RNP AR approaches

[che ci dò che ci dò!]

RNP AR non precision approach procedures are RNAV approach procedures where, to my understanding, RNP is below 0.3 NM and down to 0.1 NM, GPS is required and a special authorization is required for the operator (airline + crew + a/c tech status).
I understand that, for this kind of operation, besides the relevant RNAV approach authorization, a special authorization has to be issued by the State Authority in charge of the relevant airport.
I've checked the A320 FCOM and it states that operators are responsible for drawing contingency procedures for RNP AR NPAs.
Does any of you have any idea on when one can find an overwiew of these contingency procedures? I mean, just to have an idea on what are the actions required to perform a missed approach and to mantain the required obstacle clearence while experiencing navigation accurancy problem.
Thanks!

[Cardinal]

Our contingency procedures, as an approved RNP AR operator, are pushed to the crew. Our briefing card merely directs us to come up with a case-by-case terrain mitigation plan in case of loss of accuracy. Trained to turn toward the lowest terrain, then attempt to orient with a ground-based navaid to the extent possible. Pretty informal.

[aterpster]

che:

RNP AR non precision approach procedures are RNAV approach procedures where, to my understanding, RNP is below 0.3 NM and down to 0.1 NM, GPS is required and a special authorization is required for the operator (airline + crew + a/c tech status).
I understand that, for this kind of operation, besides the relevant RNAV approach authorization, a special authorization has to be issued by the State Authority in charge of the relevant airport.
I've checked the A320 FCOM and it states that operators are responsible for drawing contingency procedures for RNP AR NPAs.
Does any of you have any idea on when one can find an overwiew of these contingency procedures? I mean, just to have an idea on what are the actions required to perform a missed approach and to mantain the required obstacle clearence while experiencing navigation accurancy problem.
Thanks!

RNP AR IAPs can have a line of minima as high as 0.30 and as low as 0.10. Also, the missed approach can be conventional TERPS or PANS-OPS missed approach criteria or it can be an RNP missed approach. An RNP missed approach is "telescopimg" RNP segments, which presumes at least one IRU to maintain positioning throughout the missed approach in the event of loss of GPS, albeit degrading position information, thus the reason for the "telescoping" expansion.

Also, if a line of minima is used that is less than RNO 0.30 then at least one IRU is required for that purpose, independent of the missed approach construction.

So, only on an RNP 0.30 procedure with a conventional missed approach could position be lost unless a dual failure occurs (loss of GPS and IRU(s) ). Because most airplanes have 3 IRUs, this event is highly unlikely.

Here is an RNP AR procedure that does not require any IRU(s), so the loss of GPS could cause the wings to level during the final segment RF leg, thus pointing you at very high terrain.

http://aeronav.faa.gov/d-tpp/1302/05737RR30.PDF

A properly trained crew should revert to the departure procedure for this airport, which would mandate rolling back into the turn and proceeding directly to the BIH VOR to pick up the departure procedure.

If the VOR were out of service that day, then turning towards the airport, then when overhead establishing a heading the same as the departure procedure, should make a DR heading sufficently safe to fly into a less critical area of terrain.

[c100driver]

For a B737, the most likely Navigation Accuracy reduction is a GPS loss. If this occurs then the ANP will begin to increase slowly (about 0.01 every 10 seconds) more than enough time to extract the jet from a terrain rich environment.

Dual GPS failure is mandatory missed approach if still IMC.

There is plenty of time before ANP exceeds RNP with just the loss of GPS and only navigating on IRS so it's no big deal!

[che ci dò che ci dò!]

..thanks for the explanations!
Last question: on the A320 FCOM - Special Ops chapter, there's a list of requirements for PRNAV and another set of requirements for RNP AR. By Icao Doc 9905, definition of RNP AR is "a RNAV approach where RNP is below 0.3 and down to 0.1".
I understand I can fly a conventional RNAV NPA with RNP 0.3 with the same airborne equipment required for PRNAV but the relevant RNP will be 0.3 instead of 1.0.
FCOM requirements for RNP AR are applicable only for RNAV approaches with a RNP below 0.3 which call for a special authorization from the Authority in charge of the relevant airport besides the normal RNAV approach authorization from the Operator's CAA.
Have i got it right?
Thanks!

[hawk37]

aTerpster wrote

Here is an RNP AR procedure that does not require any IRU(s), so the loss of GPS could cause the wings to level during the final segment RF leg, thus pointing you at very high terrain.

Just an inquiry, I note you said "could", however, generally, would it be wings level, or would the loss of GPS cause the FMS to go into DR mode, and then continue to provide steering based on previous TAS and wind parameters?

[Ian W]

..thanks for the explanations!
Last question: on the A320 FCOM - Special Ops chapter, there's a list of requirements for PRNAV and another set of requirements for RNP AR. By Icao Doc 9905, definition of RNP AR is "a RNAV approach where RNP is below 0.3 and down to 0.1".
I understand I can fly a conventional RNAV NPA with RNP 0.3 with the same airborne equipment required for PRNAV but the relevant RNP will be 0.3 instead of 1.0.
FCOM requirements for RNP AR are applicable only for RNAV approaches with a RNP below 0.3 which call for a special authorization from the Authority in charge of the relevant airport besides the normal RNAV approach authorization from the Operator's CAA.
Have i got it right?
Thanks!

You should be extremely cautious following an 'RNP procedure' using pRNAV in theory the accuracy of navigation is the same - however, RNAV (including pRNAV) does not provide for any alerts on reduction of ANP to the extent that the aircraft will no longer remain within safe containment (normally twice the RNP).

[aterpster]

hawk37:

Just an inquiry, I note you said "could", however, generally, would it be wings level, or would the loss of GPS cause the FMS to go into DR mode, and then continue to provide steering based on previous TAS and wind parameters?

When all the criteria were being develop all FMSes without IRUs would cause the wings to go level with loss of GPS. But, who knows what some bleeding edge avionics OEMs have done by now?

There are precious few birds qualified to fly RNP 0.30 (and conventional missed approach) that don't have at least one IRU. But, there are a few.

[aterpster]

che:

Last question: on the A320 FCOM - Special Ops chapter, there's a list of requirements for PRNAV and another set of requirements for RNP AR. By Icao Doc 9905, definition of RNP AR is "a RNAV approach where RNP is below 0.3 and down to 0.1".
I understand I can fly a conventional RNAV NPA with RNP 0.3 with the same airborne equipment required for PRNAV but the relevant RNP will be 0.3 instead of 1.0.
FCOM requirements for RNP AR are applicable only for RNAV approaches with a RNP below 0.3 which call for a special authorization from the Authority in charge of the relevant airport besides the normal RNAV approach authorization from the Operator's CAA.
Have i got it right?
Thanks!

Basic RNP AR is 0.30, not below 0.30.

PRNAV RNP 0.30 is quite different than RNP AR 0.30. In RNP AR the 0.30 segments are 2 X 0.30 with no secondary obstacle clearance areas. Also, the alerting and monitoring requirements are much greater than with PRNAV or advanced RNAV IAPs.

[hawk37]

Thanks aTerpster. On a conventional Rnav approach I will try deselect the gps´s, vor and dme input and see if it will indeed roll wings level vice make a DR turn at the IF.

[aterpster]

hawk37:

Thanks aTerpster. On a conventional Rnav approach I will try deselect the gps´s, vor and dme input and see if it will indeed roll wings level vice make a DR turn at the IF.

The best test would be on an RF leg.

[FlightPathOBN]

RNP AR IAPs can have a line of minima as high as 0.30 and as low as 0.10. Also, the missed approach can be conventional TERPS or PANS-OPS missed approach criteria or it can be an RNP missed approach. An RNP missed approach is "telescopimg" RNP segments, which presumes at least one IRU to maintain positioning throughout the missed approach in the event of loss of GPS, albeit degrading position information, thus the reason for the "telescoping" expansion.

Also, if a line of minima is used that is less than RNO 0.30 then at least one IRU is required for that purpose, independent of the missed approach construction.

So, only on an RNP 0.30 procedure with a conventional missed approach could position be lost unless a dual failure occurs (loss of GPS and IRU(s) ). Because most airplanes have 3 IRUs, this event is highly unlikely.

Interesting tale, no doubt, but certainly a tale none the less.

First off, how RNP and GPS work, and how GPS and the IRU connect together. The GPS signal is not directly fed to the FMS/FMS. The GPS feed is connected to the IRU. The IRU manages the GPS signal, with the GPS signal telling the aircraft where it was, and the IRU calculating using the Kalman filter, and telling the aircraft where it is.
You cannot have a direct feed from the GPS to the FMC. Without an IRU, you have no GPS navigation.

The IRU is receiving the GPS signals from all of the antennas and balancing the results, feeding the results through the Kalman filter, and providing the update to the FMC, as to the estimate of where the ac is. This is where you see the actual vs required numbers on the FMC.

When the GPS signals degrade, or are lost, the IRU will still balance where the aircraft is, according to the other feeds it is receiving. This is where the IRS update values come from.

In the past, you had IRU systems that were not fed by GPS, but now the update value that used to be fed by the pilot, is now fed by the GPS system.

RNP procedures per the criteria, transition to RNP 1.0 from the MAP. This has absolutely nothing to do with # of systems online. It does not matter if you are RNP 0.1, 0.3, or 0.5..you still transition to RNP 1.0...this is the criteria. This transition has to happen before you start an RF leg.
With approved custom RNP, there is no transition and the missed can be at the FAS level. (the power of custom criteria!, and in reality, the only way to avoid many obstacles and terrain.)

You appear to be mixing in ANP growth due to lack of GPS signal, where the IRU is providing sole guidance. In reality, the ANP growth rate must still be below the containment area for the RNP level. As an example, for 0.3RNP the loss of GPS signal must correspond to an ANP growth rate of 0.5RNP, less than the 2x containment of 0.6.

Here is an RNP AR procedure that does not require any IRU(s), so the loss of GPS could cause the wings to level during the final segment RF leg, thus pointing you at very high terrain.

This statement has no basis in reality.

[hawk37]

aTerpster, I´m not able to test this on an RF leg, thats why I said a conventional Rnav app. We have app certified waas only, no RNP. Never the less, I will test it and let you know. Might be quite a few weeks before I can

[che ci dò che ci dò!]

..thanks for the explanations guys!
Now my question is: when I look at an approach chart and I want to know what kind of on board equipments I need, how can I recognize if the IAP is a conventional RNAV, RNAV RNP 0.3 or a RNAV AR?