I am studying for my CFII check-ride and I was wondering to know why SIDs and STARs are not RNP 1 but RNAV 1?

Let's start with the following two definitions

RNP - Required Navigation Performance: It's a PBN navigation method/specification in which the airplane achieves an specified accuracy (RNP 1, RNP 2, etc) the a 95 % of the flight time. It has an onboard monitoring system that alerts the pilot when the integrity is loss. Examples of Equipments that are able to perform RNP 1, RNP2, etc are IFR GPS receivers with RAIM or WAAS (integrity monitor).

RNAV - Area Navigation: As we know, FAA uses RNAV in chart tittles only to determine Area Navigation. But RNAV it is also a PBN method/specification in which the airplane achieves an specified accuracy (RNAV 1, RNAV 2, etc) the a 95 % of the flight time. The only difference is that the aircraft won't alert the pilot when the integrity of the system is loss. I've never used an RNAV system without integrity monitor, I guess that DME/DME/IRU is wan of those. Is an IFR GPS with RAIM capability lost considered an RNAV system without integrity monitor?

The final and most important question is: On SIDs and STARs, Why is it not required to have a system that monitors the integrity of the equipment and alert the pilot if the accuracy can't be determined? Are pilots supposed to monitor the integrity of the systems by themselves? How?



I found the following on AC 90-100A CHG2 8. RNAV SYSTEM APPROVAL PROCESS (b)(1)

"For procedures requiring GPS and/or aircraft approvals requiring GPS, if the navigation system does not automatically alert the flightcrew of a loss of GPS, the operator must develop procedures to verify correct GPS operation"

  • $\begingroup$ Which SIDs and STARs specifically are you talking about? Not every SID/STAR in the world is RNAV ... $\endgroup$ – expeditedescent Aug 22 at 16:58
  • $\begingroup$ I am not talking about conventional Ground based NAVAID SID/STAR. So, not any RNAV SID/STAR in particular, but most of them are RNAV 1. I know there are some that requires RNP 1. $\endgroup$ – Ivan Parra Aug 22 at 17:37
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    $\begingroup$ As far as I understand procedures are RNP if it's necessary for terrain separation and just RNAV if it's only for traffic flow (where controllers and TCAS can fix things up). $\endgroup$ – Jan Hudec Aug 22 at 19:48

First of all, an RNP 1 procedure requires a GPS/ GNSS. And as you have said, it requires an inbuilt navigation monitoring and an alerting system. On the other hand, an RNAV 1 procedure does not. In RNAV 1 you can get navigational aid from the IRS/ VOR/ DME or IRS/ DME/ DME.

The SIDs and STARs can either be RNAV 1 or RNP 1. RNAV 1 is the system introduced at the start of performance based navigation to cater for smaller air spaces (terminal area procedures) and it is a step towards changing over to RNP 1. Many countries these days require an aircraft with RNP 1 capability to fly their SIDs and STARs. They sometimes tend to name it unambiguously on the chart. Like a chart might state, RNAV 1 and then state that you require a GNSS. This essentially makes the procedure an RNP 1 procedure. Others state specifically that you need to be RNP 1 complaint. This means that you need to have a GPS and a monitoring and alerting system. This is mainly seen in China, where most of the airports require the aircraft to be able to fly RNP 1 procedures so that they can pack airplanes together, with minimal ATS surveillance. This is done to accommodate the ever increasing air traffic in the region.

It has been quite a while, since I have not been to Wuhan, China for obvious reasons. Wuhan airport is one those example airports which allow only aircraft that can fly RNP 1 for their SIDs and STARs.

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I think the advantage of keeping the charts and procedures RNAV is that it allows operation of older aircraft, which does not comply with RNP operations. This is I believe quite important in a place like the United States where airlines still tend to operate older aircraft and has a big general aviation sector. In other places, airlines tend to have newer aircraft, which easily fits into the current navigation standards. This makes it easy for the regulators to change their procedures to RNP.

For the question regarding losing RAIM (Receiver autonomous integrity monitoring). If you lose RAIM, you probably cannot do a GPS/ RNAV (GNSS)/ RNP approach (same type of approach, just different names). But the aircraft can still be used for RNAV navigation, as RNAV does not require the monitoring and alerting.

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