Monitoring raw data A320

[applecrumble]

The new FCOM has a reference regarding the use of NAV mode.
So long as GPS primary high
OR
Nav accuracy high
OR
Reference raw data

So is Airbus really saying that as long as accuracy high then you can blindly follow the flight directors without monitoring with raw data at all.
Are you legally required to back up with raw data?
I take that reference to mean that even if the beacon required is U/S then you can fly the SID anyway as long as you have GPS Primary.

[RUMBEAR]

According to Limitations published you can fly an NDB OR VOR approach with the ground facility unserviceable or airborne equipment unserviceable/ not installed ( with GPS PRIMARY ) providing you have regulatory approval. So yes the NAV system is capable of navigating without the raw data available.

[STBYRUD]

...and therein lies the problem. At least in EASA territory that approach would need to be approved and published as an RNAV overlay. I'm surprised Airbus writes anything about this in the limitations chapter, this is absolutely not their call to make - BMW don't write in their driver's manual that use of the indicators is optional (providing regulatory approval):}

[Denti]

Ah well, not so different from Boeing that wrote in their 737 FCOM (dunno if its still in there, but it was for several years) that GPS is actually not required for a GPS approach as long as NAV accuracy is high enough.

[sierra_mike]

you are referencing the limitation for NAV mode in terminal area. further down the chapter FCOM is pretty clear on approaches based on radio navaids. and how would you monitor RNAV only SIDs/STARs by means of navaids when they are not referenced to a navaid?
keep in mind certain equipment has to be functional for certain RNAV ops as well

[PENKO]

Ah well, not so different from Boeing that wrote in their 737 FCOM (dunno if its still in there, but it was for several years) that GPS is actually not required for a GPS approach as long as NAV accuracy is high enough.

Playing devil's advocate, isn't the RNP the same for either approach?

[vilas]

applecrumble
So is Airbus really saying that as long as accuracy high then you can blindly follow the flight directors without monitoring with raw data at all.
No! Definitely not. Requirements of using NAV mode differ depending upon where you plan to use it. On route, in terminal area, in managed approach with a NAV aid or GPS approach. Quoted from FCOM:

For RNAV(GNSS) approach: GPS PRIMARY..................................................... ...........................................................C HECK ‐ GPS PRIMARY must be available on at least 1 FMS.In GPS approach there cannot be a back up. In managed VOR approach you need to back it up with VOR although with regulatory approval the approach is permitted without the NAV aid because GPS accuracy is better than the NAV aid itself.

[applecrumble]

Thank you for your replies. I really meant regarding a SID.
Airbus allows you to free tune the nav aids so you might not have what you need to monitor the SID anyway.

[EagleA25]

While on this subject; so, with my previous operator we had the following:
Any GNSS, RNP or -AR, passing the FAF, the CDI under the PFD was visible and actually requiered.
I actually did not make of it much until a small discussions on FB about CFI’s teaching GNSS approaches, but my current operator the CDI does not appear during those approaches.
Logic dictates that “full scale CDI deviation” merits a Go-Around (if RNP is 0.3 and you have full scale on CDI you are 0.3NM or more off course/Centerline) but in our SOP’s it shows that the PM is to “monitor closely the lateral deviation” and nothing else...
Under the circumstances at my new operator it would NOT surprise me if they just too ignorant or simply don’t want to pay Airbus for the approval and made this **** up... any thoughts?

[KayPam]

According to Limitations published you can fly an NDB OR VOR approach with the ground facility unserviceable or airborne equipment unserviceable/ not installed ( with GPS PRIMARY ) providing you have regulatory approval. So yes the NAV system is capable of navigating without the raw data available.

I thought this was called GPS overlay and that it required a special kind of approval ?

[hans brinker]

In the USA GPS overlay is phased out:

https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/nas/procedures/gps_overlay/

[hans brinker]

EagleA25

In our Airbii there is no CDI at all in NAV, so we are only allowed NAV with AP and/or FD. there is a numerical cross track deviation, giving miles left/right off course, on the Nav display. I have always been amazed that Airbus certified it this way, as there is a requirement in most AIPs I have seen for a CDI which changes sensitivity based on phase of flight (enroute 10 miles down to approach 0.3 miles for full scale). I really don’t like not having a CDI, and with EFIS it would be (relatively) easy to implement.....

[Vessbot]

In our Airbii there is no CDI at all in NAV,

What... This is absurd.

[Check Airman]

Aren't most Boeings the same way?

[RVF750]

No they are not!

[safelife]

That's why Airbus aircraft are now updated with an LDEV lateral deviation indication on the bottom of the PFD, for RNP operations.

[EagleA25]

Hey hans
Thanks for your post; We do the same at my operator and I am not sure if the manufacturer actually approved this way of “doing business”.
Reading a few comments after your post actually now makes me feel nervous...
As some mentioned, I would feel a lot safer with an established LDEV attached to the VDEV, or is the Yo-Yo sufficient?

[EagleA25]

safelife

Interesting; What about GNSS?

[Check Airman]

No they are not!

Do you have any examples available? Apart from a 757/767 with an aftermarket flat panel mod, I've never seen it.

[Denti]

Can only speak for the 737. On the classic there was actually a NAVigation mode for the ND which displayed a normal CDI display with additional turning cues for both enroute LNAV operation as well as departure and approach indication. On the NG we had navigation performance scales, which displayed the ANP corridor as well as the LNAV path (same for VNAV during descent) in the PFD. For approach we used IAN which shows the same ILS symbology (including ghost pointers when still on LNAV to the approach) as a normal ILS. That symbology is completely independent of the FD or autoflight system and allowed flying without FD during RNAV or overlay approaches. GLS simply used the same ILS symbology again, but was autoland and auto-rollout capable, but not approved for that.

See also Aero 22 - 737 Approach Navigation Options (http://www.boeing.com/commercial/aeromagazine/aero_22/737approach_story.html)

[FlightDetent]

I would feel a lot safer with an established LDEV attached to the VDEV, or is the Yo-Yo sufficient? Sort of, this is my take on it:

The FMS generated lateral track, FD steering commands to follow it, and the AP control inputs are - on the modern bus - all just elcetronic signals and digital values generated by one multipart computer. Thus their targets cannot be different one from another as long as the individual but not-so-separate elements are actually working.

n.b. The B's design is most likely operationally superior. And A. will follow suit with newer designs for the more high-level RNP installations

a) There is no lateral deviation scale on PFD

b) If the aircraft does really fly off track, the A/C symbol on ND graphically shows displacement from the target (green line) path , and a numerical value of the cross-track deviation is displayed (in NM). E.g. 0,02L. This label was re-designed years ago to show that second decimal digit, which was a certification requirement.

As a pilot, during my first (and the only proper :sad:) RNAV APCH course I was trained to know by heart what is the max permissible deviation before self-declaring unable to continue.

c) If the aircraft loses certainty of where it actually is -> the value of EPE = Estimated Position Error (= ANP in Languedoc dialect) looses beyond the accuracy required, then
. there is a MCDU message NAV ACCUR DOWNGRAD
. there is a TRIPLE-CLICK audio warning
. there is a ND warning flag NAV ACCUR DOWNGRAD

This google picture search (https://www.google.cz/search?biw=1382&bih=891&tbm=isch&sa=1&ei=nzJVWviBDYi6sQHyjanICw&q=Airbus+MCDU+PROG+page+EPE&oq=Airbus+MCDU+PROG+page+EPE&gs_l=psy-ab.3...67821.68889.0.69103.4.4.0.0.0.0.96.363.4.4.0....0...1 c.1.64.psy-ab..0.0.0....0.cMwc_HOvzxE#imgrc=wlmujweczKKm6M:) shows what a PROG pg of MCDU looks like. The blue REQUIRED accuracy value is pilot-modifiable.

----

Hence from a certification point of view, the aircraft is capable of
- both monitoring and alerting the ANP v.s. RNP in forward field of vision (on ND)
- displaying the actual cross-track error to the pilot.

Functionally equivalent to CDI.