach

Old 7th Aug 2019, 11:44
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A 320 L DEV for RNP AR approach

Hello guys,

RNP and RNP AR is a new concept for me and I have a doubt about the deviation scale. Assuming we are allowed to shoot a RNP AR approach with a RNP value of 0.2 or 0.3. During the approach on the progress page the RNP value will be automatically set to 0.3 and in case we are shooting a 0.2 RNP approach we have to manually insert the value in the progress page.
I would like to know if the lateral deviation, the scale of the L DEV will adapt to the value we insert in the progress page or is it a fix value?
Like RNP AR 0.2: 1 dot would be 0.1nm and RNP AR 0.3: 1 dot would be 1.5nm?
Because in the FCOM they say: Go around in case L/DEV reaches 2 dots or XTK reaches 1 RNP. So if the scale is the same for RNP 0.2 and 0.3 then we would go around too early with 2 dots out for a RNP AR approach with a value of 0.3 nm as 2 dots if the scale is fixed would be .2nm deviation only.

Thank you for your help.

Last edited by pineteam; 7th Aug 2019 at 11:56. Reason: typo
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Old 7th Aug 2019, 12:56
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Originally Posted by pineteam View Post
Hello guys,

RNP and RNP AR is a new concept for me and I have a doubt about the deviation scale. Assuming we are allowed to shoot a RNP AR approach with a RNP value of 0.2 or 0.3. During the approach on the progress page the RNP value will be automatically set to 0.3 and in case we are shooting a 0.2 RNP approach we have to manually insert the value in the progress page.
I would like to know if the lateral deviation, the scale of the L DEV will adapt to the value we insert in the progress page or is it a fix value?
Like RNP AR 0.2: 1 dot would be 0.1nm and RNP AR 0.3: 1 dot would be 1.5nm?
Because in the FCOM they say: Go around in case L/DEV reaches 2 dots or XTK reaches 1 RNP. So if the scale is the same for RNP 0.2 and 0.3 then we would go around too early with 2 dots out for a RNP AR approach with a value of 0.3 nm as 2 dots if the scale is fixed would be .2nm deviation only.

Thank you for your help.
Scaling is supposed to conform with the RNP value in use.

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Old 7th Aug 2019, 13:20
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Donít know if there is a different A320 EIS standard available but the FCOM DSC-31-40-K we use describes the Lateral Deviation Scale ďdotsĒ as representing 0.1Nm and does not vary with the selected RNP. For an RNP-AR 0.3 approach we would call LDEV at 1 dot (because that is reached before 1/2RNP) and go around at 2 dots (still less than RNP). For RNP 0.1 LDEV is called at 1/2 dot deflection and GA at 1 dot. Confusing eh?
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Old 7th Aug 2019, 13:45
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Thank you guys!

I found the reference in the FCOM and indeed the scale is fixed regardless of the RNP value for all A320.
So I guess we go around for RNP 0.2 and RNP 0.3 at 2 dots. Thatís weird but oh well.
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Old 7th Aug 2019, 18:25
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Originally Posted by pineteam View Post
Thank you guys!

I found the reference in the FCOM and indeed the scale is fixed regardless of the RNP value for all A320.
So I guess we go around for RNP 0.2 and RNP 0.3 at 2 dots. Thatís weird but oh well.
You must go around if XTK>RNP (no visual reference), so in case of RNP 0.3 you do not need to go around at 2 dots but you can assess the XTK on the ND, unless your operator SOPs state otherwise.
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Old 8th Aug 2019, 03:42
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Hi Sonicbum,

Our SOP follows exactly what's written in the FCOM:
Go around in case of L/DEV reaches 2 dot or XTK reaches 1 RNP.
But for some guys it's whatever comes first but I don't agree with that since it's not written that way. I think like you said in case of RNP 0.3 the limiting factor should be the XTK and not the L/DEV scale. Anyway, I'm going in the sim in few days, let's see what's the instructor has to say about it.
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Old 8th Aug 2019, 04:55
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Go around will depend on the airlines SOP. At our airline we go around for RNP AR (approaches with a RF and or below 0.3) at 1 dot, 2 dots on other approaches using Final App. In the several hundred RNP AR approaches I have done I can count on 1 hand the amount of times deviation has exceeded 1/2 dot, and I have never seen it exceed 1 dot.
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Old 8th Aug 2019, 13:22
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Originally Posted by StudentInDebt View Post
Don’t know if there is a different A320 EIS standard available but the FCOM DSC-31-40-K we use describes the Lateral Deviation Scale “dots” as representing 0.1Nm and does not vary with the selected RNP. For an RNP-AR 0.3 approach we would call LDEV at 1 dot (because that is reached before 1/2RNP) and go around at 2 dots (still less than RNP). For RNP 0.1 LDEV is called at 1/2 dot deflection and GA at 1 dot. Confusing eh?
There are pertinent quotes from FAA AC 90-101A(a):

The aircraft should have an appropriately-scaled, non-numeric deviation display (i.e., lateral deviation indicator (LDI) and vertical deviation indicator (VDI)) in the pilot’s primary optimum field of view (FOV). A fixed-scale course deviation indicator (CDI) is acceptable as long as the CDI demonstrates appropriate scaling and sensitivity for the intended RNP value and operation. With a scalable CDI, the scale should derive from the selection of the RNP value, and should not require the separate selection of a CDI scale. Alerting and annunciation limits must also match the scaling values. If the equipment uses default RNP values to describe the operational mode (e.g., en route, terminal area, and approach), then displaying the operational mode is an acceptable means from which the flightcrew may derive the CDI scale sensitivity. (b) In lieu of appropriately scaled LDIs and VDIs in the pilot’s primary optimum FOV, a numeric display of deviation may be acceptable depending on the flightcrew workload and the numeric display characteristics.

(13) Display of Deviation. The navigation system must provide a numeric display of the vertical deviation with a resolution of 10 feet or less, and the lateral deviation with a resolution of 0.01 NM or less.

5. Requirements for Using Lines of Minima Less Than RNP 0.3. The AFM or aircraft qualification documentation should identify if the aircraft is capable of using lines of minima associated with RNP less than 0.3, and the required equipment configuration to achieve this capability. For example, dual autopilots may achieve a smaller RNP capability than dual FDs.
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Old 9th Aug 2019, 09:13
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Originally Posted by Out there View Post
Go around will depend on the airlines SOP. At our airline we go around for RNP AR (approaches with a RF and or below 0.3) at 1 dot, 2 dots on other approaches using Final App. In the several hundred RNP AR approaches I have done I can count on 1 hand the amount of times deviation has exceeded 1/2 dot, and I have never seen it exceed 1 dot.
Same at my airline. LDEV call at 1/2 dot. LDEV Go Around at 1 dot. (Doesnít matter what RNP the value is set to).
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Old 9th Aug 2019, 11:29
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Originally Posted by pineteam View Post
Thank you guys!

I found the reference in the FCOM and indeed the scale is fixed regardless of the RNP value for all A320.
So I guess we go around for RNP 0.2 and RNP 0.3 at 2 dots. That’s weird but oh well.

.... Our SOP follows exactly what's written in the FCOM:
Quote:Go around in case of L/DEV reaches 2 dot or XTK reaches 1 RNP.



But for some guys it's whatever comes first but I don't agree with that since it's not written that way. I think like you said in case of RNP 0.3 the limiting factor should be the XTK and not the L/DEV scale.
Hi

My take on this is: because you have an LDEV (even though alternative means of monitoring lateral deviation can be approved), you cannot exceed the FSD of the display. In other words you must maintain a 'live needle'. So you would have to go around on reaching 2 dots anyway (assuming that is FSD as seems usual for the bus).

I can see that your scale might be fixed (allowed with caveats) but, I cannot see why you need to use the fixed scale of 2 dots = 0.2 when flying RNP of 0.3. Your avionics must surely be capable of scaling to 0.3 because it is the RNP of every bog standard LNAV and LNAV/VNAV final segment.

I suspect it is only below RNP 0.3 you are stuck with a fixed scale of 0.2. When that is equal or larger than the RNP in use it would be logical to go around on reaching "2 dots or 1 x RNP", whichever comes first (as "some guys" at your company think). Or another SOP like StudentInDebt's, eg go around at x dots even if that is less than RNP because it provides an equal or greater level of safety.
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Old 12th Aug 2019, 09:02
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Thank you everyone for your inputs.

So I was wrong and at 2 DOT on the lateral scale we must initiate a go around regardless of RNP 0.2 or RNP 0.3.

Oggers,
The L/DEV scale for RNP AR is fixed on A320 Family: 0.1nm per dot. I was a bit surprised too.
Happy flying.




Last edited by pineteam; 12th Aug 2019 at 09:15.
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Old 12th Aug 2019, 20:07
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Originally Posted by pineteam View Post
Thank you everyone for your inputs.

So I was wrong and at 2 DOT on the lateral scale we must initiate a go around regardless of RNP 0.2 or RNP 0.3.

Oggers,
The L/DEV scale for RNP AR is fixed on A320 Family: 0.1nm per dot. I was a bit surprised too.
Happy flying.



That must be Your SOPs, as if you are still within the 1xRNP (0.3) there is legally no need to go around. Furthermore the LDEV scale is mandatory only for operations below 0.3.
Just make sure that if the instructor told you so during the Sim, heís actually representing the companyís message.
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Old 13th Aug 2019, 13:27
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Hi Sonicbum.

You might be right. I was not aware that the L/DEV scale was not required for RNP 0.3. I will investigate. Thanks.

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