RNP Pros vs Cons
 

Hi everyone,
Forgive me if this may have been discussed before but, I am currently working on an uni assignment where I must discuss the benefits vs limitations of RNP.

I have found a mountain of information regarding the benefits of RNP (eg: saves fuel, reduces spread of noise, lower minimums, etc). However I have not been able to find much information regarding the limitations of RNP. The only issues with RNP I have found so far are:

> Maintenance Costs - maintenance facilities may have to meet a higher standard of maintenance. (source: http://www.aea.net/AvionicsNews/ANArchives/RNPJan08.pdf )

> RNP cant be coupled to auto-land (i dont remember my source for this)

> A somewhat lack of contingencies if sudden failure were to occur on approach at such low minimums (AC90-101) "The operator’s contingency procedures need to address at least Failure of the RNP system components, including those affecting lateral and vertical deviation performance (e.g., failures of a GPS sensor, the flight director or automatic pilot)"

I guess this is why its called a 'research assignment' - yet i simply cant find anything in regards to 'cons' of RNP

Any help is really appreciated :ok:

Required Navigation Performance
 

Tom,

You seem to regard RNP as a single aircraft system, ie a box which is bolted to the aircraft somewhere which provides 'RNP'. In reality however RNP is a concept, a state of mind if you like, more than anything else.

RNP specifies how accurate the navigation solution has to be at a given time.

The article you linked to says that aircraft manufacturers are working towards RNP certification of 0.1 or 0.15 in the future. 0.1 nm is about 180 metres, a standard runway is 45 metres wide. RNP certification of 0.1nm will not provide a navigation solution which I would want to rely on to perform an auto-land.

Tom, look up Baro VNAV and you will find the biggest limitation to current RNP operations.

Until a better altitude measuring device is used RNP operations will not approach the accuracy and reliability of an ILS. WAAS and LAAS can increase the accuracy but it is the vertical that limits it.

Also human failure to report the correct QNH will affect the Baro VNAV solution. Human failure in setting, reading and transmittim QNH is about 10 -3 reliability verse FMC reliability at 10 -7.

Make no mistake I am a big fan of RNP AR operations and fly them almost every day of my working life in some very interesting places, but it still does not replace an ILS yet as a better approach. It is close but for most operations it is not quite there.

LAAS approaches (GLS as boeing calls them) do not depend on barometric altimetry anymore and can achieve up to CAT IIIb precision. However that is still in development and only CAT I is approved so far. WAAS is CAT I equivalent currently in some parts of the world and Baro VNAV limits other operations to NPA values.

Flying GLS approaches is basicly the same as any ILS, however the approach path can be curved and support different descent angles in one approach. In practical flying it is very very smooth indeed and allows autolands allready (at least in 737s, delivered with GLS standard autoflight system).

C100Driver:

The real advantage of RNP AR is the lateral accuracy and RF legs to get access with reasonable minimums at airports with significant terrain issues.

Baro VNAV became the vertical component because of politics; i.e. the airlines all had Baro VNAV and none of them had SBAS as a sensor.

To the contrary, WAAS (SBAS) provides a far better vertical solution than Baro VNAV. In the US we now have LPV approaches to 200 1/2. The future will see RNP AR that switches from linear RNP and Baro VNAV to LPV at the FROP (final roll-out point). The result will be greatly improved vertical and lateral accuracy to values that cannot be achieved with linear RNP. With LAAS there is no reason that an auto-land couldn't be a part of the future RNP AR IAP. The limiting factor today is airline economics rather than technology.

At terrain rich locations the lateral solution is far less forgiving of screw ups than the vertical component. Altimeter missettings started with the Wright Brothers.:)

Until LAAS becomes part of the RNP AR solution, it will be "apples" and ILS will be "oranges."

What concerns me is that the "greenies" have kidnapped the U.S. RNP AR program. The concept a few years ago was "RNAV everywhere and RNP AR only where needed." Now, it has become a vehicle where very complex equipment is required to save a few pounds of jet fuel on each approach. Unless abated this will place the non-RNP AR operators (the majority of business aviation) at an inappropriate and unfair disadvantage. And, at airports without terrain or serious obstacle issues the use of VNAV and RF legs on a conventional RNAV platform will provide the "green solution" without requiring all the conditions and restriction imposed by RNP AR. But, the FAA thus far has been resistive to what I call RNP AR overlays with conventional state-of-the-art RNAV. For example, they won't permit non AR state-of-the art RNAV to perform RF legs in the final approach segment or within two miles of the FAF. That, plain and simple, is regressive policy.

RNP AR is great at a places like Rifle, Colorado, Glacer Park, Montana Runway 20, or Monterey, California Runway 28L. But, it is like shooting flies with an elephant gun at places like Baltimore, Maryland, and such.

Major Tom:

Here are a few cons:

1. Equippage with at least one grossly expensive IRU for RNP of less than RNP 0.30 or missed approach of less than RNP 1.0 It's a freebie for the airlines because 3 IRUs are cost effective in an $80 million airframe and needed for oceanic or remote land mass operations.

2. Expensive independent database validation of all RNP AR IAPs on a continuing basis. The only practical solution for the non-airline operation is to pay a Honeywell twice for the same database; i.e., it makes the RNP AR database "golden."

3. Extra training costs and extensive line qualification requirements. And, the invarible struggle with the occasional obnoxious FAA inspector who doesn't like you so he makes it very difficult to obtain your approval.

Really appreciate the help all

Thanks C100 for the info on Baro VNav. As IRRenewal says "RNP" is in a way a state of mind, and not a physical 'box' (like that of a DME or VOR).

Its just hard getting my head around the 'concept' of it (I only have 120 hrs TT) and working on this as a uni student sure makes it an interesting challenge.

In regards to the certification of RNP is there anything from ICAO that is out there? I have some documents from the FAA and CASA which discusses the certification, however I cant find anything from ICAO

Its nice to hear from you guys (actual pilots) what your thoughts on it are rather than listening to the works of Naverus and aviation authorities. If anyone has reports/essays on RNP that have been written from a pilots point of view it would be awesome if you could link them or PM me.
eg; Required Navigation Performance (RNP)

thanks :)

ICAO/IATA stuff:

http://www2.icao.int/en/pbn/ICAO%20D...20Approval.pdf

and

http://www2.icao.int/en/pbn/ICAO%20D...Doc%209613.pdf

Hope the links work...

Major Tom:

State of mind? No. Performance based navigation (PBN) that in theory is not tied to a specific sensor; yes.

But, as a practical matter the present sensors are GPS and secondarily at least one IRU. Further, there has to be at least two FMSes, sometimes three, and for lower RNP values a tailored EGPWS/TAWS. Roll steering is also required instead of CDI steering for RF legs. And, generally for RNP values of less than RNP 0.30 a roll steering autopilot must be used, and probably with a second autopilot available.

That is a whole lot of hardware.

Each OEM has some latitude as to how they make their airframe qualify.

I am very skeptical of all the claims of future efficiency gains based on the use of RNP and Continuous Descent profiles and all the other associated "NextGen" systems and concepts. Even today the vast majority of commercial airplanes flying have good RNP capabilities, but ATC cannot handle them by simply clearing them for a SID, route, STAR, and approach. There are so many vectors and altitude restrictions and speed constraints assigned that even a simple TOD calculation in an operative FMS is unlikely to be accurate by the time ATC gets done with it all.

I just don't see how a couple more/different boxes are going to make so much difference that all that vectoring and constraining is going to go away...

Aterpster

Is it true that, under the FAA AC, an operator and crew have to certified for each RNP SAAAR approach to be flown. If so, this will be a great burden on biz aviation and non-airlines.

GF

galaxy flyer:

No, the burden is on validating each approach in the database. An airline might have only 30 RNP AR IAPs in their ops specs, and they have the expertise in-house to keep precise track of those database entries.

The biz aviation guy wants to be authorized on all of them so he has to buy Honeywell or Jeppesen's database validation service.

But, operationally, once the crew has finished their quota of practice RNP AR IAPs, they are good to fly them anywhere, with the database caveat. The crew cannot validate the database in the cockpit. The procedure is much more complex than merely going through the legs page.

aterpster wrote:
You have a lot of interesting information. A few questions:

How is that a regressive policy? Have they allowed that operation in the past and they are now regressively retracting it?

How is this this an unfair policy toward business operators?

It seems to me that RNP started out as a way for Alaska Airlines to get into Juneau and Sitka more often. Now it's a "Greenie" method to trash BizAv?

Imbracable Crunk:

Not sure what you mean by "that operation." The overuse of RNP AR for non-terrain-related purposes is quite exclusionary. The "green" objectives could be far better served by using non-AR advanced RNAV with unrestricted use of RF legs by all aircraft equipped with modern avionics.

When the RNP AR IAPs become the preferred method of access to busy airports, the non-AR aircraft will go to the holding pattern.

Exactly. The way Alaska Airlines uses RNP AR is only when conventional procedures are limited by terrain. Now, however, other airline operators have chosen a method that requires equippage they have, but most non-air carrier aircraft do not have. The result is not because of a deliberate plan of exclusion, rather it is a plan that only considers the equippage that some airlines happen to have and one operator chose to spend a fortune to upgrade, now they are forcing the overuse of RNP AR to become national aviation environmental policy.